Living Mulch

Mulching garden beds has been a consistent ‘must-do’ for many gardeners.  Mulching reduces moisture evaporation, suppresses weeds, adds nutrients to the soil, reduces erosion and in many cases improves moisture absorption. Wood and bark chips are the most common materials employed.  Recently I have seen and read more about the negative effects of wood chip mulches. In particular when these mulches are over applied and allowed to compact, they can actually retard rain water absorption.  Thickly applied mulch can retard the growth and expansion perennials.   In his book The Know Maintenance Perennial Garden, Roy Diblik discusses how unnatural wood chip mulch is.  In areas that get adequate rainfall to support a tight matrix of plants, a living, or green, mulch is the ideal.

For many sustainable and naturalistic gardens the goal is to create this ‘green mulch’ or ‘living mulch’ by filling the space between garden features or larger plants with more plants.  ‘Green mulch’ is living plant material that performs all the functions of wood mulch such as weed suppression and soil moisture and temperature moderation.  In addition properly selected green mulches offer additional benefits like supporting wildlife and insects with food and cover, lower maintenance (since they do not to be replenished or broken up on a regular basis).  This green mulch may be a single species or a variety of plants that form an interlocking matrix. This matrix is most like what you would encounter in a meadow or woodland setting.

An example of a classically mulched bed. 
Each plant is distinctly identifiable and  there are no
random plants to confuse the composition.

One problem with green mulches is that they can look weedy due to lack of readability, no clear design or cluttered appearance.  Many folks are more comfortable with a landscape or garden where the features are clear and recognizable.  Having beds with clean edges is a quick way to make a garden more legible.  This human desire for legibility is also seen in the preference of many for plants neatly separated by oceans of mulch. 

One means of creating a green mulch is to install plants closer together so that they quickly grow together to create a continuous green carpet of foliage. This is one of the themes of the book New Naturalism, by Kelly Norris, published in 2021.  The trick here is to create plant communities that a good match to your site conditions and that the plants play well together. 

An example of a green mulch.  Creeping phlox, Phlox subulata,
and wild strawberry, Fragaria virginiana, have grown together
 to form a 4-6" deep ground cover.  Shrubs and perennials,
like the wild geranium, Geranium maculatum, here,
 are able to grow through.

Another route to a green mulch is to plant lower grow species as a matrix between your feature plants.  The trick here is to keep your garden readable so that the matrix does not obscure your design intent.  By way of example consider what bindweed does to a garden.  While it grows quickly to fill all the voids in the garden it also grows up and over taller plants resulting in tangle of plant material that has no discernible form, no starting or ending points and little textural contrast. (Kudzu does this on a much larger scale, draping a woodland edge with green vines, effectively removing all contrast between the trees.)  Consider instead a perennial bed where the spaces between plants are filled with low growing violets.  The glossy round leaves of the violets don’t interfere with the forms and textures of taller perennials or shrubs.  A list of some of the lowest growing  Mid-Atlantic natives includes:  Pussytoes (Antennaria sp.), wild ginger (Asarum canadense), Green and Gold (Chrysogonum virginianum), wild strawberry (Fragaria virginiana), barren strawberry (Geum fragarioides), Meehan’s mint (Meehania cordata), partridgeberry (Mitchella repens), golden ragwort (Packera aurea), various species of phlox (Phlox divaracata, stolonifera or subulata),  foamflower (Tiarella cordifolia), common violet (Viola soriana),  and  short sedges like rosy and ivory sedge (Carex rosea  and  C. eburnea ).

Golden ragwort is excellent native ground cover.  It spreads rapidly
 in moist, partly sunny locations.  It also seems to suppress invasive
 weeds like Japanese stilt grass and garlic mustard.

 

Using spring ephemerals for this purpose is a very natural means of creating a matrix.  These perennials naturally flourish early in the year when tree and shrub canopies are open, then slowly go dormant as the canopy closes up.  They return again the following spring.  In the Mid-Atlantic you may find Dutchman’s breeches and squirrel corn (Dicentra sp.), mayapple (Podophyllum peltatum)  and Virginia bluebells (Mertensia virginiana)  as common examples of these.  A limitation with using ephemerals is that they do totally disappear after setting seed.  They are ideal in an established perennial or shrub bed where you just need something in early spring while the larger plants are leafing out.  

Ephemerals, like cutleaf toothwort, Cardamine concatenata (in bloom),
 and Dutchman's breeches, Dicentra cucullaria, fill out in early spring.
but disappear as the tree canopy fills in. 

Using short-lived species that survive primarily by reseeding is another means to establishing a sustainable living mulch.  By nature these are opportunistic gap fillers.  As the longer lived perennial and shrub layers get established these reseeders tend to be squeezed out as their preferred, open habitat disappears.  Yellow fumewort (Corydalis flavula), American pennyroyal (Hedeoma pulegiodes), common yellow woodsorrel (Oxalis stricta), and self-heal ( Prunella vulgaris) are some of the shorter species that can be used in this way.  Taller, showier species that can be used as temporary gap fillers include wild columbine (Aquilegia canadensis), Partridge pea  (Chamaecrista fasciculata), fleabanes (Erigeron annuus and E. philadelphicus), and black-eyed Susans (Rudbeckia hirta).

Wild columbine, growing about 2' high, is relatively
 short lived in gardens due to competition and rich soils.
It produces a lot of seed so that it can reappear in other suitable
 locations where seeds can make soil contact and
have access to sunlight. (unmulched areas).

Yellow wood sorrel is considered a weed in many cases.  However,
 this native plant satisfies the requirements of a living mulch quite well. 
It grows quickly and produces lots of seed to fill gaps, is relatively short lived,
and is easily displaced by larger plants. As a native it also supports wildlife,
 particularly bees and birds.  Its lax, floppy habit detracts from it appearance,
making it appropriate around much larger plants and shrubs
or where this flop is not a distraction.

Oxalis stricta is very similar to its European relative, O. europea. 
Each has the same common name, common yellow woodsorrel. 
The North American species can be distinguished by the horizontal
disposition of the seed stalks (indicated by arrow). 
O. europea has seed pods on ascending stalks.

So if you are willing to give up the repetitive chore of mulching your garden beds, consider having your plants do that job for you and establish a living mulch.

Managing Invasives 2022

This is pretty typical of an 'invaded' tree with
Japanese honeysuckle and oriental bittersweet
 twining among its branches.

We’ve been on our central Maryland property for about 9 years now.  From day one we have been battling a slew of invasive species that were covering the ground and trees through the woodland landscape.  While we still have a ways to go I feel that we have made some good progress.  As I am getting ready to start another season managing the landscape I thought I would organize my plan around actions and timing, rather than looking at one species at a time. 

First 

My first activity for the year will be to treat any visible garlic mustard, Alliaria petiolata, with a 2% glyphosate spray.  I'll do this sometime in the middle of March, when the temperatures  will be above 40°F with no rain for a couple of days. I started doing this 2 years ago and it seems to have helped me get the upper hand.  Prior to that I had  been only been pulling up plants in mid-spring as they grew tall prior to blooming. This link to my post on garlic mustard control options describes winter spraying in more detail and shows some of the desirable native species that may be visible at that time.  It also summarizes control options throughout the year.  The timing is critical as it is before the spring ephemerals, like spring beauties and Dutchman's breeches begin to sprout.  This lowers the chances of harming native species.   

Here you can see what a small garlic mustard, Alliaria petiolata,
looks like in winter.  Indian mock strawberry, Potentilla indica,
is also green through the winter.  Unfortunately, that weed is not
 as sensitive to glyphosate as the garlic mustard. 
See this link for methods to manage it.

Second

Cutting woody invasives and vines and stump treating with 20% glyphosate can be done anytime of the year that the temperatures will be over 40°F for a few days in a row.  Winter is a good time to do this as it is easier to get to the base of many or these plants and there is less chance of getting the herbicide on desirable plants.  Particularly troublesome on my property are multiflora rose, Rosa multiflora, oriental bittersweet, Celastrus orbiculatus, Japanese honeysuckle, Lonicera japonica, and to a lesser extent Autumn olive, Eleagnus umbellata.  An excellent reference for managing invasives in the Mid-Atlantic region is Plant Invaders of Mid-Atlantic Natural Areas. 

In doing dormant season treatments it is critical to be able to distinguish friend and foe.  Below are some images of multiflora rose and other native species that it might be confused with.  

Here is multiflora rose in the winter.  Some distinguishing features are
its round olive green branches and its curved thorns.  The most
distinctive feature are the fringed stipules at the base of the petiole,
shown in the inset above.  It is the only species with this type of stipule.

