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!!!

Late Summer's Larger Blooms

Now that summer is nearly over I thought I would do a run down of some of the showier native plants that I have around the property.  Most of these are found naturally in Maryland.  A few are true wild flowers that have arrived on their own, though most of these have been introduced to this site.

The most common wildflower to this area is Wingstem, Verbesina alternifolia.  It is tall (4-6 ft) in full sun, though shorter in shadier locations.  The bright yellow compound blooms have been evident since the beginning of August.  In this area it is currently the dominant roadside wild flower.

This wildflower was just fading out at the beginning of October last year and I could not figure out what it was.  The northern edge of it range is in New York state, so it would be uncommon in the Boston area.  The form of the plant is similar to Sneezeweed (Helenium) but the shape and quantity of the petals are different.  Sneezeweed has wedge-shaped petals tightly arrayed around the center disk.

The arrows point out the winglike appendages
on the stems, hence the common name.  

Before the Wingstem was in full bloom, Common Evening Primrose, Oenothera biennis, was dominating the roadsides.  This is a true biennial.  It spends the first year as a low growing leafy rosette.  The second year the flower stalk shoots up to 6 ft or more with spikes of yellow blooms.  Each flower opens in the evening and only lasts a day (maybe a little longer if it is cloudy).  I have a bunch of these growing around our patio, while the flowers are nice and bright, they have unsightly lower stems.  For this reason I would banish them to the background.  The biennial lifecycle is well adapted to highly disturbed environments.  The seeds need an open sunny area to germinate and for the low-growing rosette to get plenty of sunlight.  So this is a good plant for open roadsides that get cleared once or twice a year.  But not so good for an area with an established dense undergrowth.

Evening Primrose gets way too leggy
to use near the front of a border.

I noticed that the Japanese beetles were eating a lot of the Evening Primrose leaves.  I collected quite a few in my soapy water jug (aka, Jug of Death).  Could these plants be used as a magnet to draw the beetles away from more desirable garden crops?




This next group of wildflowers are native to Maryland, although I am pretty sure they are not indigenous to our little valley.  New York Ironweed, Veronia noveboracensis, is scattered around our house, but I have not seen it growing in the woods or along the roadside.  The deep purple flowers are long lasting and provide a wonderful contrast to all of the yellow flowers that are dominating the gardens at this time.  In protected areas these wildflowers can get quite tall, more than 6 ft.  In open areas the deer have pruned them back to 2-3 ft tall, but they have still managed to bloom.  At this smaller size they actually fit in better to the garden scale.

This Ironweed was not eaten back by deer and
grew to about 5 ft in partial shade.

Speaking of yellow flowers, there are lot of Yellow Cone Flowers, Rudbeckia fulgida, growing in the neighborhood.  Probably most are the 'Goldstrum' cultivar.  They all grow to the same height and bloom at the same time, making a definite statement in the garden.  As a advocate for native annuals and biennials (i.e. short lived, freely seeding plants) I have been using more of the Black-eyed Susans, Rudbeckia hirta.  These have a more relaxed habit and a longer blooming cycle than the more common cone flower cultivars.  (note some variation to the flower shape).

These Black-eyed Susans were raised from seed this year.
They are not all identical, note the double-side blossom on the right.

In the shadier areas of the garden I have added them more demure Brown-eyed Susan, R. triloba, and the shade tolerant Elm-leaved Goldenrod, Solidago ulmifolia.  This is the first year that this goldenrod has bloomed.  It is an early bloomer and really brightened up the shady area, but the color only lasted a couple of weeks.  I just planted in some Bluestemmed Goldenrod, S. caesia, to beef up the appearance.  I just read that these two species can hybridize - so I guess that the bloom times will be similar.

This plant was eaten back once.  If unpruned
it would reach 304 ft tall.

This is an early blooming species of Goldenrod
that does well in shadier locations.

I found that I had problems with the deer and possibly rabbits eating back both the Rudbekia and Solidago.  I gave these plants a little assistance with some hot pepper spray (cayenne pepper) and some scent-based deterrents like 'Repels-All.'  I need to get on a regular application schedule with products because waiting for the plants to be eaten is usually too late.  I have also gone too far with over application on new plants resulting in killing much of the tender foliage with the liquid sprays.  For these plants I am favoring the pelleted products.

Partridge pea, Chamaecrista fasciculata, is a true native annual that I planted this spring.  I had better results with plants started indoors than outdoors, but much of this was due to grazing by unwelcome animals.  After a little hot pepper spray I was rewarded with stems full of loose yellow blooms.  These open blossoms were visited primarily by larger bees.  These peas need a specific symbiotic bacterium to assist with nitrogen fixation.  This was supplied with the seeds I bought.

The leaves of the Partridge Pea will fold up when it is dark or particularly windy.

In the vegetable garden I'm growing the 'Stampede' cultivar of Jerusalem Artichoke, Helianthus tuberosus.  Now that they have reached 10-12 ft they are just starting to bloom.  I selected this cultivar for its more easily harvested tubers.  I've seen Jerusalem artichokes recommended for use in edible landscaping; however, to be effective a shorter earlier maturing selection would be more appropriate.  Before I put up the deer fence around the garden these plants were being grazed down to the ground on a regular basis.

