Home&Garden
feature article spring 2008
Weed Wars
Weeds are ancient foes of gardeners, but they can be controlled. Here are five of the worst offenders, along with
eco-friendly ways to banish these invaders from your landscape.
By John Cretti
Weeds! They tenaciously compete with our lawn and plants for water and nutrients, and they’re extremely proliferate. “Know thy enemy” is good advice, if your goal is to win the war against these adversaries. Because weeds are a natural part of the landscape ecology we’ll never eliminate them completely, but we can control them.
Death Knell for Dandelions
The dandelion (Taraxacum officinale) is a vigorous perennial with a very strong and persistent taproot that makes itself at home almost anywhere. Every spring, bright yellow flowers adorn the landscape and soon transform into the feathery globes kids love to pick and blow, inadvertently helping this relentless plant disperse its progeny. To make matters worse, the seeds remain viable for decades. And dandelions don’t need a rest cycle, though thankfully they grow very little in the dead of winter. Dandelions do have one redeeming quality, however: The young leaves are deliciously spicy and make a nice addition to salads.
Aside from salads, how else can we deal with this weed? Most young dandelions are easily pulled after a good rain or deep watering, but you must get the roots to prevent re-growth. Grasp the dandelion as close to the ground as possible; if it breaks off at the base, slide a sod knife or dandelion digger underneath it and pry and twist as you pull. If uprooted dandelions haven’t produced seed heads, let them dry in the sun before retiring them to the compost pile. If they do have seed heads, dispose of them in a garbage bag.
Some natural weed sprays are effective against dandelions. Ones that contain clove oil or eugenol work well on young emerging dandelions, while soap-based herbicides delay growth by damaging a dandelion’s protective leaf cover. But for well-rooted plants, both of these provide only temporary control. To eliminate dandelions, try products that contain acetic acid (the compound that gives vinegar its sour taste). Use caution during application, as acetic acid can damage neighboring desirable plants. Shield these with paper bags, and only apply the spray on non-windy days.
Thumping Thistles
Canada thistle (Cirsium arvense) is perhaps the most dreaded perennial weed, because it produces new plants from buds on a wandering root system that can spread everywhere. It also sheds thousands of seeds in late summer and fall that remain viable for 20 years or more. This thorny plant with purply-pink spherical flowers recovers from almost any stress conditions, including control attempts, because of the reserved food in its root system. Therefore, the most effective way to manage Canada thistle is through a combination of cultural, mechanical and herbicidal controls. Canada thistle must be removed promptly, before it gains a foothold. As soon as you spot it, dig it up and dispose of it in garbage bags. Be sure to wear a flexible pair of thick, waterproof gloves to protect your hands from the thorns. If you’re unable to dig up the plants, regularly mow them to weaken the roots and reduce seed production.
One effective biological control is
the Canada thistle stem weevil (Ceutorhynchus litura), which is being released in public lands and open space. Its larvae devour the weed’s main leaf veins, then travel down to the plant’s crown. Although they can’t wipe out an entire thistle population, weevils stress the plants and reduce their vigor. Canada thistle stem gall fly (Urophora cardui) is another insect that stresses thistles by feeding on them, triggering galls that prevent the plants from flowering and producing seeds.
Soap-based herbicides dehydrate thistle leaves by damaging the protective cutin layer. This control option is very effective on young thistles, but only temporarily sets back well-rooted plants. To maximize the spray’s effectiveness, spray young weeds when temperatures rise above 70º F.
Binding Up Bindweed
You can’t mistake bindweed’s showy white and pink-tinged blooms. This plant is found throughout the state and thrives anywhere, except in the deepest shade. Also known as wild morning glory, bindweed (Convolvulus arvensis) is a perennial vine that sports green, arrow-shaped leaves and inch-wide morning-glory-type flowers. If it weren’t a noxious weed, it would be an ideal ground cover for the most difficult sites. Like Canada thistle, bindweed is extremely persistent and produces new plants from seeds and buds that develop on its roots. It’s not unusual for roots to spread 30 feet beyond the parent plant.
