Pigweed! Weeds can arrive in landscapes via many different pathways. They may be little volunteers at the base of a potted plant brought home from the nursery. Sometimes a bird will drop seeds as it flies overhead. It is possible to find weed seeds in bulk seed containers. Even humans and our pets can be the villains, carrying seed capsules on our boots, the tires of our truck, or the coat of the family dog. No matter how they reach our gardens, once they get in, they may be difficult to remove. This is one reason that people apply more herbicides (weed killers) to gardens, golf courses, and commercial landscapes than any other pesticide.
One weed that seems more prevalent lately than it was in previous years is pigweed, a plant that can be the bane of gardeners. It evolved in eastern North America, but it has spread all over, extending even into Europe, North Africa and parts of Asia. This annual is a member of the amaranth family and can tolerate high temperatures and drought. It is also a serious health problem for cattle if they are allowed to eat it in large quantities. The genus has several relations that are weeds, but it also includes some attractive ornamental plants, a species grown for its edible leaves, and another whose seeds can be eaten as a grain.
The plant we call a weed is edible, and it is possible to prepare it similar to spinach or beet greens. These are all horticultural cousins. No members of this group are known to be toxic unless they are grown in very high nitrogen soils, which will cause nitrates to accumulate in the leaves. Leaves contain measurable amounts of oxalic acid, but this is also true for spinach, parsley and some members of the broccoli family. Experts usually consider pigweeds to be an agricultural pest, but they have spread into home gardens.
It is easy, even tempting, to see a weed, and then reach for a spray to wipe it out. Many times that is an effective management strategy, but it could also be the equivalent of using an elephant gun to shoot a BB. One result of the overuse of herbicides is the alarming increase in resistant species. There are species of pigweed that can withstand treatment of four or more different herbicides. Another good reason to hold back on the chemical controls is that since most of these weeds are annuals, pigweed will usually die after it has produced seeds. Killing a plant that is already dead, or nearly so, seems to be a waste of resources.
Once the plant is well established, it is not easy to remove. Simply mowing is not sufficient, as it will generate new stems and flowers from its base. As with most weeds, it is easiest of control when the plant first emerges from the soil. If there is only a small infestation, probably the best management strategy is merely to pull or use a hoe to get the plants out of the ground. If it has spread over a wider area, or if hand pulling is not possible for the gardener, then other means, like an herbicide for broadleaf plants, become necessary. Horticultural vinegar can burn down the foliage, so if the plant has not flowered and the infestation is not too large, this is a useful tool.
The most important thing to remember is to do the major control efforts before flowering. For pigweed, this may take some close examination. Many of the common ones bear flowers on tall terminal spikes, but others put their insignificant flowers in the axil where leaves meet the stem. Wherever they are, if the plant has flowered, then it has more than likely produced a new seed bank, which means the problem will continue into the next warm season.
Now for something much cheerier than pigweed infestation: the South Valley Rose Society and University of Nevada Cooperative Extension are starting the new fall season with a Rose Festival. Beginning in late afternoon (about 4:00) on Thursday, September 24, rosarians will be leading tours of the small, but lovely rose garden behind the Extension office. There will also be demonstrations of cut and potted rose plants. Several of the rosarians also plan to bring in samples of their own flowers.
If you have a rose bush, you might not know much about it other than the color of the blossoms. This can be the opportunity to identify the variety and learn how best to care for it from expert growers.
Email or call Angela O’Callaghan, Social Horticulture Specialist at 702-257-5581.