 

Most native roses do not have green branches in
winter. Thorn shapes vary.  Here, swamp rose has
straight needle-like thorns.

Cat briar has bright green branches in winter
and it has straight almost pyramidal thorns.

Wild blackberry has smooth red stems in winter
armed with stiff red spines. Older branches
are square with indentions on each face.   

Black raspberry has red canes with somewhat smaller thorns. 
It is distinguished by the white bloom on the older branches 

You can read more about  dormant season treatment of multiflora rose at the link.  This technique is also effective on English ivy, Hedera helix.  Even if you don't use herbicides to treat the cut stumps, cutting vines climbing trees and over shrubs is helpful in controlling the spread of these invasives.  This is because many of these species are only able to bloom on vines that are elevated and/or exposed to plentiful sunlight.

Oriental bittersweet can twine against itself to get
stiffer and climb higher.  I cut these a while back
but left them long so I could easily find them. 
I'll cut them shorter and treat with glyphosate later.

This is typical of the damage that Japanese
 honeysuckle can do to a tree.  This vine can be
 recognized in winter in that it still has leaves and
 the older branches have shaggy bark.  

Third

Around the end of March I will use a brush cutter and mower to cut down last years growth of vines and undesired woody plants in the meadow and woodland edges.  The biggest problem in my meadow is wineberry, Rubus phoenicolasius. In addition to the wineberry there are the aforementioned multiflora rose, autumn olive, and oriental bittersweet.  

Spring mowing and pulling has been pretty effective at reducing wineberry in shady areas. This is a short-lived species and not deeply rooted so it is usually easy to pull up.  Cutting to the ground in spring seems to keep it from blooming, but it is still able to reproduce by it ability to put down roots wherever a branch touches the soil.   It is recommended that mowing/cutting should be done several times each season to be truly effective.  This is particularly true in sunnier areas where the cut plants can rebound quickly.  Oriental bittersweet can also resprout easily after being cut.

Wineberry is easily recognized by its dense
coating of stiff hairs mixed with red spines. 
While formidable in appearance these are
easily crushed with a gloved hand.

I will return about a month later and do a foliar spray on the wineberry sprouts. While burning is not considered an effective option of wineberry control I will test out targeted ‘cooking’ of individual crowns with my garden torch to see if that kills them in place.  (Standard burns do not selectively kill the wineberry, rather it clears out the competition and allows it to grow unhindered.

Fourth

At about the time that the forsythia is beginning to bloom is the time for me to get started on Japanese stiltgrass, Microstegium vinineum, control.  Stiltgrass has definitely been reduced in the lawn by use of a pre-emergent herbicide originally used for crabgrass control.  It should be applied when forsythia are beginning to bloom.  If you want to be more precise you can use a growing degree day tracker geared toward turf management. like GDD Tracker 4.0. The product I use contains only dithiopyr (Dimension™) and no added fertilizer.  Most of my lawn is fescue based and not that hungry for added nutrition.  This link lists some other preemergent products that have shown effectiveness against stiltgrass.  I wrote about my year long plan for controlling stiltgrass in this post.  In the woods pulling and weed whacking, particularly in late summer has reduced, but not eliminated the amount of stiltgrass.  As a result of thinning out the stiltgrass, I am seeing more native species filling in such as white avens and Virginia jumpseed.  I am also seeing an increase in perennial grasses (perhaps a Glyceria species) in areas where stiltgrass had dominated. 

In the sunny meadow stiltgrass has been harder to eliminate.  The preemergent has not been as effective on the rougher soil and I am hesitant to use it every year as it may negatively affect the growth of desirable plants from seed.  Summertime pulling and the addition of tough native grasses and forbs is helping to displace the stiltgrass.  Weed whacking close to the soil level in late summer as the stiltgrass is beginning to bloom is effective.  However this will also damage other desirable species.  The best method or methods to use depend on the situation in a given location. 

Rest of Year

Mid-spring is the time when I will be watch for the rapid growth of garlic mustard as it prepares to flower.  Pulling it out and leaving it in the sun to dry is my method of choice at that time.  

Late-July and August are the time for pulling out stiltgrass as it prepares to bloom.

So there appears to be an awful lot to do, but it is encouraging that I have seen some progress.  I realize that I am talking about using a lot of herbicide, but these treatments are targeted on the actual plants and done at a time that has little negative impact on native species.  In this battle I feel it is necessary to properly use all the tools that are available.  Another aspect of invasive species control, is limited resources, especially time.  It is better to do one area really well, then move on to the next, rather than doing a little bit everywhere.

