Pre-Emergent Herbicides
by Bill Meyer
    Some time ago a product for gardeners called Preen entered the marketplace. It was called a "pre-emergent herbicide" and was touted as stopping weeds in the garden before they came up. Its promise of an end to the tiresome chore of weeding was offset somewhat by its high price, and no one seemed to know much about it. The use of pre-emergent herbicides in the home garden actually goes back further than Preen, as "crabgrass preventer" fertilizers did their preventing by using pre-emergent herbicides to stop crabgrass from germinating.
    A pre-emergent herbicide is a chemical that burns the roots of all plants in the zone in which it is applied. At normal application strengths, it will burn the tips of emerging roots of weed and other seeds, thus preventing them from growing. It sits in the upper 1/2 inch or so of soil and breaks down slowly over the course of a season. For practical purposes we will have a look at only two pre-emergent herbicides here in this article. The most commonly used chemicals are Surflan (trade name) or Oryzalin (generic name) and Treflan (trade name) or Trifluralin (generic name). Surflan is sold in liquid form for use in sprayers, and Treflan is a granular form. Treflan is the active ingredient in Preen.
    Before going any further, I know that some are already getting nervous about the word "chemical". Many are afraid to use "chemicals" in the garden for fear that they will be toxic to themselves, their children, or their pets. In numerous independent toxicity studies, both of these chemicals have proven remarkably safe to humans and animals. In these studies, highly concentrated forms were tested on animals in such ways as dripping it into eyes, ingesting large amounts, and prolonged contact with skin that had been abraded. The worst thing reported in those studies was mild irritation that lasted a few hours. For comparison, consider what ordinary table salt would do in similar tests. These are both very safe products to use in the garden, and should not be a cause for worry. Even the most stringent environmental organizations have found no safety issues with either of them.
    The most important thing to know about using either of these products is that timing is everything. They will only affect the tiny weak first roots of seedlings, and only in the shallow layer in which they are present in the soil. This means they must be applied before weed seeds begin to germinate in the spring, so that they are in place when the soil warms enough to cause germination. For the Delaware Valley area, this would mean before the end of March. Common lore says that they must be applied before the Forsythia bloom. If you wait too long to apply them, they will be ineffective because the weed seeds have already gotten their roots further down into the soil where the herbicides have not penetrated. It is also important to understand that they must be watered in. This is necessary to aid them in penetrating the surface of the soil. Without watering in, they would just lay on top of the ground and the weed seeds would germinate underneath and come up as normal.
    If applied at the higher rates recommended on the label, they can last until late in the summer before breaking down into harmless compounds. Specific amounts and breakdown time will vary as different soils require different applications, so some fine tuning is necessary to get long term results. Overuse beyond the recommended rates can cause them to build up in the soil from year to year. They burn all roots in their active zone, but the damage to anything other than delicate seedlings is minimal. Applying them to two-leaf seedlings in April showed no effect. Growing plants, including weeds, are safe from any serious harm. Some larger seeds, like tree seeds, can break through the protective layer, but virtually all small seeds will not germinate and will die. If you want annuals and biennials to reseed, do not use pre-emergents in those areas. If you disturb the soil where they are used, they must be reapplied immediately because buried seeds will be brought up to a level where they can grow.
    Though easy to find in garden centers, Preen is generally too expensive to use for more than small areas. For larger areas, visit your local nursery supply store and buy a professional product like Surflan if you have a sprayer, or Snapshot if you prefer a granular. Snapshot is the choice of many nurseries, and it includes some extra protection against some difficult weeds and grasses. Professional products are only sold in fairly large quantities, so you may want to split a purchase with a friend or two. These products are not restricted in any way, and no license is required to buy or use them.
    So, how well do they work? The answer is very, very well indeed. The results of using Surflan in several hosta gardens I have seen were nothing short of amazing. In one large garden, several annual weeds had gotten completely out of control and there were probably millions of seeds waiting to germinate when the weather warmed up the following year. Surflan was sprayed at the higher recommended rate in all the garden beds, and by mid-July there were hardly any weeds to be seen there. The surrounding lawn had not been sprayed and there was a wide border of grass around the beds where weeds were coming up in the thousands, where they then met the lawnmower.
    March in the garden is not normally the busiest of seasons, so it is a good time to set aside a day to apply a pre-emergent herbicide. A few hours work then will save you many hours of work later in the season when you want to be enjoying the garden rather than weeding day after day. If you are overrun with common annual weeds like Oxalis and Purslane, you can expect dramatic results. When you have totaled the cost of using pre-emergent herbicides, consider whether it is worth that much to safely and easily reduce your yearly weeding to a minimum.
Cost Analysis at heaviest recommended rates (for maximum control):
     For generic Oryzalin purchased in a 2 1/2 gallon jug, approx. $3.00 per 1,000 square feet. Will need a backpack sprayer at approx. $100.
     For Snapshot purchased in a 50 lb. bag, approx. $10.00 per 1,000 square feet. More effective on more weeds than Preen.
     For Preen purchased in a 17.5 lb. container, approx. $11.00 per 1,000 square feet for same coverage as Snapshot.
     For Preen purchased in a 6 lb. container, approx. $15.00 per 1,000 square feet for same coverage as Snapshot.

Reading Room
Library Homepage