The very qualities that endeared shamrocks to Celtic farmers make these plants a most unlucky addition to your lawn or landscape. Fields of clover can be a real nuisance to get rid of, and cousins of the shamrock are a hazard to domestic animals.
Shamrock or Oxalis?
If you see three-leaved plants, you'll first need to identify the species to know how to proceed. Shamrock is a fast-growing three-leafed clover that is excellent grazing for livestock. These plants can be cut to the ground and will grow back rapidly, which is why they were a favorite of the Celts.
Oxalis is another three-leaved member of the clover family. Like the shamrock, it grows faster than most grasses and weeds, allowing it to quickly dominate a landscape or crowd out a lawn. Unlike the shamrock, oxalis is toxic to horses, dogs and cats, causing kidney failure if enough of the leaves or tubers are ingested.
Controlling the Plants
Clovers, including shamrocks, grow from tubers below the ground. These tubers look a lot like small potatoes, and some varieties of the tubers are harvested for food. The leaves of oxalis and shamrocks can also be used in cooking; if you've ever had sorrel, you've eaten clover, which is rich in protein and Vitamin C.
The tubers present the biggest problem for the gardener. They grow and divide rapidly, sending up new stems with leaves whenever the old ones are cut back. Simply cutting or yanking out the leaves often isn't enough to get rid of clover; you need to eliminate or kill the tuber to kill the plant.
Whenever you're preparing garden beds, be sure to dig down six to eight inches and dispose of any clover tubers that you find. Leaving them there is inviting a problem. Newly established clover can often be controlled by yanking out the leaves and stems every few days, until the young tuber dies.
For lawn areas, cut the grass close to the ground every two or three days until the clover stops growing back. If this doesn't work, you can use a broad-leaf herbicide to target the plants without harming your grass. This won't work in flower beds, since you'll kill annuals and perennials as well. Use caution with herbicide in areas that border flower beds, as it's easy for the poison to spread and kill plants you want to keep.
Most varieties of clover grow during the winter, rather than the spring. You'll see fresh sprouts poking up as early as February in some parts of the United States. This is a good time to dig down and get rid of the tubers or to apply herbicide. You may lose some ornamental plants, but the damage won't be as noticable in winter, when most plants are dormant.
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