The Crabgrass of July
There is always some morning in July where I bring out a small stool, a large trug, and several tools and clean the front of the borders. I pick a starting point, sit down and weed and dead-head my way around the edges. I don't leave many seed heads because they look dead and I don't want a touch of November in the summer. I tend to be ruthless about deadheading the last few blooms of a post-peak plant, and sometimes I bring along a vase to capture the last stokesia or daylily. After a while I become less sentimental about the late bloomers and attractive seed heads because of the huge volume of crabgrass.
Weeding crabgrass out of the border is a lot of work, but it is really gratifying. The soil is pretty loose, so huge mats of shallow rooted weeds come up pretty easily and fill the trug quickly. Honestly, I don't think it is any less attractive than most "dwarf ornamental grasses" and it can give a garden a meadow-like lushness that is currently in vogue. However, garden fashionistas will sneer at my crabgrass while gushing about some equally sloppy poaceae with a better pedigree, and it will choke some favorites while creating millions more seed. Also, when I am done form, texture, and rhythm decisions I have made really shine through and I can congratulate myself for being so clever.
Crabgrass is an annual weed that seems to appear overnight, but the stealthy little seedlings get their start in earlier in the year. The general rule is that crabgrass germinates when the forsythia blooms, and the reason is that the soil temperature rise that cues the forsythia to open its buds is the same temperature that cues the crabgrass seeds to germinate. Supposedly, if the lilacs are in bloom, the seeds have mostly germinated and by mid summer your choice will be to weed or surrender.
So the crabgrass is there in May and June, but it takes a few hot days to make it grow at an offensive rate. I have tried pre emergent weed killers, and they work fairly well. They work by preventing sprouted seeds from developing more complicated roots, so they die quickly after germinating. Corn gluten is the most common source is the consumer friendly version (sometimes it looks almost exactly like cornmeal). If you ever get annoyed at someone rhapsodizing about the goodness of nature, remind them that corn murders plant babies.
I don't use the pre emergents that often because I think they are more trouble than they are worth. They don't work perfectly so you still have to weed. They can be washed away and they need to be applied pretty uniformly to really work. Also, the timing is bad: they need to be applied at the same time that a million other things need to be done, and applying pre emergents is usually near the bottom of my to-do list. I have several plants that need to self seed, and pre emergents kill their seedlings too. So I usually use them at the front of the borders because most of my self seeders are tall and airy and too untidy for the front edge.