First, thanks again for all the help and practical advice y'all have offered so far. (By the way, Puck, you're tip on digging a pit turned out to be golden. Muchos gracious, and huevos rancheros.)
Now I seem to have a new problem. It started, I think, when I accidentally forgot to set the barrel limit on my seed stockpile to zero. I hate it when I do that. So suddenly I look over and there's a barrel with all my seed bags stuck in it.
So I remove the old stockpile and redesign a new one, careful to set the barrel limit to zero this time. And my stupid dwarves respond by filling it with single seeds. So I remove it, and start a new one again. Only I forget to set the barrel limit to zero. And so on. In other words, I've had my dwarves shuffle my seeds around a few times. Now I have a couple of seed barrels as well as a stockpile with seed bags. I keep hoping that eventually some creative dorf will eventually even take the bags out of the barrels and put them on the stockpile, but that seems to be a bit above everyone's pay grade.
The problem was that if I hit Q-s and looked at a farm plot, towards the end of fall, several of my crops (Quarry bushes, Cave Wheat, and Pig Tails) were in red text. And I noticed my crops weren't being planted, despite having much available seed of all types. In the Stocks screen, all the seed was listed and available (nothing in red). It's moved on into winter now, before I had time to post a question, but I'm worried about what will happen next spring.
Anybody out there have an explanation for this strange behavior?