Friday, December 31, 2021

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


Stellaria media


Ground ivy

Glechoma hederacea


Hairy bittercress

Cardamine hirsuta


Indian strawberry

Duchesnea indica



Fatoua villosa


Ladies thumb

Polygonum persicaria


Nut sedge

Cyperus esculentus 


Persian speedwell

Veronica persica



Native Annuals & Perennials:

Common Name

Botanical Name


Annual sunflower

Helianthus annuus


Common Milkweed

Asclepias syriaca


Butterfly weed

Asclepias tuberosa


Common and Daisy Fleabanes

Erigeron philadelphicus and annuus



Cynanchum laeve


Wild Bergamot

Monarda fistulosa


Scarlet beebalm

Monarda didyma


Purple Coneflower

Echinacea purpurea


False sunflower

Heliopsis helianthoides



Solidago sp.

Yes (some species, at least)

Browneyed Susan

Rudbeckia triloba


Wild Blackberry

Rubus sp.



Prunella vulgarus


Pennsylvania smartweed

P. pensylvanicum



Verbesina alternifolia


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

Garden Vegetables:


Botanical Name



Eruca vesicaria ssp. Sativa



Onicum basilicum



Brassica oleracea var. viridis



Cucumis sativus


Green Beans

Phaseoleus vulgaris



Lattuca sativa



Capsicum annuum



Cucurbita pepo


Swiss chard

Beta vulgaris var. cicla



Solanum lycopersicum



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!!!

Saturday, October 30, 2021


What are Nativars?

Nativars are the subject of much discussion in the native plant community right now.  The comments I've seen have ranged from mild interest to deep concern.  I originally formed my opinion on these cultivated native plants over ten years ago.  Before writing this post I thought I should do a little refresher on the subject and I learned that the world is a complicated place.

Nativar is not a true botanical term but rather a word coined by Robert Armitage for a cultivar of a native plant.  It’s a combination of native, for native species, and cultivar.   A cultivar is short for cultivated variety and is reproduced by asexual means, not from seed.  Anytime you see a botanical name followed by a capitalized word (or words) in single quotes you are dealing with a cultivar.  Alternatively, cultivars may be call using common names.  For example Jim Dandy Winterberry Holly is the same thing as Ilex verticillata 'Jim Dandy'. 

Here are two inkberry cultivars, Ilex glabra 'Nigra' on the left and 'Shamrock' on the right. 
These have been in the ground here for about 3 years.  'Nigra' has darker green
 leaves and nice branching.  'Shamrock' has narrower bright green leaves and
is also well branched.  'Nigra' should grow to be over 6' tall, 'Shamrock to about 4'.

This is a wild type inkberry.  I don't know how big
 this will get, but it should grow to 6-8' tall.  Unlike the
cultivars this one has few lower branches.

Advantages of Nativars:

  • May possess improved traits like disease resistance or improved blooming (size or quantity).  For example, mildew resistance in phlox.
  • Predictable performance as design elements for color, shape and size.  Size is particularly important when you are working close to structures.  You can avoid a lot of  maintenance pruning by electing a properly sized cultivar.
  • Greater commercial availability as a result large scale, asexual reproduction.
  • As a nursery propagated cultivar there is no risk that it was collected illegally from the wild.
  • For dioecous species the gender is often known, e.g. male and female hollies.  This is important if fruit production desired.
  • Good introduction to native species for the novice.


Concerns about Nativars

There is concern that cultivated natives may not be as good of a food source for pollinators and herbivores.  The potential impact is both on pollinator preference and acceptability as host plant.  In some cases plant breeders have gotten creative and, in order to create some very interesting forms, have sacrificed ecological function.  Examples of this include double coneflowers such as Echinacea 'Pink Double Delight' which does not produce significant amounts of nectar, pollen or seeds.  There is also evidence that unusual colored foliage, particularly reddish, is less attractive to larval insects.

This wild-type purple coneflower grew from seed. 
Its parents were about 10' away.  It is 'fully functional'
producing  pollen, nectar and seeds.

When you get into the details of a native ecosystems it can get complicated.  Native insects depend on plants, mostly native ones, for food.  This food consists of  not only the pollen and nectar from flowers that everyone loves, but also on the leaves, stems and other plant parts that not our focus when we think of how we want to support our insects.  Insects consuming biomass is a key step in how sunlight is able to nourish all life on earth.  Insects convert plant matter into a higher protein form that that is consumed by retiles, birds and other small predators, particularly those that are feeding their young. These first line predators then supply food energy for larger predators and so on.  If these insects are not able to survive then the food web comes crashing down.

As clones, nativars lack the genetic diversity of wild-type, seed-grown plants.  In sufficient quantity the cloned population may have an adverse effect on the wider eco-system.  This may be from introducing either under or over-performing traits to the population.  This possibility of introducing an overly advantageous trait is an aspect I had not considered, an example of which was brought out in an article found on the University of Maryland Extension website by Dr. Sara Tangren which identifies many of these issues around nativars.  The article cites a case where the stronger growth habit of 'Shelter' switch grass, Panicum virgatum 'Shelter', may allow it to out-compete other meadow species and throw off the ecological balance in that system.

