Maximizing success with foliar sprays

The most common method of applying plant growth regulators (PGRs) is using foliar sprays. There are several reasons why sprays are the most popular application method. First, sprays are already familiar to many of us since a majority of insecticides and herbicides are applied as foliar sprays. In addition to being a familiar application method, foliar sprays are also popular because all the PGRs on the market are absorbed by shoots (leaves and/or stems). Finally, for post-transplant growth control during finishing, foliar sprays can be more time-efficient when compared to drenches. To make sure you get the most out of your PGR sprays, this article will review some important best management practices to remember.

Mixing it up

Like every other type of PGR application, proper measuring prior to mixing will help avoid problems, such as insufficient or excessive growth control from mixing errors.

Start with the highest quality water available, one with low alkalinity and a lower pH. The alkalinity in water can reduce the effectiveness of PGRs, as can a high pH for some. Starting out with high-quality water is the first step in maximizing effectiveness. Beyond mixing the right amount of active ingredient at the right time, consider enhancing your formulation. If permissible, as indicated on the PGR label, add a non-ionic surfactant or “spreader-sticker” to improve the efficacy of your foliar sprays. The strong surface tension of water causes drops or beads to form on foliage. By adding a surfactant, the surface tension of the water is broken and instead of drops, a more even film will form on the leaf, increasing contact between the leaf and the PGR solution, thus promoting increased uptake.

Also, be mindful of the time that will lapse between mixing PGRs and applying them. Some PGRs (such as ethephon) can lose activity if they sit in a tank too long,

Application volumes

One of the most important aspects of foliar spray applications is the volume of solution applied to plants. A common recommendation for PGRs has been to “spray to runoff", where solution is sprayed until it is running off leaves. The main problem with this approach is that it is very subjective. “Runoff” will look different to different people. A great illustration of this is an activity I have done before with people. I ask a group of different people to go apply foliar sprays to the point of runoff on crops. The amount of water or “solution” that is sprayed is calculated afterwards and, along with square footage of the area sprayed, we have calculated the volume of solution applied by each individual on a square foot basis. Results have ranged widely, from less than 1 quart per 100 ft2 to over 4 quarts per 100 ft2.

Compared to the recommended application volume of 2 quarts per 100 ft2, it is easy to see how using a subjective indicator such as “runoff” can result in applying less than half to over twice the recommended volume of PGR solution.

Keeping it even

One of the biggest challenges when using foliar sprays is to keep the coverage uniform within crops. While we would all hope the proper volume of PGR solution (i.e. 2 quarts per 100 ft2) is being distributed across all plants evenly by the applicator, the uniformity of spray applications can’t be ignored.

Applicators should practice spraying with water prior to applying PGR sprays. This will allow applicators to “calibrate” the speed or cadence of their steps with their arm movement and volume of spray to achieve uniform coverage of their crops.

Yet, it is not just people who miss plants — equipment used to apply PGR sprays can miss, too (Fig. 1). Same as with human applicators, check the coverage with equipment like booms before and after spraying, being sure to follow-up and spot treat if needed.

Foliar sprays are an effective application method for every active ingredient found in PGR formulations. Take a few extra steps to ensure accurate application of PGRs in the greenhouse.

Application environment

Another important factor in maximizing product effectiveness is the environment that the PGRs are applied in.

For other application methods such as substrate drenches or liner and bulb dips, the application method is generally not something you need to be concerned with. However, the application environment will affect the drying time of PGR solution on shoots. The longer the PGR is maintained in solution on the surface of leaves and stems, the longer plants have to absorb and uptake the active ingredient.

Maximizing the drying time is important for all PGRs applied as foliar sprays, but the time it takes to be taken up varies with the different active ingredients. While it is essential to maximize the drying time for PGRs which are absorbed slowly, it also benefits PGRs that are absorbed quickly. Some of the best ways to increase drying time are to minimize air movement and avoid warm or hot temperatures and low humidity.

One of the first steps in maximizing drying time if to try and time applications for early in the morning before the sun is too high and temps rise too much, or late in the day as the sun is going down and the afternoon heat has passed. The middle of the day should be avoided, unless it is a cloudy or rainy day.

Additionally, turn any horizontal air flow fans off to reduce air circulation during application and subsequent drying. However, once the foliage has had sufficient time to dry, do not forget to turn the fans back on to resume air circulation. And be sure to avoid irrigating plants after treating them - you do not want to rinse off any active ingredient.

Take-home message

Foliar sprays are an effective application method for every active ingredient found in PGR formulations. When making applications, keep in mind that taking a few extra steps to ensure foliar sprays are made with the best solutions, applied at the correct volume, and allowed ample time for absorption and uptake will ensure your PGR sprays are successful each and every time.

Christopher is an associate professor of horticulture in the Department of Horticulture at Iowa State University. ccurrey@iastate.edu