I wish you all good luck as another growing season is upon us!

Plans for 2022

For 2022 I have a smaller list of plants that I will be adding to my landscape than in years past.  There are a number of reasons: I’m running out of space to tuck in new plants, I’m moving more seedlings of successful native species around, rather than purchasing new plants, and I am getting more selective about using regionally native species.  However, I am not beyond adding a few species for decoration.  New plants that I am adding are prairie willow (Salix humilis), creeping lespedeza (Lespedeza repens), a wild type smooth hydrangea (Hydrangea arborescens) and I’m trying winecups (Callirhoe involucrata) again in a less competitive environment.

I have been looking for native willows to replace the mounds of forsythia on my property.  Willows bloom early in the spring, like forsythia.  But unlike them, willows do provide a benefit to the native insects: early flowers for bees and as a host species for a number of insects.  Early on I put in some native pussy willows (Salix discolor).  These are doing well but mine tend to have a taller upright form (nearing 20’) rather than a more rounded shape that I was looking for.  Prairie willow has the form I’ve been after, 4-7’ tall with a loose vase-shaped habit.  It is also tolerant of drier conditions than most willows.  I’ve got some on order, so we will see later this spring if my search is finally over. 

Pussy willow blooms at about the same time as
forsythia (shown here in the background).  

Another species that I have been trying to expand in my landscape is smooth hydrangea.  Most of mine are the huge mophead type, most likely ‘Annabelle’.  These grow really well with lots of flowers, but since most of these flowers are sterile, they offer little benefit to wildlife.  Two years ago I planted planted some tiny pots of the ‘Haas Halo’ cultivar.  These were highly rated in a recent study done at the Mt. Cuba Center.  These have large lace cap flowers which consist of mostly small fertile flowers in the center surrounded by a ring of large sterile flowers.  The plants grew to over 2’ tall last season, so I’m hoping I might see some flowers this year.  To build on this diversity I ordered a wild-type smooth hydrangea to add to the mix.  Wild-type plants usually have smaller flowers than the cultivars, but I’m hoping to see a benefit in the production of some seeds that could help the birds.

These wild hydrangeas are growing along the Potomac River
in Maryland. This photo was taken in late April. 
It seems kind of early for buds to be forming, considering
that 'Annabelle' hydrangeas normally bloom in June.

An ongoing project that I have is to find replacements for vinca, particularly in dry shade.  I am expanding the scope to include a replacement for the yellow archangel (Lamiastrum galeobdolon) that is spreading in my pine woods.  Last year I got some round leafed tick trefoil (Desmodium rotundifolium) for which I had been searching for 10 years (we’ll see how that has overwintered). It trails along the ground sporting three round leaflets on each stem. While examining catalogues I came across another trailing plant for dry shade, creeping lespedeza (Lespedeza repens).  While not super attractive on its own, it may work nicely in a matrix planting. 

I didn't have a photo of the Lespedeza, so I'm sharing an image
from my botanical sketchbook.  Included are some notes on
how to tell Desmodium and Lespedeza species, both
members of the pea family, apart.

The wine cups are an exception to my focus on regional natives.  These are native more to the mid-west and southern plains, than to the east, but I became enamored with them ever since I saw them in a field in the Ft. Worth area.  I was able to grow them in the Boston area and had them for awhile here in Maryland.  The problem was that they were not able to compete with dense growth of asters, goldenrods, and Virginia creeper that surrounded them.  This time I will plant them where they have a little more of their own space to get established.

The magenta flowers of wine cups are hard to miss.  It has a
sprawling habit and can form a ground cover 
where competing vegetation is sparse.

As I mentioned above (and last year at this time) I will also be moving many native seedlings out of my vegetable garden, particularly Rudbeckia sp., Monarda fistulosa, Asclepias tuberosa, and Echinacea purpurea, and into the beds where I am fighting Japanese stiltgrass.  I started that process last year.  It’s too soon to see a difference, though some of the transplants appear to have overwintered.  Their real value will be if they are able to reseed themselves and become self-sustaining.

