Weeds are the biggest challenge for landscape gardeners. There’s no single magic solution to weed control in your landscape, but if you understand weeds you can defeat them using a simple system of controls. First you need to know some weed basics.
There are three basic kinds of weeds; Annual, biennial and perennial. Annual weeds come up from seed each year and die before winter. Biennial weeds grow the first year and bear seed the second year, and then die. Perennial weeds survive the winter and continue growing stronger each year, spreading underground and also by seed.
Complete darkness is the most effective weed control. We spread three to four inches of mulch on new plantings, and renew it every year with another two or three inches of fresh, sterile mulch. This is key; weather will erode your mulch over the winter, and new weed seeds will plant themselves constantly, so unless you renew your mulch every spring you will have weeds.
Many gardeners spread weed barrier fabric on gardens and cover it with mulch. Fabrics are more effective than mulch by itself, since they allow water and air to reach the soil but block sunlight. They will control all three types of weeds at first, by preventing existing weeds and seeds from penetrating the fabric, but eventually new weeds will sprout from seeds spread by wind, birds, and blown lawn clippings, and invade your beds above the fabric layer. Unless you have enough mulch to block sunlight, weeds will sprout through your fabric.
Using weed barrier fabric can be a negative if you neglect to prepare the soil before putting down fabric and mulch. Fabrics hinder many plants from spreading and “colonizing”, which pushes out weeds. Fabrics can also be a nuisance if you want to cultivate or change your beds. Any time you disturb the fabric or bring up soil on top of it you invite weeds.
Healthy, happy plants are your best defense against weeds, since they are aggressive competitors and they shade the ground. Spacing plants so that they eventually crowd each other is very effective in controlling weeds. If you insist on having a space between plants, weeds will thrive there because they have sunlight and no competition. We believe in groundcovers like pachysandra, vinca, ivy, carpet juniper and liriope, which shade the ground like a blanket and suppress weeds.
We recommend using Glyphosate-based weed killers like Roundup (glyphosate) to kill all weeds and grass completely before making new beds. If you till biennial or perennial weeds they will multiply; you must kill them down to the deepest roots before tilling. Certain weeds, like nutgrass, clovers, wild violets, need specialized weed killers since Roundup won’t kill them.
Another important step is to cut away soil along sidewalks and bed edges, so two or three inches of mulch can be added right to the edge. Skimping on mulch allows sunlight to activate weed seeds in the soil. Before mulching we always clean foundation walls, walks, curbs and other adjacent places where dirt containing weed seeds might contaminate the new mulch after it’s spread.
The final line of defense is “pre-emergent” weed control chemicals like Treflan, the active ingredient in Preen. These products create a shield that kills emerging weed seedlings. We spread it on beds just before mulching, since exposure to air and sun reduces its potency.
We like to spray weeds in our gardens instead of pulling them, if there’s any doubt they will come up by the roots. Pulling weeds brings up soil and weed seeds from below your mulch, making the problem worse. Deep-rooted weeds like dandelion and thistle are hard to pull. Some weeds, like nut-grass and wild onions, leave the next generation behind when you pull them. Spraying is best, as long as you can do it without spray drift onto desirable plants.
Eliminating weeds in the midst of existing perennials and groundcovers is a ticklish job, because most weed killers will damage desirable plants. We’ve come up with a solution we call the “Roundup glove”. To do this trick, simply wear a rubber glove under a cheap cotton work glove. Dip your hand into a pail or bowl of Roundup, make a fist to squeeze out the excess so it won’t drip, and then wet the weeds with your fingers. Everything you touch will die in about a week.
Steve Boehme is a landscape designer/installer specializing in landscape “makeovers”. “Let’s Grow” is published weekly; column archives are on the “Garden Advice” page at www.goodseedfarm.com. For more information is available at www.goodseedfarm.com or call GoodSeed Farm Landscapes at (937) 587-7021.
