ARCHIVES

ARCHIVES

Weeds: Eradicating these monsters can take a lot of work

By Joan S. Bolton/Contributing Writer

The only good weed is a dead weed.

At first glance, that might seem like a reasonable motto.

But the problem is, even a dead weed can spread its kind through the garden, if you haven’t removed it yet. That’s because prior to dying, annual weeds set an inordinate number of seeds to ensure the success of future generations. Perennial weeds may also appear dead on the surface. Yet in addition to producing copious seeds, their roots are very much alive below the surface — and ready to spring forth with a vengeance next year.

There’s not a lot of planting to be done in the garden in August. So now’s a good time to get the upper hand on weeds. If your yard is already pristine, congratulations. Put up your feet and enjoy the sunshine for the rest of the month. But if you’re like the rest of us, there are at least a few undesirables lurking at the edges. Preventive maintenance this month can head off woes later on.

What You’re Up Against

Annual weeds, such as grasses, wild mustard, common groundsel, sow thistle and spotted spurge, come up once a year, run through their entire life cycle, then die. They produce a prodigious quantity of seeds in rapid order.

The key to control is to remove the weeds before they set seed. Dormant seeds from previous seasons may still sprout next year. But if you persist over a few years, you’ll deplete those reserves, too.

Getting rid of an annual’s roots isn’t particularly important, because a new plant won’t sprout from them. You can yank out the top by hand or hoe the area. However, hoeing roughs up more soil than hand-weeding, and if there are many dormant seeds in the bed, the disturbance can bring those older seeds to the surface, prompting them to sprout.

Biennial weeds, such as blessed milkthistle, salsify and prickly lettuce, root out and produce leaves and stalks the first season, then flower the next. So you have some warning before they set seed. Be sure to dig out any tap roots. Otherwise the roots may bulk up and send up even bigger leaves, flowers and seeds.

Perennial weeds, such as creeping wood sorrel, spreading dogbane, Bermuda grass and bindweed, have all sorts of devious ways to ensure their survival.

Creeping wood sorrel, for instance, may change the color of its leaves from mid-green to dark red, to blend in with neighboring vegetation. Then when its tiny, cornucopia-shaped seed pods ripen, they explode with such power that they can hurl their zillions of black seeds 5 to 10 feet in any direction. Spreading dogbane and Bermuda grass multiply via rhizomatous roots, which have growth tips at each underground joint.

Bindweed sends out such a tangle of long, lateral roots that it’s nearly impossible to find them, until they’ve popped up as new plants.

Gaining Control

Pulling and digging are among the most ancient of methods for getting rid of weeds.

To slow down new weeds, cover the soil, as most weed seeds need soil contact and sunlight to germinate. Lay down 2 to 3 inches of bark, wood chips or gravel, taking care not to mound the mulch over the tops of existing plants.

If you’re working with a new landscape and the area was previously infested with weeds, consider laying down landscape fabric before applying the mulch. The woven or spun fabric allows air and moisture to penetrate the soil, yet stops most weeds from popping through. Do not use black plastic, which cuts off air and moisture flow to beneficial microorganisms in the soil, may cause drainage problems and eventually breaks down and makes a mess.

In lawns, the fewest weeds pop up in the healthiest, greenest spreads. Fill any thin spots with topper and grass seed, so that weeds can’t gain a toehold. The same goes for ground covers: the most tightly knit plantings are least likely to allow intruders.

Chemicals are powerful allies as well.

Pre-emergent herbicides, such as corn gluten, interfere with a seed’s ability to germinate. They’re typically most effective in late winter or early spring.

Post-emergent herbicides kill actively growing weeds. The weeds die by direct contact or by absorbing the chemical and then dying from within. If you’re going after biennial or perennial weeds, use a systemic product such as Roundup. The active ingredient, glyphosate, is absorbed into the leaves, then travels into the roots to kill the entire plant. The full effect may not be apparent for a week to 10 days.

Especially vigorous woody plants, including bamboo and ivy, may require more effort. Try “hack and squirt”: slash the stems, then spray the open wounds with glyphosate. Late summer and early fall are generally most effective, when the plants are moving nutrients down into their roots to store for growth the following year.

A more natural approach is to spray white household vinegar (5 percent acetic acid). Vinegar works best during warm weather and may take two to three applications.

Non-chemical controls include enlisting goats or grass-eating geese to clean up your property.

Bring On the Heat

Several local governments, including the city of Santa Barbara, kill weeds in parks and playgrounds with steam. Commercial machines use high-pressure nozzles to blast the steam: The heat instantly ruptures the weeds’ cells. However, the steam only targets the top quarter inch of soil. So the systems work best on annual and young perennial weeds, rather than well-established weeds with deep taproots or extensive root systems.

Home gardeners may be tempted to use a hand-held propane torch, sometimes called a flamer. However, unlike steam, which delivers super-heated water, you’re dealing with an actual flame. The searing heat bursts the cell walls of green weeds, and can set anything else on fire. Absolutely do not use a flamer in an area with brush, or even in areas with mulch. Instead, the device is only appropriate for vast expanses of hardscape where not even a single leaf or twig is present — hardly a desirable garden.

Far safer death-by-heat can be conducted with a teakettle.

Boil a kettle of water. Then pour the water over the top of any persistent weeds growing in cracks in your pavement. The method is time consuming, but every bit as effective.

HOW WEEDS MIGRATE:

Talk about resourceful. Weeds may:

— Hitch a ride on animals or humans via stickers, burrs or thorns;

— Produce fragile seeds attached to silky threads that are blown and scattered by the wind;

— Bear seed capsules that explode upon ripening;

— Supply seeds that birds or other animals eat, digest, then eliminate and scatter;

— Spread below ground, via rhizomes;

— Travel above ground, via rooting runners. — Joan S. Bolton

ATTACK OF THE KILLER WEEDS:

If weeds in your garden seem more relentless than ever, you can blame global warming.

The associated rise in carbon dioxide in the atmosphere encourages weeds -- including poison ivy, ragweed and cheatgrass -- to grow faster than ever, according to research by the US Department of Agriculture.

For instance, ragweed "grew faster, flowered earlier and produced significantly greater aboveground biomass and ragweed pollen at urban locations than at rural locations," according to the National Science and Technology Council.

In addition, the USDA found that invasive weeds, especially those that reproduce by vegetative means, such as roots and stolons, respond more vigorously to increasing carbon dioxide than do agricultural cash crops. — Joan S. Bolton

Joan S. Bolton is a local freelance writer and garden designer. Her In the Garden column appears regularly. She can be contacted through her Web site at www.santabarbaragardens.com.