 In most cases there is a lack of science based evidence for actual damage, but there is a strong basis for concern.  The Budburst project is using citizen scientists to collect data on pollinator preference between cultivars and wild-type plants.  In general the more that the cultivar resembles the wild-type plant the more similar the interaction with pollinators. Factors to consider are bloom size, quantity of blooms, seasonal timing, color and the accessibility of pollen and nectar to pollinators.  This addresses one piece of the puzzle.  The effect of the narrower genetic diversity of nativars on the surrounding natural communities will be much harder to establish and I expect it will be different for each cultivar.  

One study out of the University of Delaware has shown clear evidence that as foliage color shifts from green to purple hues (increasing anthocyanin) that feeding by insect herbivores (e.g. caterpillars) decreases.  Other features of cultivars like varigation, fall color, disease resistance, habit  and fruit size did not significantly affect how insects fed with these plants compared with their wild type relatives.  This would be a factor in choosing among the new red-leafed ninebark cultivars.  This study was focused on insect feeding on woody plants and did not get into pollinator behavior.  

A much more positive spin on nativars can be found in this article from Nursery Management Magazine by Jolene Hansen.  It cites the findings in the University of Delaware study.

An article  by Rhonda Burnett and Carol Davit on the Grow Native! website also takes a cautious and critical view of nativars.  But it also offers one of the best, though least satisfying, answers to the use of nativars, from Douglas Tallamy, "It depends."

Problem is when there is an imbalance.  Questions to ask: Will my planting adversely affect the surrounding ecosystem?  Is there a local native population that may be affected.  What is the scale of my installation.  Is it a single clone or is it from open pollinated seeds?  Another quote from Dr. Tallamy (from the GrowNative! article) is the assertion that “It’s not the presence of cultivars, but the absence of natives that’s a problem.”

A major concern with cloned native species is their lack of genetic diversity.  The question is whether a large-scale injection of one set of genes will negatively impact the larger population.  This impact may be seen as a change in the native population or as a loss diversity leading to a decreased ability to adapt to environmental changes.  Plant genetics is complicated.  In many cases observable traits are passed on by the interaction of multiple genes.  

To make this issue even more complicated consider that many nativars and hybrids do not reproduce true to type (if they can reproduce at all).  Anecdotal evidence from my yard is that seedlings from 'October Skies' aromatic aster, Symphyotrichum oblonifolium 'October Skies' have very different forms.  One is gangly and another is nicely rounded but about 50% larger.  To my knowledge there are no other aromatic asters within 200 yards or mine (probably further).  So these are probably the result of self pollination.  So despite this being a cultivar, there is still a lot of variability packed into its genes.  I watched the insects visiting the flowers, there was no obvious preference among the three plants.  One advantage of having both was that the effective bloom time was increased over just having a single plant.

October Skies is a compact, early blooming cultivar of aromatic aster, seen here
in the foreground.  To its right is another aromatic aster that is likely a seedling
from the cultivar.  It bloomed a couple of weeks later, with fewer flowers
and it has a more open habit.

This is the first seedling I got from the original planting
of October Skies.  It is about 25% taller, floppier and
blooms a little later than its parent.  

Reversion of an individual cultivar to the wild form is not uncommon, dwarf Alberta spruce and Henry Lauder walking stick are two examples.  Though when being developed for commercial sale genetic stability is an attribute that is selected for.  An obviously unstable plant will not make it to market (or stay there long).

How and when to use Nativars

For many residential gardeners in urban and suburban developments there may not be much of a remnant ecosystem nearby that would be adversely effected by your plant choice.  Closer to natural areas and especially when doing large scale restoration work, using locally adapted, or even locally sourced plant (local ecotype) material becomes much more critical. There are several companies that can help with suppling large quantities local plants.  For example Ernst Conservation Seeds, CardnoCarolina Native Plant Nursery, and  Archewild.  When concerned use seed grown native species from local eco-types.  This is easy to say but much harder to do.  

At this time there are many more questions than answers, but studies are underway to begin to answer some of these concerns.  Based on examples of disastrous plant introductions such as kudzu and Japanese knotweed it is understandable to be cautious when introducing new plant materials, especially when we are doing it in quantity and for what we believe is a 'good reason'.

For me, in my rural location I will the following guidelines.

  • Within 50' of my house where design aspects such as color, shape and particularly size are important, cultivars are fine to use. 
  • As I move outside that designed space I will use mostly wild-type or straight species.  Most, if not all of these are seed grown and are species native to this ecoregion (foothills of the Blue Ridge Mountains).  
  • As I get into natural spaces I try to avoid adding any new species not already present in the area.  In many cases I am just managing the native species that are existing on-site (or have blown in) and most of the work involved is the removal of undesired and invasive species.

So I've written a lot of words but I'm afraid the conclusion comes down to the less than satisfying 'it depends'.