You are correct in areas that small they will never make it to maturity. But for the cost of a bag of sunflower seeds plus the crude protein they will get from the sunflower sprouts I don't see a reason not to do it.I would be doing two separate areas. One being 3/4 of an acre and another being 1 acre.
I figured the sunflowers would get wiped out long before they mature but I could buy a bag fairly cheap so I wasn’t sure if it was a good idea to mix or just stick strictly with buckwheat.
Heck, I'd plant 30-40 lbs of buckwheat and 5-10 lbs on sunflower. Not sure where you got that seeding rate on sunflower but its very light. The two are compatible and if the buckwheat doesn't smother the sunflower it will grow up over the top. Though as said if you have many deer, once they figure out the sunflowers they will eat them all overnight. I was also surprised to see heavy grazing on buckwheat last year on my farm though they had never seen it.
I assume you are broadcasting?
It’s expensive but sunn hemp is another good companion with buckwheat and does good on marginal soil too. Iron clay cow peas is another companion option also
That calculator is a neat resource. Should I broadcast first and use a drag harrow with a log or two or drag first, broadcast and then drag?Smartmix Calculator on Green Cover Seed website says the seeding rate should be just over 7 pounds of black oil sunflower and just under 27 pounds of buckwheat (per acre). Those numbers were calculated with broadcast application. Broadcasting with incorporation reduces the seeding rate down to 6.3 and 23.7 pounds respectively.
Unless you incorporate, or cover with thatch (Throw and mow) I can say that you will lose a lot seed to birds. Birds will hammer those varieties of seed if laying on the surface.
Keep soil temp and frost in mind for timing the planting. Neither tolerate frost and sunflowers do best when planted in 60-70 degree soil.
Sunflower seeds are not durable like hard-seeded stuff like clover, brassica, etc. You can damage seed I suppose.That calculator is a neat resource. Should I broadcast first and use a drag harrow with a log or two or drag first, broadcast and then drag?