- by Bill Meyer
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.
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.
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.
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.
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.
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.
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
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
For Preen purchased in a 17.5 lb.
container, approx. $11.00 per 1,000 square feet for same coverage as
For Preen purchased in a 6 lb. container, approx. $15.00 per 1,000
square feet for same coverage as Snapshot.
Back to the Reading Room