Squirrel baffle

Our squirrels have figured out how to get into the bird feeder that was hanging from a branch of a tree. This was despite a series of impediments that I put in their way. It was a somewhat tolerable situation for awhile; however, when they figured out how to knock the entire feeder off of its hanger and onto the ground, it was time for a change. So we moved it to a shepherd’s hook. I thought the narrow metal pole would be hard for them to climb. NOT SO! After a day they were climbing the 1/2” pole and clearing out the bird seed. Searching for another solution, I saw baffles advertised online and that seemed to be a good solution, but I didn't want to shell out upwards to $50 on something that I was not certain would work. So to test whether this would out with our squirrels I looked into making one from materials I had on hand.

Most baffles that I saw for sale were essentially sheet metal cones at least 18" in diameter that are attached to the central pole. The conic shape keeps debris from getting trapped on top and makes it a little harder for the squirrels to get a hold of. I was able to make this one from 10” aluminum flashing left over from a roof repair.

1) Here are the two semicircles with the notch
cut in the center. Note the triangular tab
on the left hand piece made by bending
down the edge.

To form the baffles I cut semicircles 20" long x 10" wide from my stock of aluminum flashing using tin snips. I cut a a notch about 1//2" wide x 3/4" deep on the edge of each piece of the stock 10" from the end. This will fit closely to the pole. To increase rigidity of the final cone I bent a triangular tab from the center cutout to the edge, about 3/4", on each piece. (Image 1)

To join the pieces together I overlapped one seam by about a half inch and drilled holes for two small #6 machine screws and bolted them together. To get the fit just right I put a 1/2" rod into the center hole and pulled the two free edges together to form the cone. Now I could drill the final two holes along the edges of the stock to ensure a good fit. (Image 2)

2) The two pieces were joined along one edge with
 machine screws and nuts.  To ensure a proper fit,
 the holes for the second pair of screws were
 predrilled after fitting the baffle onto a 1/2" post.

3) Here's a closeup showing the two taps formed by
 folding down the edges.

Mounting the cone on the shepherds hook was easily accomplished by attaching a slightly oversized hose clamp at the desired height on the pole. The cone was mounted by removing two of the machine screws, opening the cone, then fitting it on the pole above the clamp. The screws were reattached, in place, to secure the cone. Having the cone 'float' on top of the clamp makes it more difficult for the squirrels to get hold of. (Images 3 & 4)

4) I had this hose clamp on hand that was large
enough to keep the baffle from sliding down.

Success!  Here's the final product with
birds happily at the feeder and a pair of squirrels
on the ground waiting for seeds to fall.

With the homemade baffle in place the squirrels were no longer able to get directly into the feeder.  Instead they just hung around the edge waiting to see what hit the ground.  Serendipitously I was given a nicer looking store-bought baffle at the end of last year.  I will be using that one in 2022.

This store-bought baffle has all the features of my
homemade one, except maybe the rugged good looks.

Why Won't My Garden Grow?

 I’ve noticed that I’ve been having less success growing vegetables from seed in my garden over the past few years.  That is particularly the case for small seeded crops like lettuce and chard.  This is in spite of increased organic fertilization.  True, I don’t water as much as I should for seedlings, but I never do.  I tend to rely on rain.  Still I have become suspicious that something else may be going on.  This has led me to consider allelopathy.  My question is whether there is something that is growing or has grown recently in the garden is affecting the ability of new seeds to germinate and develop?

Garden in mid-June: Vegetables still small in
 relatively clean beds, but Monardas
and Rudbeckias are coming on strong.

Allelopathy refers to the beneficial or harmful effects one plant has on another. It arises from the release of allelochemicals from plant tissues though leaching from leaves or roots, volatilization or decomposition of plant parts in or on the soil.  Through these allelochemicals one plant is able to suppress germination or development of other plant species in the immediate area.  In some cases these chemicals may have a beneficial effect on some neighboring species while having a negative effect on others.  There is a lot of research in this area, particularly to find crop plants that are able to produce their own weed suppressive chemicals rather than relying on added herbicides.

There are a couple of non-traditional practices that I have been employing in my vegetable garden that I am beginning to question.  One is that I allow native species to run rampant along the edges and between the rows of vegetables, particularly wild bergamot (Monarda fistulosa) and black- and brown-eyed Susans (Rudbeckia hirta and triloba).  The other is that I leave the roots of the previous year’s plants in place.  I just cut the old plants off at ground level and throw them into the compost pile.  I do this as a means of increasing the organic matter in the soil and reducing soil disturbance. 

