The Wonderful World of Pre-emergent Herbicides
By Bob Chapman
Webster defines herbicide as "Any substance used to destroy plants, especially weeds, or to slow down their growth."
In the not too distant past, the main substance used to kill noxious weeds was used crankcase oil, liberally applied. This caused problems, however. The smell was bad, the used oil tended to sterilize the soil, and the area with oil remnants was left bare and very unsightly. Help was on the way, although we didn't know it at the time.
Science to the rescue
Plant scientists found that an application of certain chemicals forced many plants, particularly those with a large leaf surface, to grow faster than their roots could manufacture food and send it to the leaves. This caused the plant to literally starve to death. Now most gardeners take it for granted that applying the post-emergent 2,4-D and its derivatives will rid our lawns of dandelions, clover, dock and a host of other weeds. The wonder is that it works in quick order and does not harm grass at all.
Pre-emergents developed
But the ability to kill weeds by spraying chemicals on the leaves is only one form of herbicides now available. The scientists found that certain chemicals would inhibit the growth of newly emerging seedlings. These chemicals are called pre-emergent herbicides. By correctly applying pre-emergent herbicides at the proper time of the year, gardeners can have weed-free lawns, flowerbeds and shrubbery borders. They will not have to pull or dig out common weeds.
How they work
A pre-emergent herbicide is a chemical applied to the surface of the soil that will kill the new shoots sprouting from seeds. Pre-emergents do not kill existing weeds. You must rid the areas to be treated of weeds first. The pre-emergent herbicides can either be in a liquid or granular form. To be effective they must be applied prior to seed germination. A chemical barrier formed at the surface of the soil prevents plant germination for extended periods. In some mild winter areas, pre-emergent herbicides must be applied twice yearly, once in the fall to prevent the winter growing weeds and in the early spring to control those that grow in spring and summer. After the pre-emergent is spread, do not disturb the treated soil lest you break the chemical barrier.
Species-specific pre-emergents
Some pre-emergent herbicides are species-specific. They are usually applied to control one type of weed, such as crabgrass, in lawns. Crabgrass weed preventers are incorporated in lawn fertilizers. This crabgrass preventer/fertilizer combination should be spread on the lawn in February.
Broad-spectrum pre-emergents
Others are broad-spectrum types that will prevent up to thirty of the most common garden weeds from growing in flowerbeds, shrubbery borders and tree basins. Currently, the commonly available pre-emergents are Preen, Amaze and Weed Stopper.
Care when using these helpers
Some words of caution when using pre-emergent herbicides: They must be applied before the weed-growing season starts. They cannot be used in flowerbeds where flower seeds are to be sown. Some vegetables and some plants cannot tolerate these chemicals. Pansies and violas, for example, may show signs of damage.
A wise gardener will always carefully read and follow the directions and warnings on the labels before using these chemicals in the garden.
The many benefits of using these wonderful products
It takes thought, timing and minimal preparation when using these chemical wonders, but they can save a gardener many, many hours of hard work. By spreading or spraying pre-emergents, a gardener can have a virtually weed-free lawn and garden. Thus it allows your lawn, plants, shrubs and trees to utilize all the food and water you give them and these are not shared with the weeds. And, it sure makes a yard so much prettier when it remains weed-free all year 'round.