The official definition of a weed is: a plant that is not valued where it is growing. That can include that nice ground cover that is now covering the sidewalk and is heading for the nearest interstate. That shrub rose that looks like it was hit by a bomb. The cilia (squill) that you planted in the grass and now cover all the available soil in the tulip garden each spring. In other words, it is not just dandelions and creeping Charlie.
Summer annuals germinate in the spring, grow, flower in the summer and die. They are: morning glory, crabgrass, ragweed, purslane, wood sorrel, spurge and sweet clover. Again a pre-emergence herbicide will kill the emerging seeds. The escapees need to be sprayed with a post-emergence herbicide. Do this only when plants are actively growing, there is adequate soil moisture, and the day is still, sunny and between 45 and 85 degrees. Dave Barry once said “crabgrass can grow on bowling balls in an airless room and there is no known way to kill it that doesn’t involve nuclear weapons”.
Biannual weeds need two years to mature. These are bull thistle, garlic mustard and wild carrot. The first year they are either just a clump of leaves or a rosette of them. Since they go to seed the second year, kill them as soon as you recognize the weed, fall or early spring.
Some perennial weeds reproduce from seed, like dandelion, plantain, chicory and curly dock. If possible, kill them before they go to seed. This can be nearly impossible for dandelions as they produce seed almost year round. Best to dig them up if possible.
Then there are the monsters – perennial weeds that reproduce vegetatively, a fancy word that means they spread from underground stems, above ground stems, or bulbs, corms and tubers. These include creeping bellflower, bindweed, creeping Charlie and quackgrass. Bellflower was once a favored ornamental. It can still be found around the foundations of old farm houses. It not only spreads by roots, but also has a tuber as deep as six feet to deal with. These plants are best killed as seedlings, usually impossible as they are up and running by the time we see them. Kill them in the fall. Start spraying Labor Day and continue once a week until freeze up. This should at least, slow them down.
The grasses can be killed with a herbicide made to just kill grasses. Yes, even in the middle of iris without damaging the iris. It takes several applications and you will still have to cut or pull the dead grass out. Unfortunately, unlike weeds, it doesn’t just shrivel up and die. It just sits there and glares at you, dead and brown.