Actually 3" spacing is 18 plants per square foot so you've "underplanted" if you only put in 12 per square. : ) Since a 4x8 = 32 squares and you can plant 18 per square, you could have planted 576 plants.

My only "concern" is that there might not be enough "air space" for your roots in your soil mix which is why the "real" SFG method doesn't use the existing "dirt" but a mix of 1/3 peat moss, 1/3 coarse vermiculite & 1/3 blended compost (4-5 sources) to aid in water retention and keep enough air space in the soil. This also allows you to grow in just 6" depth of mix.

You may want to take this opportunity to experiment a bit. Keep some squares at the 18/square level, some at 12/square, and a few at 8/square. This way you can see what works best for you.

WOW! I can't even imagine a 4x8 of cowpeas. As a general "rule of thumb" a 4x4 gives 1 adult salad for a season (16 different crops). Another 4x4 gives 1 adult supper veggies for a season (again 16 different crops). A third 4x4 gives 1 adult extra for canning/freezing/sharing, etc.

Let us know how it goes.

If the package indeed says to plant every 3 inches, your doing it right. When I had my first SFG many years ago, I had the same concern when I was planting things like carrots or radishes-is this too much in one square? The 3" spacing will mean 16 plants per square inch. You will be fine with air space needs. The reason why is your vermiculite that you have in your soil. It creates a lot more of those little air pockets that you wouldnt have otherwise. Our friendly single row gardeners probably wouldnt be so lucky...

All the peas are up a good 3-4 inches now and I've taken the bird net off the bed (this was my 2nd planting, birds ate all the sprouts the first time around). It is a 4x8 bed but I only planted 4x6, leaving 8 sqft for baby limas. I actually have 270 plants in the 4x6 area, losing a few that didn't germinate or died for whatever reason. I'm a little anxious about this first raised bed/sqft attempt, just because it's so much in such a small area. Thanks for the encouragement! I'll post a pic one day if I can ever figure out how to do it.

Well, it's been about 6 weeks since I posted this. The peas are looking great and just beginning to bloom. I covered the bed today with about 1" of worms castings, then mulched over that with some finished compost. Can't wait to see if it all turns out well... also, anybody got suggestions for grasshopper control? I got a ton of them!

Your doing exactly what you are supposed to. I planted a 4X8 bed of pink eye purple hulls at 3inch spacing too. It came out to 512 plants. Here is a post showing mine from May 29th. The plants are around 18 inches now. I love cowpeas too, I call them field peas.

http://georgiahomegarden.blogspot.com/2011/05/2011-garden-may-29th.html

Here is a link that might be useful: My Garden Blog

I'm a bit of a rule breaker, but I don't like to do mine as close as the book says because I think it makes it more frustrating to harvest.
Purple hull peas aren't so bad since the grow out of the top.

It's great you're getting good results from it. I overload peas too, but not all germinate so it works fine. Keep letting us know how it works out. Congrats on your first experiment with SFG. It works well. Plants don't mind crowding.

Sorry, we don't get grasshoppers here so I have no idea. Would a fine mesh shade cover over hoop covers work? I use my PVC hoop covers for everything. More info on my blog if you're confused.

Here is a link that might be useful: Sinfonian's garden