This is going to be a long post with more photos than text, I hope. But the bottom line is that I waged a long and losing hand-to-hand battle with crabgrass on the patio last year. This year I intend to try a different tactic, though I am not particularly sanguine that it will work. Still, as you will no doubt agree by the end of this post (come on!), anything is better than digging crab grass out with a teaspoonhandle.
Crab grass germinates when soil is between 50 and 60 degrees. On the east coast, that is approximately after forsythia blooms and before the lilacs.
What I have learned after a fair amount of research over the winter, is that short of bombing the whole patio with round-up, which I am not willing to do just to solve a crab grass problem, my problem could perhaps be solved over time with diligent applications of pre-emergent herbicide, which in theory would prevent the crab grass seeds from sprouting. Corn gluten is a naturally occuring pre-emergent herbicide that is rumored to have some effect. Well, to be more specific, there are people who say it does and people who say it doesn't.
Corn Gluten is applied at 20 pounds per 1,000 sf right around the time when the forsythia blooms. It needs to be watered lightly after application, but if there is heavy rain it will get washed out and be ineffective. Also, there seem to be disagreements about whether the powder or pelletized forms work better.
Now you're getting a handle on the problem, oh boy:
And, finally, I start to shoot photos of the reason I will not use round-up. There are many lovely and beautiful and non-invasive tiny creatures in the patio. Here's one, a slender, minute grass that springs up only when there is damp: