By: Keith Santner, Published in NM Pro, July 2008

Spring shipping season has passed, and now we’re in the throes of summer. It seems those pesky weeds germinate in an instant and get out of hand in the blink of an eye. So what should you do?  

Life will be much easier and your nursery will be much healthier if you can prevent weeds from gaining a foothold. If it’s too late to prevent weed germination at your nursery, then it’s time for some general eradication followed by prevention in both the non-crop and crop-growing areas. After all, it’s impossible to control weeds in crop containers if nearby non-crop areas are infested.

Cleanup and Prevention
In non-crop areas adjacent to the nursery, spray weeds that have already germinated with a post-emergent herbicide containing glyphosate, then treat the non-crop areas with a pre-emergent herbicide containing oryzalin. Many times, the post-emergent herbicide can be tank-mixed with a pre-emergent herbicide so both products are applied simultaneously. If the label doesn’t provide instructions for tank-mixing the products you want to apply (or prohibits tank-mixing altogether), conduct a jar test for compatibility.

Remove the dead weeds so that any mature seeds are moved off-site. For non-crop areas with grass, mow to the proper height and treat with herbicides labeled for turf maintenance, such as Corral pre-emergent grass herbicide from Scotts. If turf maintenance isn’t practical, consider replacing it with gravel or groundcover – or at least mow the turf low enough to minimize week seed production.

Wind breaks (such as Leland cypress or arborescent junipers) can also be beneficial if they decrease the number of seeds that land in crop-growing areas. Living windbreaks may require insecticides or fungicides so they don’t become a refuge for other pest problems. Also, make sure not to plant a prolific seed-producing plant because your windbreak may become your No. 1 weed problem.

When crops are grown in containers, herbicide selection is more limited. Obviously, post-emergent herbicides can injure crops and should be used with extreme caution, if at all, to prevent damage.

Most nurseries rely on hand labor to pull weeds after they’ve germinated. Even pulling weeds must be done correctly or you’ll get poor results for your labor investment.

1. Make sure workers remove all of the weeds, including those next to the plant stem.
2. Place the extracted weeds into a container and throw them into the trash. Weeds can disseminate seed even after they’ve been pulled out, so don’t leave them in the field to re-infest the cleaned crop.
3. Synchronize the weed extraction with a follow-up application of pre-emergent herbicide to kill the remaining weed seed as it germinates.

One more point on pulling weeds: Since no herbicides are labeled for use in propagation houses, liners coming out of propagation are notorious weed carriers. An investment in cleaning up those liners by hand will pay generous dividends later.

Watch for Injury
As we all know, weed prevention in nurseries is largely based on pre-emergent herbicides. Used properly (always refer to the label for details), pre-emergent herbicides can give months of control with little risk of phytotoxicity. But not all plants tolerate herbicides in the same way.

A good rule of thumb is, the more tender the plant tissue, the more likely it will be injured by a pre-emergent herbicide. Thus, annuals are more sensitive than perennials and perennials are more sensitive than woody plants. In practice, this means if you can safely use a herbicide on perennials, it is also probably going to be safe to use on your woody plants. However, as with any plant protection product, always read the label for details.

It’s possible for herbicide manufacturers to include a comprehensive list of ornamental species, much less specify specific treatments that cover all growing conditions. So it’s good practice for you to do on-site trial work before implementing a change in your herbicide program. The best-case scenario would entail applying the new herbicide to a representative selection of plants at the time of the ear and under the same conditions you’d anticipate applying it if adopted.

Always make sure to compare treated plants to untreated (or plants treated with your existing herbicide program). Most damage will show up within two weeks, but it often appears much more quickly.

Granular or Sprayable Forms
With the exception of pre-emergent herbicides that include the active ingredients isoxaben and prodiamine, few sprayable pre-emergent herbicides are tolerated by a large list of crops. This brings us to another good rule of thumb.

Sprayable forms are roughly 10 times more likely to cause phytotoxicity than granular forms. That is why key active ingredients like oxyfluorfen are only labeled for use in containers when in a granular form. Certainly, the sprayable forms are excellent choices for non-crop areas, but with few exceptions, risk increases when switching to a sprayable form.

Timing is Everything
A few years back, James Altland wrote an article on the Top 10 reasons pre-emergent herbicide programs fail. He concluded that the No. 1 reason for failure is that growers applied pre-emergent herbicides to weeds that had already germinated. In my experience, there are two main reasons this happens:

1. Poor hand-weeding. This tedious task is expensive, boring and important and it takes concentration (as well as good eyes). It’s critical first step in ensuring that you’ve removed all germinating weed seeds from the area. 
2. Waiting too long to apply the pre-emergent herbicide after hand-weeding. Weeds can germinate in a couple of days. Pre-emergent herbicides are absorbed through either the roots or the shoot of a germinated seed. As the weed seedling ages, the roots penetrate the soil below the herbicide barrier, and the shoots thicken and become resistant to herbicide uptake. Therefore, if the herbicide isn’t applied before the seedling grows past the susceptible stage, there will be little or no control.

Always follow the label instructions, but in general, the herbicide should be applied immediately after the growing medium has settled (the first watering) or within 24 hours of the initial hand-weeding. Wait to apply herbicide after the plants have been moved to the final growing site. Moving them after application will disturb the herbicide barrier.

Once the herbicide has been activated (see label for instructions), don’t disturb the crop for at least 90 days or the period of time specified as the length of control on the label.  Reapply at appropriate intervals, and rotate active ingredients where possible to avoid building a population of weeds immune to the herbicide’s active ingredients.

The bottom line is that it’s easier to keep a nursery clean than to let it become infested and then try to clean it up. If weeds get out of control, aggressively deploy the resources necessary to eradicate the weed population. Prevent new infestations with the judicious use of pre-emergent herbicides in both crop and non-crop areas.

This article was originally published in the July 2008 issue of NM Pro magazine