Valentine’s Day is great time to put out pre-emergent herbicides for summer weed control. When it comes to the application of pre-emergents, timing is everything. Many of the summer weeds that compete for space in the lawn begin germinating as soil temperatures rise above 50 degrees Fahrenheit.
Pre-emergents applied to the soil are activated by a light watering just before the weeds sprout. Applied too early, pre-emergents may become useless as they become degraded by exposure to weather, sunlight and microorganisms. Applied too late and they may miss the chance to destroy newly sprouted seeds. Once the warmth of late March arrives, most weeds have already become established beyond the ability for pre-emergents to control them. Pre-emergents applied after March may be a waste of money and time. The term pre-emergent is not entirely accurate. In fact, seedlings usually need to emerge from their protective shell before the herbicide can kill them.
To be effective, pre-emergents must be watered into the soil. Too much water will cause them to be weak, while too little will limit the amount of area they can protect. Although liquid pre-emergent formulations may have a slight advantage over granules, they may still require follow up by an irrigation cycle. However, for the homeowner, granules are usually the easiest to use since they can be applied with a properly calibrated fertilizer spreader. Regardless of the formulation, homeowners should always read and follow the directions on the product label.
Pre-emergents may be less effective in areas that receive a lot of activity immediately after they are applied. Lawn equipment, ditching, sports and other pedestrian traffic all have the potential to disturb the protective shield offered by the herbicide. More importantly, state and federal laws usually have clearly defined time periods that must pass before people are allowed to enter a treated area since herbicides can be a potential human health hazard.
Pre-emergent effectiveness is aimed at the actively dividing cells of young vigorous plants, therefore most established species and mature plants experience no ill-effects from them. This makes them an important part of your overall weed control strategy. On the other hand, post-emergent herbicides containing glyphosate, 2, 4-D and atrazine (active ingredients in products such as Roundup, Weed-B-Gon and Shotgun respectively) actively destroy cells with a much broader capacity, limiting their usefulness around the mature plants you want to keep.