Following the adage, “a month in the laboratory can often save an hour in the library” (Frank Westheimer) I decided to do a little research first.  I searched the internet for information on the allelopathic potential of all the native and non-native species that are growing in my vegetable garden.  After searching those, that I turned to looking at the vegetable themselves. 

The tables below list many of the weeds, native species and lastly the vegetables that are common in my garden along with the existence of any documented evidence that these plants possess any allelopathic properties.

Garden Weeds:

Common Name	Botanical name	Evidence of Allelopathy
Chickweed	Stellaria media	Yes
Ground ivy	Glechoma hederacea	Yes
Hairy bittercress	Cardamine hirsuta	No
Indian strawberry	Duchesnea indica	No
Mulberryweed	Fatoua villosa	No
Ladies thumb	Polygonum persicaria	Yes
Nut sedge	Cyperus esculentus	Yes
Persian speedwell	Veronica persica	Yes

 

Native Annuals & Perennials:

Common Name	Botanical Name	Allelopathic?
Annual sunflower	Helianthus annuus	Yes
Common Milkweed	Asclepias syriaca	Yes
Butterfly weed	Asclepias tuberosa	No?
Common and Daisy Fleabanes	Erigeron philadelphicus and annuus	Yes
Honeyvine	Cynanchum laeve	No
Wild Bergamot	Monarda fistulosa	No
Scarlet beebalm	Monarda didyma	Yes
Purple Coneflower	Echinacea purpurea	Yes
False sunflower	Heliopsis helianthoides	No
Goldenrods	Solidago sp.	Yes (some species, at least)
Browneyed Susan	Rudbeckia triloba	No
Wild Blackberry	Rubus sp.	?
Sealheal	Prunella vulgarus	No
Pennsylvania smartweed	P. pensylvanicum	Yes
Wingstem	Verbesina alternifolia	No*

*Golden crownbeard, V. encelioides, a native of western North America does have allelopathic properties.

Garden Vegetables:

Vegetable	Botanical Name	Allelopathic?
Arugula	Eruca vesicaria ssp. Sativa	No
Basil	Onicum basilicum	Yes
Collards	Brassica oleracea var. viridis	Yes
Cucumber	Cucumis sativus	Yes
Green Beans	Phaseoleus vulgaris	Yes
Lettuce	Lattuca sativa	No
Peppers	Capsicum annuum	Yes
Squash	Cucurbita pepo	Yes
Swiss chard	Beta vulgaris var. cicla	Yes
Tomato	Solanum lycopersicum	Yes

 

While doing these searches I came across some interesting studies on the potent allelopathic effects of many invasive species.  This is one of the factors that allow invasive species to outcompete native ones.  One study treated radish seeds with the aqueous extracts from leaves of a number of invasive species.  The radish seeds were then evaluated for germination rate and root growth.  The following list is ranked in order of negative effect on germination, most to least:  Ailanthus altissima, > Microstegium vimineum, Alliaria petiolata, Celastrus orbiculatus,> Ligustrum vulgare, Rosa multiflora, Rubus phoenicolasius, and Acer platanoides.  There are multiple studies that document the allelopathic effects of Japanese knotweed, Polygonum cuspidatum.  It seems that many species of the genus Polygonum have some allelopathic character.

From this research it looks like the fleabanes (Erigeron annuus and philadelphicus) and purple coneflower would be the most likely allelopathic culprits among the native species in my garden.  However, most of the allelopathic candidates on these lists are from the weeds and garden vegetables.  Of these, ground ivy, nut sedge and ladies thumb are the most prolific weeds in my garden.  Among the vegetables in my garden, collards, peppers and cucumber are the most likely suspects to be causing some troublesome effects.

Allelopathy experiments.  In general experiments that measure allelopathy are tricky to interpret.  Many are based on bioassays, where the allelopathic effect is measured by such things as the germination rates and root growth of a target species.  In many cases radish or lettuce seeds are used.  Many of these studies use extracts of leaves, stems or roots of the species being studied applied at various concentrations to isolated seeds of the target species in a Petri dish.  Another approach is to test the soil itself.  Due to the complex nature of plant-soil and plant-plant interactions, most studies on allelopathy are correlative rather than causative. 