Use a spading fork to dig up bindweed and expose its roots to sunlight. Dispose of as many of the root ganglia as possible to curtail the plant’s spread. Start digging about a foot from the plant’s center to loosen the soil, then lift the vine from beneath. This reduces the number of roots that are likely to break off and re-grow.
Organic herbicides containing vinegar provide temporary control, but they must be frequently applied. Persistence is the key to controlling bindweed with organic treatments.
When all else fails, apply a conventional herbicide, like glyphosate (found in Roundup and other trademarked formulations). Mix the product according to the label instructions and apply it directly to the foliage with a paintbrush. Do this on a day when temperatures are in the seventies and rain is not expected for at least 48 hours. Be sure to shield desirable plants by placing a bottomless cardboard box over the bindweed clump before applying this potent, nonselective herbicide.
Perish Purslane
As soon as the soil warms up, purslane (Portulaca oleracea) appears. This annual weed’s red stem thickens and branches from a central point, producing rounded reddish-green leaves that look like a jade plant’s. Purslane eventually forms a mat up to a foot in diameter. Pull or dig it up when the soil is moist, but be sure to remove all of the plant, because this little succulent can survive for several days after uprooting and quickly re-grows roots from its nodes. Like dandelion, this is an edible weed. A single serving of fresh leaves (a little less than a cup) provides the minimum daily requirement of alpha-linolenic acid, a type of omega-3 fatty acid.
Pulling up purslane is effective, albeit time-consuming. Portable flamers provide faster results. Spot burn the plants, but only for about a tenth of a second. Flamers should never be used where mulches are present. For more information on flamers, log on to eweedcontrol.com.
Because purslane is an annual, a pre-emergent herbicide will control it. It must be applied before the seedlings emerge, so timing is crucial. An organic pre-emergent is corn gluten, which can be safely applied to established lawns and perennial beds. However, don’t use corn gluten on newly seeded lawns or in garden beds where you plan to sow seeds.
Crimping Crabgrass
Relentless crabgrass (Digitaria spp.) invades our yards, consumes bare spots and thrives in cracks. This grassy weed develops several prostrate branches that choke out lawn grasses and nearby plants. Crabgrass colonies spread aggressively in hot weather, with spikelet seed heads that resemble turkey feet. But since crabgrass is an annual, it can be stopped early in its growing season.
One of the best ways to stop it is to maintain a thick, healthy lawn. A dense canopy of grass blades shades the soil, making it tougher for crabgrass seeds to sprout. Always mow cool-season grasses—Kentucky bluegrass, Kentucky bluegrass hybrids, tall fescue and perennial rye—
to a height of 2 or 2.5 inches.
When you
mow too short, you reduce the lawn’s
vigor and give crabgrass seeds a chance
to sprout. You can mow warm-season grasses—buffalo grass, native mixes and Bermuda hybrids—a tad lower, but it’s really a matter of aesthetics and personal taste. Leave finely chopped grass clippings on the lawn to mulch and fertilize it.
Always avoid fertilizers that promise “quick green-up.” Excessive nitrogen (the first number in the fertilizer analysis) actually weakens lawns over time by increasing thatch and making the lawn susceptible to weeds. Instead, use an organic or organic-based lawn fertilizer that contains a slow-release nitrogen.
Corn gluten is a very effective, nontoxic way to control crabgrass. Apply it in mid-spring, before the seeds germinate. Portable flamers also send crabgrasses to their doom, and are very useful for eliminating weeds between pavers and cracks. Apply heat for only a tenth of a second, and never use a flamer where mulches are present.
Remember, in the war on weeds, your best arsenal is persistence. Stay ahead of invaders by taking preventive steps, like mulching, and regularly checking for weeds. Then swiftly implement controls that best suit your goals and ecological values. John Cretti is a regional horticulturist and a garden talk show host on “Gardening & Living with an ALTITUDE” on KHOW 630-AM (Saturdays from 8-9 a.m.) and author of The Colorado Gardener’s Guide, Rocky Mountain Gardener’s Guide and Month-by-Month Gardening in the Rocky Mountain Region.
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