Media-active plant growth regulators (PGRs) are very important in the production of many greenhouse crops. Unfortunately, all of them have potential residue issues that can cause problems later in production.
Residue problems arise when PGRs are very active in the growing media. Additionally they are very persistent in soil and water and on hard surfaces like plastic and concrete. Also, these PGRs are not very soluble and only dissolve slowly in water, which means once they dry on a plastic tray, they are not easily washed off.
Although some PGRs can be very persistent, residue can be fairly easy to diagnose and remove. A quick bioassay test with broccoli seeds can help prevent possible stunting from residue.
The most common residue situation is found when plug trays are reused. Crops such as impatiens and petunias usually receive large amounts of PGRs. After the trays are washed, much of the PGR remains as residue. If a sensitive crop like begonias is one of the first crops started the next year in those residue-covered trays, its growth will be affected. Residue problems might also occur with water-collecting trays and when pots are reused.
A potentially serious situation exists with subirrigation systems and residue. If sprays are applied when there is considerable exposed bench or floor surface, the spray dries and leaves a significant residue. During later flooding, water picks up the residue, and it goes back to the holding tank. When other crops are irrigated with this water, it acts like a drench. I have seen situations where the nontarget crops are affected more than the crop that was sprayed.
Applying PGRs in subirrigation is becoming more common, and the residue left by the first subirrigation does not seem to be a problem if there are several irrigations applied to the same crop. However, when the treatment is applied near the end of a crop, the residues might still be on the surface and/or in the irrigation water when the next crop is put down.
Because of the longevity in soil, PGR residues can occur in composted soil and plants. In unusual situations, residues can build-up in landscape beds where multiple plantings of heavily treated crops like impatiens are tilled in and then a sensitive crop is put in the bed. Also, there have been residue issues with in-ground crops where one crop is sprayed heavily with PGRs that run into the soil where the residue can affect the next crop.
A few years ago Jeff Million at the University of Florida developed a bioassay test to determine PGR levels in soil and water. He found broccoli was a good test plant because it is sensitive to PGRs, and the seed are easy to handle and germinate quickly. Growers can use a similar test to check for PGR residues. The seed can be sown directly into small pots or cell packs.
For the test, use these three treatments or groups of seedlings:
1. Control treatment. Consists of clean containers, clean water and new media.
2. Spiked treatment. Same clean material as the control except the PGR in question is applied either by drenching the media with the PGR in question at about 0.1 ppm or irrigating with 0.01 ppm at each watering.
3. Suspect treatment. Use the same clean materials as above except use either the water or soil suspected of containing the PGR residue.
If after 2-3 weeks the seedling growth in the suspect treatment is similar to the spiked treatment, this is a positive indication there are PGR residues in the water or soil. If the suspect seedlings are similar to the clean control group, then any PGR residues that may be present are not high enough to cause a problem on the most sensitive crops.
For residues on hard surfaces, the procedure is a little more involved. With containers, the seedlings will not start picking up the PGR until the roots reach the sides of the container where the residue is. For the spiked treatment, apply the drench when roots reach the side of the container. In subirrigation situations, grow the seedlings for the three treatments on the subirrigation bench or floor. Remove the control and spiked treatments during irrigation and replace them when the surfaces are dry.
There also is an alternative test for containers and trays and one that works for sprayers and irrigation equipment. Soak them for 2-3 hours in as small a volume of water as possible. Then use that water to irrigate the suspect treatment in the test.
What to do if you have PGR residue on:/p>
Soil — There is not an easy way to remove the residue from soil.
Water — A sand filter will not remove residue. Water can be used to irrigate crops that need a lot of PGR. Also, residue can be diluted with clean water to a level where the PGR is not detectable.
Hard surfaces — The PGR will dissolve slowly into water, so a quick wash does not remove residue. Soaking the surface for 1-2 hours and then rinsing with clean water removes 80-100 percent of the residue. Cleaning or disinfectant products can be used but are not required.