Weeds, just like flowers, can be annuals, biennials, or perennials. Annual weeds are generally the easiest to control as they only live a single growing season, do not develop extensive root systems, and reproduce only by seed (seed reproduction), as opposed to reproducing by roots, stems, tubars and stolons (vegetative reproduction). About 80 percent of all weeds are annuals and can arrive by lying dormant in your soil, being brought in with a bale of hay or straw you used as a mulch, or being blown in by the wind or arriving via a bird. Pigweed is an annual weed whose thousands of seeds can live for years in your soil, only a few of which will germinate each year, thus assuring the plants survival.
Biennial weeds, such as Queen Annes lace, take two years to produce seeds. During the first year they produce lots of leaves, and the second they send up a flower stalk containing all of the seeds. They can overwinter even here in Wisconsin.
Perennial weeds are the most difficult to eradicate, dandelions, quackgrass, bindweed, nutsedge and poison ivy being good examples. If you cut off the tops, the plants continue to reproduce vegetatively, usually from nodes on underground stems and roots, stolons and rhizomes which is another way of saying that you must dig out the entire plant, including the roots, to kill it.
The best form of weed control is prevention. Black plastic is without exception the best form of prevention ever invented. Weeds hate black plastic because they cant get through it, and spotting weeds that do pop-up through errant holes, slits and tears is easy. Black plastic is normally not feasible for large gardens, but for small ones it works fine. Clear plastic allows weeds to grow vigorously, so avoid this if possible. Do not use herbicides to destroy weeds in your garden, as it will do more harm than good. Simply pull the weeds by hand and throw them into the trash. We never use a hoe because our garden isnt that big, and we have never had many weeds. Again, an ounce of prevention is worth a pound of cure by preventing weeds from gaining a foothold in your garden by using a mulch.
Quackgrass, also known as witch grass or couchgrass, is a perennial that exhorts an allelopathic effect on seedlings, thus killing them. It is a stubborn, constant nuisance throughout the northern USA and Canada, and, once established, is difficult to eradicate. Certain vegetables, such as potatoes and corn, can be stunted by the presence of quackgrass resulting in poor yields. Never use a tiller to try to kill quackgrass, as each small part that you chopped-up will produce more quackgrass. To kill quackgrass, use a plastic mulch, perhaps topped with an organic mulch, and try to smother it.
How do you destroy perennial weeds that are deeply entranched in your garden? One way, (not approved by organic gardeners), is to use a product called Preen which sets-up a small soil imbalance that prevents germinating weed seeds from sending out feeder roots for nourishment, resulting in their demise. Preen is known as a preemergent herbicide, meaning that it kills newly germinated seedlings. If used in moderation and only as directed, this product has been reported to be effective and safe for your vegetables.
Some weeds, such as Creeping Charlie are very difficult to eradicate. One trick is to wait until the first heavy frost in fall and then spray it with a herbicide containing a combination of dicamba, 2,4-D and MCPP. This product is sold as Ace Lawn Weed Killer, Dragon Lawn Weed Killer, Fertilome Weed-Out Lawn Weed Killer, Super KGro Broadleaf Weed Killer, Ortho Weed-B-Gon for Southern Lawns/Formula II and Spectracide Lawn Weed Killer. However, check the labels to see if they have changed the formula.