These towering blooms are supposed to have a chocolaty scent.
I will need to pull some down to find out.

The color of the bracts ranges from pale green to pink.
On close examination  you can see the black dots
on the yellow flowers between the leafy bracts.

Among the deer resistant plants I put in around the garden were two native species of Bee Balm.  The Wild Bergamot, Monarda fistulosa, has been growing larger but has not bloomed this year.  However, the Spotted Bee Balm, M. punctata, has grown and flowered in the first year from seed.  These two plants have been untouched by the deer while the nearby Rudbeckia have been routinely munched.


While not native to the east coast the midwestern Blue Giant Hyssop, Agastache foeniculum, has been an excellent garden plant.  It is long blooming, attractive to pollinators, nicely textured and untouched by deer.  The biggest concern with this plant is its tendency to set a lot a seed.  We'll see what happens over the next couple of years.

There were lots of bluish spikes at the end of July.
These have faded to a mauve color in September,
but have maintained their shape.

In addition to these plants I have observed a number of truly wild flowers including Heal-all (Prunella vulgaris), Maryland Hawkweed (Heiracium marianum), Spanish Needles (Bidens bipinnata) and several goldenrods that I still need to ID.

As we transition into Fall there are a number of asters coming into bloom as well as a variety of goldenrods.  As I walk through the woods I will keep an eye on the ground for anything new

Early Summer Blooms

Bees visit American Bellflower in both sunny
and shady locations

Now that we are getting into the hot days of summer, blooming of many native plants and the associated pollinator activity are picking up.  Here at home I saw my first Monarch Butterfly in many years.  You'll have to take my work for it, since it was gone by the time I got my camera.

One plant that has been blooming for awhile is the American Bellflower, Campanulastrum americanum.  This is particularly attractive to a medium sized black bee, probably a Miner Bee.  This plant blooms nearly as well in the shade as it does in the sun.

This Miner Bee is the primary visitor to the Bellflower

This Miner Bee draws nectar from the open face of the flower.  To do this it lands on the stamen and style of the flower.  Apparently the flower's stigma is situated to receive pollen from the bee as it makes its approach to the flower.








Nearby, on the sunny side of the driveway the Echinacea is in full bloom.  These attract a variety of pollinators, such as this Green Sweat Bee.  The Meadowsweet, Spiraea latifoia, has finished it's first round of blooms.  This year I cut some of the plants back significantly to keep the growth in check.  (I'm pretty sure it will put out a second growth.)  So for now the bees will be visiting other flowers for pollen and nectar.

It's interesting to note that with all the activity on the native flowers, I have seen very few insects visiting the flowers on my nearby shrub rose (other than a couple of Japanese Beetles).

Another early bloomer in the dry shade of my Norway Maple is Rosin Weed, Silphium integrifolium.  I chose this species of of Silphium because it does not get as big as the more familiar Cup Plant, S. perfoliatum.  This plant has slowly been expanding its mass, but I have not seen it show up in other parts of the garden.

A Hover Fly monitoring a cluster of Rosinweed blooms.

The flowers on this particular plant tend to form on the shady side.
This makes for a difficult photograph.

A new native annual that I'm trying out this year is Partridge Pea, Chamaecrista fasciculata.  This plant will grow in poor, dryish soils.  So far I'm favorably impressed.  One grouping that I planted near a highway is growing and blooming, despite receiving no additional moisture, other than the small amount of rain this summer.  Like its relative the Sensitive Plant, its leaves will fold up when the plant is handled roughly.  The leaves also fold up when it gets dark.  I wonder if this behavior helps it to survive under dry conditions (by limiting transpiration).

Some drifts are still intact, like the lavender-color Beebalm, Monarda fistulosa;
the orange Butterflyweed has blown over from another part of the
Wildflower Meadow at Mount Auburn Cemetery.

A Monarch Butterfly passing over
some Beebalm and Hoary Vervain

Over at Mount Auburn Cemetery, in Cambridge, MA, there is a good sized native wildflower meadow installed about 5 years ago.  Here I have seen many more butterflies than in my urban backyard.  This meadow features a number of native grasses as well as many showy flowering plants.  The original planting had the plants arranged in drifts, but the management plan is to let the plants move around as they will, to create a dynamic garden with plants finding there best locations.

In another part of the cemetery I noticed this Bottlebrush Buckeye, Aesculus parviflora, in full bloom.  While native to the Southeastern US, this shrub is very attractive to the bees up in the Northeast.  I spent some time watching how the bees interacted with the flower.  It was more like a mugging than a gentle approach to sip some nectar.  The bee grabs onto to the outside of the flower and extracts nectar from between the petals and the calyx.  In the process the bee's abdomen rubs all over the anthers and the stigma, thus achieving pollination of the flower.  

This bee on the Buckeye flower is about 1.5" long.

Other flowers are about to open up here, like the Scarlet Sage, Woodland Sunflower and Prairie Coneflower, so the show has only just begun.