Can Sunflower Seed Hulls Harm Plants – Are They Allelopathic

Sunflower seeds are good for feeding birds. They are relatively cheap, easy to handle, and birds love them. There is only one problem – the hulls. By spring the ground under bird feeders is covered with sunflower seed hulls.

Very little seems to grow under a feeder and many have heard that the sunflower hulls are allelopathic and that the allelochemicals prevent other plants from growing. There are even lists of plants on the internet that are affected by these chemicals.

There is also a concern about disposing sunflower seed hulls. Is it safe to compost them? Should they be put into the garbage? Can they be used as mulch? In this post, I’ll answer these questions by looking at the science.

What is Allelopathy?

Wikipedia says, “allelopathy is a biological phenomenon by which an organism produces one or more biochemicals that influence the germination, growth, survival, and reproduction of other organisms. These biochemicals are known as allelochemicals and can have beneficial (positive allelopathy) or detrimental (negative allelopathy) effects on the target organisms.”

That definition is correct but many gardeners simplify the definition to something like, allelopathy is the process where one plant prevents another one from growing. In most gardening situations we don’t talk about any positive effects, and we normally only care about the plants.

The classic example of  allelopathy is the walnut tree. Everybody talks about the fact that nothing grows under a walnut tree because it produces juglone. That is largely a myth which I have discussed in Walnuts, Juglone and Allelopathy.

Are Sunflowers Allelopathic?

There has actually been a fair amount of research on this subject since sunflowers are an important agricultural crop and it is known that rotations with the wrong crops can cause problems.

This has also been studied to see if sunflower plants could be used for weed suppression.

Much of this research work is still preliminary and many reports point out that more work needs to be done, but the following summarizes what we think we know.

Allelopathic Effects on Seedlings

Most of the work has been focused on germination and seedlings. Leaving plant residue in the field from one year to the next can have adverse effects on the germination of some species. Sunflower extracts also affect seed germination.

Effects are Species Specific

Allelopathic effects are species dependent. Some species are affected more and others not at all.

It is incorrect to make a blanket statement such as “sunflowers are allelopathic and stunt the grow of other plants”, and unfortunately these kind of statements are common in the gardening world.

Weed Control

A 5 year study found that weed density was less in fields that grew sunflowers but another 4 year study showed no weed reduction. It is not clear if this was due to less seed germination or slower growth of plants. Both could be affected.

Effect on Plant Growth

Field studies found that “the presence of a preceding sunflower crop decreased plant height, dry matter, and yield of all subsequent crops (sorghum, pearl millet, maize, cotton, sunflowers, cluster beans, cowpeas, and green gram) as compared to those sown in fallow plots. The greatest reduction in growth and yield was observed in cotton and sunflower, and the least in cereals while moderate in legumes.”

It is interesting that sunflowers affect the growth of future sunflowers. For this and other reasons, commercial growers tend to use crop rotation to grow sunflowers.

What is the Real Sunflower

You will note that my discussion above talks about sunflowers as if they are all the same. This is also common practice in popular internet articles, but there are several species of sunflower and hundreds of cultivars. Their allelopathic effects are not all the same.

To really clarify things you need to specify the plant being tested. Research papers usually do this, but gardening information rarely does.

Allelochemicals in Sunflowers

More than 200 natural allelopathic compounds have been identified from different cultivars of sunflower.

It seems as if all parts of the plant contain them to some extent.

Limits on What We Know

A lot of allelopathic work is done in pots using seedlings and work with sunflowers is no different. It is next to impossible to extrapolate these results to what happens in your garden.

A lot of the studies used concentrated plant extracts to see an effect. This might be a suitable practice for controlling weeds, but it does not really apply to most gardening situations. Remember, dose makes the poison.

There is almost no research done on sunflower seed hulls. This is of interest to gardeners and birders, but not agriculture and research money is scarce.

Plant Lists

If you check the internet you will find some lists of plants that are immune to sunflowers, and other lists that are affected by sunflowers. None of the ones I found had a reference cited. One online list of unaffected plants added this at the bottom; ” if anything seems to be misshapen or stunted, it’s the toxin. Try another plant on the list”, essentially admitting that they don’t have confidence in their own list.

I did not find any research on horticultural plants, and so I would not have much faith in any of these garden lists, although quite a bit is known about agricultural crops.

Does Composting Eliminate Allelochemicals?

In general composting will reduce allelochemicals as microbes degrade them. When it comes to sunflower allelochemicals we still don’t know exactly which ones are a problem and we don’t know how stable they are in a composting process. Added to that is the complexity of composting – everybody does it differently.

When plant refuge is left in the field from one year to the next, it still has allelopathic effects, but the studies which looked at this didn’t correlate it with a degree of composting. Plant material laying on the surface until spring and roots in the ground will almost certainly compost slower than in a compost pile.

Sunflower hulls compost slowly and it is possible the allelochemicals in them lasts longer.

At best all we can say is that the chemicals will degrade in both compost and the soil over time.

Under Bird Feeders

A lot of the concern with hulls is a result of feeding birds. The sunflower hulls accumulate under bird feeders. Many report that not much grows under these feeders and they blame the allelopathic nature of the hulls – which might be true.

Some weeds do grow under bird feeders, and you have to remember it is also a spot with a lot of bird and animal activity. My squirrels are constantly digging around there for a missed seed. Wild turkeys are also common visitors.

A thick layer of hulls will also act like mulch, keeping light from seeds and preventing germination. So, is the lack of plant growth due to the allelopathic nature of hulls, animal activity or the simple effects of a mulch?

Which Plants Don’t Produce Allelochemicals?

Gardeners get excited about a few well known plants that demonstrate allelopathy, such as sunflower, walnut and garlic mustard, but the reality is that most plants produce these chemicals, at least to some extent. Plants are great at making a very wide range of chemicals and we know they produce exudates from their roots to control microbe populations in the rhizosphere. It is very likely they also produce exudates to control competing plants.

This is all fairly new science and we don’t have many answers.

We have been growing all kinds of plants in the same garden and if they all produce allelochemicals, and we have not had significant problems so far, maybe all the hype about allelopathy is a bit exaggerated – from a gardeners perspective?

What Does It All Mean to the Gardener?

Unfortunately there is little science about this topic in a horticultural setting so all we can do is guesstimate the facts.

Every part of the sunflower contains allelochemicals. You should keep them away from important seedlings.

If you grow sunflowers, keep an eye on the plants around them to see if their growth is stunted. Some may be affected.

I would not be too concerned about composting sunflower plant parts, including the hulls, provided it is a small part of the compost pile. If you use the compost as a mulch, it is unlikely to cause a problem. Don’t use it in the vegetable garden where you tend to plant seeds.

Sunflower seed hulls may reduce weeds and so they should make a good mulch. Larger mature plants and shrubs are least likely to be affected by any allelochemicals, leeching into the soil.

I see no reason to put the hulls in the garbage. Why waste good organic material?