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Target a True Garden Enemy: Weed Seeds!

(Which Weeds Germinate When?)

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Weeding. If anything can suck the fun out of gardening it’s weeding. And the ultimate irony is that, the more you pull, the more weeds there seem to be.

There’s good reason that many gardeners feel overwhelmed by weeds.

“Pulling weeds disturbs soil. Right now, buried in many gardens, there are millions upon millions of weed seeds ready and waiting for an opportunity to surface, sprout, grow and produce millions more seeds,” says Dr. Richard Old, a weed identification specialist for Washington State University and the University of Idaho.

“Weeds are the most adaptable plants on earth,” he adds, “and the ability to thrive in disturbed environments -- like gardens -- is among their most important adaptations.”

To the layman, it can seem as if weeds are sprouting everywhere at once in a swirl of confusion. In fact, each type of weed has its own unique germination schedule. Understanding how weeds behave can give gardeners a real advantage in the war on weeds. Most important, says Dr. Old, when fighting weeds it’s critical to zero in on the seeds. “Stop the seeds,” he says. “That disrupts the reproduction cycle of annuals and biennials.”

To stop weed seeds from germination, cover garden soil with a three-inch layer of mulch to deny weed seeds the light they need to germinate. For added long-term protection, apply a pre-emergent weed preventer such as Preen on top of the mulch. Preen stops weed seeds from sprouting in the top layer of garden soil and in the mulch itself by creating a weed-fighting barrier that keeps seeds from sprouting for up to three to four months per application. Ideally apply Preen in early spring and then again mid-season. There are always more weed seeds to target, as different weeds germinate at different times spring, summer and fall.

Most troublesome garden weeds are annuals, says Dr. Old, which means they sprout, mature and set seed in the same year. The first wave of summer annual weeds germinates in early spring, around the time forsythia blooms. Others sprout later. By mid-summer, these weeds, which include smooth pigweed, lambsquarters and common ragweed, have already matured and set seed for the following year. Winter annuals, such as chickweed, henbit and shepherdspurse, sprout in late summer or fall and survive the winter, thus gaining a head start on the next growing season. To stop mid- and late-summer germinators, reapply Preen after three to four months to extend its effective period into fall.

Perennial weeds live for more than two years. Some, such as dandelion and pokeweed, form a deep taproot that re-grows each spring. Other perennial weeds spread with creeping roots as well as seeds. Examples include Canada thistle, milkweed, ground ivy, creeping buttercup and quackgrass.

“Some of the most unpredictable weeds are biennials,” says Dr. Old. The seeds of a biennial weed can germinate at almost anytime during the growing season. These plants normally produce a small rosette of leaves the first year, then flower, mature, and die during the second year. A true biennial never produces flowers or seeds the first year. Troublesome biennial weeds include wild carrot, common burdock, bull and musk thistle, and poison hemlock.

Some weeds are so adaptable that even weed experts can’t predict what they’ll do. “Common mallow is a weed that can behave as an annual, biennial and perennial,” says Dr. Old. “Its seeds germinate at almost any time of year, depending on the location and the environmental conditions.”

Is it possible to conquer these wily weeds? “Not entirely,” says Dr. Old. “But gardeners will be much more successful if they know which weeds they’re fighting and how those weeds reproduce.”

For gardeners interested in winning the war against weeds, online Weed IDs are handy tools. The Preen Weed ID site at is geared to home gardeners and offers multiple images of 115 common weeds and a state-by-state search feature which helps gardeners hone in on the specific weeds most likely to plague their own gardens. Besides images, each weed’s ID identifies solution-based products and videos. The site also includes links to top U. S. university weed resources.

Those interested in a more comprehensive weed identification reference might like Dr. Old’s App for Android phones and tablets: 1,000 Broadleaf Weeds of North America $9.95 at:

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