To help me understand whether there was a problem with the soil (and all the things that come with it), or the cultural effects like moisture and sunlight, I decided I would try a bioassay comparing surface soil samples from various parts of the yard and garden.  I tested to see how each of these soils affected germination and growth of some lettuce seeds which had performed well when started indoors this past spring.

Germinated seeds:  Control (top);
near wild bergamot (middle);
 under English walnut (bottom)

I adapted a procedure from Environmental Inquiry/Cornell University for performing lettuce seed bioassays using materials I had on hand.  I took three samples of surface soil, 0-1/2” deep, from each area of interest.  This is the zone that I normally plant small seeds like lettuce in.  I mixed the soil samples in a baggie then pulled out about a tablespoon’s worth.  This was put into a fresh baggie (as I didn’t have any Petri dishes) and moistened with a few drops of water to make the soil damp, but not wet.  I disinfected the lettuce seeds in dilute bleach, according to the procedure, and then put 8 seeds into each baggie.  The baggies were sealed, put into the dark for 5 days at 60-80°F.  I used a moistened, peat-based seed starting mix as the control.  Samples were taken in November so there was no active growth of vegetables except for the sample near actively growing collards.  I tested the following areas: 1) garden soil under green beans, near daisy fleabane; 2) garden soil under wild bergamot; 3) garden soil near where tomatoes and cucumber had grown; 4) garden soil near collards; 5) typical lawn soil (fescue and weeds); 6) under an English walnut with vinca groundcover.

After five days I separated out the germinated lettuce seeds, counted the number of germinated lettuce seeds and measured the length of the roots.  I found I was able to pluck out the germinated seeds from the soil in the baggies by suspending the entire sample in water with a little dish soap.  This allowed the soil to fall away from the roots without damaging them.  The root growth measurements had a lot of variation.  This is not too surprising considering the many uncontrolled variables, like variable soil moisture and soil contact within each baggie, and small sample size.

Graphical comparison of various soil samples on germination and root
growth of 'New Red Fire' lettuce seeds.

Results and Conclusions

While the differences in root length and germination rate among these samples are not highly significant, due to the small samples size, there is an indication that the soil around the collards is not as favorable to seed germination and growth as any of the other garden samples and is on the order of that for soil under an English walnut tree.  (English walnut, Juglans regia, is much less allelopathic than our native black walnut.)  These results also indicate that the presence of wild bergamot (Monarda) is not detrimental to the initial stages of seed growth.  Another indication was that root growth in the soil around the green beans was less than the control sample.  This is consistent with a study that looked at the effect of leaf extracts of several lines of common beans on seedling growth, including lettuce.  That study made no mention of the effect on seed germination.

Lettuce or radish seed bioassays are also good for checking for herbicide residues in soils and mulches.  General information about testing whole soil samples for herbicide residues can be found at this link from NC State.

Based on the literature it seems like many of the plants in my garden have some degree of allelopathic behavior.  This brings up another important question; how long does that effect last.  The sense I get is that it’s more on the order of weeks than months.  In field trials some brassica family cover crops have shown allelophatic effects on the order of weeks to months after being tilled in. I found another post discussing the use of cover crops, many of which are allelopathic.  It suggested waiting 3 weeks after tilling them in before planting.  Leaving allelopathic plant residues in place as a mulch, will increase the time over which they have the suppressive effect.

 So, based on my little experiment and all the literature I scanned, it seems there is no single clear cut culprit exerting a negative influence on my gardens fertility.  This research has led me instead to a list of suspects and actions to control their effects.   It may be that my practice of leaving roots in the ground, particularly the collards, over the winter may be negatively affecting my gardens ability to grow grow directly from seed. I will need to dig these out at least a month before planting in the spring.  I will also pull out any remaining roots of the curcurbits and chard which remain, as well.  The ubiquitous weeds, particularly chickweed, ground ivy, and ladies thumb need to be removed, especially during the growing season.  As much as I like the prolific flowers of the native daisy fleabane, I will be removing those from the vegetable beds, as well.

Another less traditional garden practice I have been using is flame weeding.  This involves using a hand-held propane torch to kill weed seedlings before they can establish.  This was done prior to planting, or transplanting crops.  An advantage of this practice is that it doesn’t disturb the soil surface.  I have not looked into any literature on possible negative effects of this practice, but for next year I will not use this method in the growing beds themselves. 

So with these changes we will see if I get any better results in the garden, or will I need to dig deeper?

Wishing you all the best for the New Year!!!