Pre-emergents are a wonderful thing. For us lazy gardeners, they are essential.
As far as I am aware, there are two types of commercially available pre-emergents. One is chemical, “Preen” and it’s competitors. The other is organic. Big box stores are starting to carry organic pre-emergent. Brand names are inconsequential.
Pre-emergents, as the name implies stop plants from sprouting. The Result is very few weeds to pull. This is done with chemical pre-emergents by inhibiting the germination of the seeds. Organic pre-emergents actually allow the seed to sprout, but kill it before it develops true leaves.
If you think about these two processes, one is clearly better. When the chemical pre-emegent retards sprouting, the seed is still there, still viable, and when the chemical wears off; guess what? It sprouts. Chemical pre-emergents therefore must be reapplied on a rigid schedule to keep the chemical retarding substances at a functioning level.
Organic pre-emergents, on the other hand, let those seeds sprout. So when the organic pre-emergent wears off, you have a few seeds that did not sprout and any new seeds brought into the area. Obviously even organic pre-emergent has to be reapplied. But it is much more forgiving of doing it a week later than you should.
With chemical pre-emergent you are always going to need to make those regular applications. With organic pre-emergent as the seasons roll by you can cut back on the use and still get good service out of it just because you are only dealing with new seeds, not seeds from 5 years ago.
What’s in them? The chemical stuff, I don’t know. I do know it burns my nose. Organic pre- emergent is pure corn gluten, a by product of commercial corn milling that contains the protein fraction of the corn. Its use poses no health risk to people or animals. In fact, because it is 60% protein, corn gluten meal is used as feed for cattle, poultry, fish, and dogs. In addition to the 60% protein, corn gluten meal is 10% nitrogen, by weight, which is an excellent fertilizer and soil builder. Applying corn gluten at the recommended rate of twenty pounds per 1,000 square feet, twice a year, gives your garden a total of four pounds of nitrogen annually - more than enough to meet its needs. This saves you money in two ways. 1) You don’t need to add extra nitrogen, and 2) you apply it less often. The example and the directions say to apply twice a year, but here where we have fairly warm Winters, I apply it a third time in late October to eliminate winter weeds.
Pre-emergents do not affect growing plants. It may be applied around shrubs or food crops. It is primarily marketed, however, as a crabgrass “killer” for the American monoculture lawn.
Technically speaking, gluten is found only in wheat because it is difficult to wash a cohesive protein mass even from rye, the closest relative to wheat, let alone from barley or oats or corn. Unfortunately, the term “gluten” has been misused by the corn industry and has become common in recent years. So if you are a Celiac (Gluten intolerant) like me, do not concern yourself about whether it will harm you. It will not.
The best part is…now that it is being sold at the big box stores, it is cheaper than the chemical pre-emergent when you compare coverage rates. Add on to that less usage and free fertilizer, using organic corn gluten is by far less expensive, not harmful to the environment, and IMHO works much better.
The rule of thumb for spreading pre-emergents is, in Spring; do it when the Forsythias bloom. Do it again in June. And again, if you have warm winters apply a thrid dose in October.