| The active ingredient in Preen is trifluralin, which was sold originally under the brand name Treflan. It acts by killing weeds when they germinate by preventing the shoots and roots from elongating. It is generally safe to use on legumes which are planted from seed (beans, peas), and onion sets and potatoes. It can be used on tomatoes, peppers, and members of the cabbage family that are transplanted into the garden. Theoretically, it should be safe on other garden plants as long as they have emerged and are growing well before the herbicide is applied to the ground, although I'm a bit leary of applying it to any members of the squash family, including melons, and to corn. It does a good job of preventing weeds in raspberries and asparagus. Trifluralin does an excellent job of controlling kochia, lambsquarters, redroot pigweed, purslane, and annual grasses. You can look up the label to see what all crops it is registered for to get an idea of what you can use it on and what you can't. It is degraded by sunlight, so should be shallowly worked into the soil as soon as possible after applying, or should be watered in with 1/4 to 1/2" of rainfall or sprinkler irrigation. Under some conditions (particularly applying at too high a rate), it may last in the soil into the next year, which can cause problems if you plant sensitive crops in treated areas. This happened to me one year when I planted sweet corn where I had treated the prior year. Corn came up, then didn't grow and looked drouthy, and was easily pulled up to display 1" root stubs. I like to use it in a small garden which gets all my tomatoes and peppers since I don't have to worry about planting anything sensitive there or chancing carryover. The larger garden with everything else I don't treat because I don't grow really large quantities of anything and don't want to risk carryover, although I will sometimes apply it in the large open spaces around squash and melons after they are starting to vine. |
