Is it safe to compost/use as mulch Sunflowers?

[Susan BBPM]

I have read that sunflowers are allelopathic. For those, like me, that don't know what this means - they produce a chemical that inhibits growth of competing plants. I've read conflicting advice about whether this is an issue in practice.

I had planned a 3 sisters variation in one of my 1m square raised beds - I wanted sunflowers with beans climbing up them and a summer squash as mulch at the bottom. The bed was full of very rich soil, manure etc. Yet nothing thrived (except the sunflowers). The beans died and two separate courgette plants (one sown by me, and second bought as a seedling) failed to grow. I planted some celery and herbs where the other plants had failed and whilst they're still alive they're tiny. I've watered well, fed....but only the sunflowers thrived.

i have lots of sunflowers in my garden. I'd planned on using them as mulch (using a wood chipper) in my food forest and putting some in the compost heap. But if they are indeed allelopathic could this stunt my plants???

I should add that I've researched this and some say it's nonsense and have no problem growing beans up sunflowers - others say don't do it.

edit: Article here talking about use of allelopathic plants for weed control - this suggests to me that composting of sunflowers (among other things) COULD be problematic? (It quotes one study that shows sunflowers work as well as herbicides on some weeds - but what about other veg and flowers? I've frequently read that beans don't do well near it....what about beans grown in sunflower compost - this article suggests you'd essentially be making a compost that acts like a weedkiller to some plants?

https://www.researchgate.net/publication/303631547_Allelopathic_potential_of_oil_seed_crops_in_production_of_crops_A_Review

 

[Susan BBPM]

Sorry for blethering away to myself here, but as I'm researching I'm finding info that might interest others or spark other lines of thought.
This article seems to be a really good primer on the whole issue of allelopathic plants.

@Meadowlark - it explains a lot of the things you talk about....not sure if you're knowledgeable about the topic, or whether it's just observation of what works that have led you to the right practices?

Sunflowers (according to my research thus far) and other allelopathic plants are great weed suppressors. But if you grow them in the same place every year the toxins will build up to the point where even sunflowers can't grow!

It seems that it's OK to compost them, or to chop and drop around fruit trees etc. But you don't want to be leaving them to rot down in veg beds as they may well be still releasing chemicals in the spring when you plant again. Does this mean I need to dig the roots out of my veg beds?

It might be worth laying down the sunflower stalks BETWEEN beds to suppress weeds on paths etc? Can you even create a weedkiller by making a 'sunflower leaf tea'?

It seems that beans and potatoes are particularly sensitive to sunflowers, but lots of anecdotal evidence to suggest that they can wreak havoc with all sorts of plants if you allow them to build up in the soil over years. I adore sunflowers and have scores of them - this has scuppered my plans a bit, but I think it's just a matter of working out how to make them work for you.

Allelopathic Plants: What Are They, The List, and How To (and Not To) Use Them

We take a look at allelopathic plants, what they are and what they do, plus tips on using and preventing allelopathy effectively in the garden.

morningchores.com

 

[Meadowlark]

Excellent research. That's what I call "doing your homework".

I'm aware of the limited allelopathic properties of sunflowers...and my use of them is always on the outside borders of my gardens where they do suppress weeds. I do not believe they have anywhere near the power of walnuts, for example, in terms of allelopathic properties, but I don't use them as garden mulch, nonetheless. The heads go to feed the birds and the stalks ring my garden. There are three other properties you didn't mention that I find are important on sunflowers.

1) They are a great insect trap especially for the dreaded stink bug. For years I had problems with stink bugs on my tomatoes and squash, but sunflowers have enabled an acceptable fix to that for me. Wherever I plant tomatoes and squash (always rotate, rotate, rotate location in garden) I also locate sunflowers nearby...off to the side of the garden. The squash bugs collect on the heads where I kill them 100%. I discovered this property a few years ago quite by accident but have found it to be most effective for me.

2) Another property not mentioned in the above research is nitrogen fixing. Sunflower is one of the top fixers of nitrogen in the soil. To me, this is just a bonus property for this terrific garden tool.

3) Last, I'm told that they absorb the toxic heavy metal contaminants (including arsenic, zinc, lead, manganese, and copper) leaving your soil healthier. To me this is an "oh, by the way" factor, as I've never had a problem with heavy metals as far as I'm aware. This factor is kind of the reverse cover crop, removing rather than adding.