Do you feel like the weeds in your garden or flowerbeds just keep growing, and growing, and growing, no matter how many you pull, hoe, or spray? Well, it's true, they do. And they will continue to do so. It is one of the injustices in a gardener's life. But understanding the hows and whys of the situation can help in weed management.
One reason weeds are so persistent is because of the soil seedbank, the collection of weed seeds that have been produced in the past, sometimes in the distant past. Most soils contain millions of weed seeds. Researchers in England once dug up and sifted through the top few inches of soil on a plot of land. In one acre, which is 208 feet on a side, they counted more than ½ million seeds of prostrate knotweed, 700,000 shepherds's purse seeds, 1 1/3 million chickweed seeds, and 6 3/4 million annual bluegrass seeds. And this is only a partial list. Numbers of seeds in the soil are similar in the US. This not surprising when you consider the numbers of seeds that one plant can produce in a season.
For example, a single lambsquarters plant can produce 39,000 seeds in one season. A garden plot in which the lambsquarters plants were not controlled for a single summer would have several million seeds deposited in the seedbank in one year. Redroot pigweed can produce 120,000 seeds in a summer, plantain 36,000, crabgrass 8,000, and mullein 220,000. Common purslane can produce an astounding number of seeds in a single day!
So this answers the question of why there are so many weeds in your garden. But you say you have pulled at LEAST that many weeds already. So why do they continue to keep sprouting? The answer lies in the weed's survival strategy of seed dormancy. Not all of a weed's seeds will germinate at the same time. This means if environmental conditions suddenly become unfavorable, such as a drought or frost, the weed has not used up all it's chances to reproduce; it is holding other seeds in reserve. The length of time they can be held in reserve is amazing for some species. Seeds have been found that are still viable and able to germinate after several thousand years. Most, however, last only a few years or a few decades. So it is no wonder that we are still pulling lambsquarter after controlling the weeds in our gardens 15 years- their seeds can still germinate after 40 years. Some other examples of length of seed viability are 20 years for foxtail, 3 years for redroot pigweed, 1-6 for quackgrass, 40 for field bindweed, and 21 for Canada thistle.
Luckily, weeds seeds also die in the seedbank. Once seeds have been incorporated into the seedbank, they can die due to natural causes or due to the action of microbes and pathogens. Invertebrates such as ants, carabid beetles, and cricket can consume a large number of seeds. For example, in a laboratory condition a single female field cricket consumed more that 250 pigweed seeds per day!
Understanding these factors can help you plan
the most effective weed management strategy. The importance of preventing weeds
from going to seed becomes clear. And so does the virtue of patience. There are
many sources of good information on weed management. The Extension service,
yard & garden books such as Common Sense Pest Control by William Olkowski,
and horticultural publications are all good sources of help.
References:
Danneberger, T.K. 1993. Turfgrass Ecology and Management. G.I.E. Inc., Cleveland, OH. 201 pp.
Handbook of Integrated Pest Management for Turf and Ornamentals. 1994. Leslie, A.R., ed. U.S. Environmental Protection Agency. Lewis Pub., Boca Raton, FL. 660 pp.
Ross, M., and C. Lembi. 1985. Applied Weed Science. Macmillan Pub. Co., New York, NY 10022. 340 pp.
Watschke, T., P. Dernoeden, and D. Shetlar. 1995. Managing Turfgrass Pests. Lewis Publishers, Boca Raton, FL 33431. 361 pp.
Written by Sherry Lajeunesse, Extension Urban Pest Management Specialist. Sept., 1997
Categories: Weed, Integrated Weed Management
Date: 04/28/2005