sunflower the ideal dry land no till cover crop for vegetables?

I've been doing some thinking about sunflowers as a cover crop. They grow even with no irrigation in Denver, and still produce a lot of biomass and a small yield of seeds. They are very pretty, have a deep and extensive root system, and smother weeds, particularly bindweed.

I was thinking that a stand of sunflowers could be left to freeze out, and the tops harvested, while leaving the stems standing in place. They wouldn't absorb or intercept too much water, but would shade the soil and keep the water in, particularly by preventing rapid sublimation of the snow. In the spring, they could be knocked down and planted through, by using either transplants or large seeds such a squash. The stems would be easy to move aside for planting. I'm thinking that they would be a good mulch because they wouldn't absorb much water, shedding it rapidly into the soil.

I imagine they would work even better combined with some type of frost sensitive grass or if a fall legume was seeded into the standing stalks, though fall is a hard time for crop establishment here, and sunflowers are allopathic.

The two drawbacks I can think of are that they would be a little hard to walk on, and hard to weed with a hoe; but of course they would have helped to suppress weeds the season before, and by using large, vining crops, the bed might not need much weeding.

I've spend lots of time thinking about mulches which would shed water into the soil instead of absorbing it. What do you think?

I know that I don't know your system or what your goals are for that parcel, however, here are a few thoughts I might share. What I have learned about cover crops and what I understand about soil leads me to believe that a seed blend might be a better thing to do instead a monoculture cover crop. With a diverse cover crop that you're simply using as a mulch and to add bio mass later, a diversity of plants gives you a diversity of roots which exudes a diversity of carbohydrates which attracts a diversity of soil biota, which in turn will be there to feed your later harvest crop. Diversity in cover crops also is a hedge of protection (no pun intended) against pests because there's not one big food source for a pest to be attracted to. A diversity above ground will also give you a slight increase in solar energy capture than one kind of plant that stands the same height uniformly. Diversity of root system also gives you an improved soil structure and better water infiltration.

Here is a really interesting seed mix calculator that Green Cover Seed offers its customers to use.  It is for larger scale no-till, cover crop farms, but helps you see what is possible out there.
https://smartmix.greencoverseed.com/

In my neck of the US sunflowers grow tall and die before October, not the best for a "cover crop" description.
When we plant sunflowers really close together as you would need to do for a true cover crop, there is a lot of stem die out with an ending of a plant living about every foot or so, and the leaves don't last long enough to be considered shading of the soil.
As I said, this is just the conditions and observations of where I live but from what I know about sunflowers, they tend to grow the same, if you are comparing the same species, just about everywhere.
We grow them for seeds so we are using species that grow large, heads. These have fat stalks too, they can even be used for flutes if harvested and dried before the plant head begins to die.

I like a thick growth of cover crop, I don't want plants that need to be more than a few centimeters apart and even that I would like to be millimeters instead so I plant a cover crop mix of grasses, brassicas, legumes and clovers, I get complete shade at soil level this way so no unwanted plants get a chance to grow.
I can chop and drop two or three times to add a lot of mulch to rot down into the soil as well as feed the earthworms and my soil microorganisms and I don't have to worry about rain runoff erosion.

Ill just echo what others have said. Growing sunflowers for oil this year I saw basically zero shading effect on the soil and a thick blanket of weeds developed underneath them. So while I think sunflowers could be a great addition I wouldn't recommend planting only them

It might be possible with one of the "bush" sunflowers such as "baby bear" but I still doubt they would produce much shade. And I don't know how well that type would deal with drought, we grow them in a swamp!

It seems to me like sunflowers are highly allelopathic. So growing them as a cover crop would tend to supress germination of crop species and/or interfere with growth.  

The sunflowers I plant in my Back to Eden garden, I chop off a couple of heads to dry for seeds, then I leave them alone until the goldfinches are finished with the rest of the seeds. Then, I use my loppers to chop them for mulch, but I leave some of the sturdier ones as a support for some of my spring/summer transplants that need staking.

Thanks for the comments everyone!

I agree that they would not shade the soil sufficiently on their own. As Joseph points out, they are allelopathic. What low growing crops would do well under them to add diversity and suppress weeds? Would the left over stalks act as a water shedding mulch like I would hope, or not? Ideally, the companion crop would be a legume and warm season.

In my neck of the US sunflowers grow tall and die before October, not the best for a "cover crop" description.

Bryant, do you mean that they wouldn't suppress erosion? Wouldn't all the biomass, and particularly the root systems, hold the soil fairly well?

It seems to me like sunflowers are highly allelopathic. So growing them as a cover crop would tend to supress germination of crop species and/or interfere with growth.

Joseph, how long does this effect last? I know rye is allelopathic, but that makes it a better cover crop; it suppresses weeds, and the effect breaks down in about six weeks. Does the sunflower effect last longer?

There's an interesting discussion in the thread Can a Sunflower Befriend a Long Island Cheese (squash)?

I believe that I have not had noticable issues because I don't plant an enormous amount of sunflowers in my vegetable gardens. I do, however, have fields of sunflowers that I plant just for wildlife to enjoy. I also don't plant seeds of plants that will be using the sunflower stalks as support, I plant transplants.

Gilbert Fritz wrote:Joseph, how long does this effect last? I know rye is allelopathic, but that makes it a better cover crop; it suppresses weeds, and the effect breaks down in about six weeks. Does the sunflower effect last longer?

Last time I grew sunflowers and left the stalks in the garden to fall down overwinter, the area was still allelopathic for the next two growing seasons. I forgot to check the growing season after that...

I think sunflowers would make a decent cover crop. They might not make the ideal cover crop, but considering their other attributes I think combined it would be hard to beat. My wife and I have actually considered this, but mostly because we have highly erodible land on some steep ground, and because the sunflowers are drought resistant, but just slightly less in forage tonnage then corn. As I said, they have a lot going for them...

It might depend on the kind of sunflower you are thinking of.

We planted oil sunflowers for the doves.  As soon as DH took down the electric fence the deer ate everything except for the stalk.  Then we were left with several hundred/thousand stalks to dispose of.

Sunflowers are heavy feeders, and the coarse stems take a long time to rot back into the soil unless you can chop them finely (run them through a shredder, perhaps).  I prefer cover crops that will add something back into the soil, such as legumes, and that rot quickly when turned under, such as buckwheat.

The root system of Sunflowers is not what I would want for erosion control, it tends to be very sparse in both density and depth, just about like corn not much root system to hold soil in place.
Also as Joseph mentioned the sunflower is allelopathic in the extreme and as Kathleen mentioned it is a very heavy feeder of N, P and K along with several trace elements.
Any of the grasses (grains are grasses) do a far better job of holding soil in place some (alfalfa for an example) put roots down to over 4 feet and spread just as wide in a near circular pattern.
The plants also knit their roots with other plants so you get a thick mat of roots that doesn't allow soil to wash away.

I see there are a lot of downsides . . .  

Thanks for all the responses!

Mr. Fritz,

I also live in an area where 18 inches of rain is a really good year.  It's hard to know what is ideal this year because it's always different than last year and we can't count on getting enough rain to grow the good cover crops.  The sunflowers would mean a living root system in the ground, some windbreak, and at least partial shade.  I like your idea.  You mentioned combining it with a frost sensitive grass.  Were you thinking something like spring barley or oats that would winter kill in your area?