Growers looking to control plant growth have several non-chemical options to turn to.

Controlling plant size is one of the most important responsibilities for greenhouse growers. Plant growth retardants are commonly applied to inhibit gibberellic acid synthesis and, subsequently, stem elongation.

However, there are a variety of non-chemical growth control techniques also used to control plant growth, such as restricting irrigation, reducing the amount of nitrogen and/or phosphorous in fertilizer programs, and brushing plants. Perhaps one of the most well-known strategies non-chemical strategies for reducing unwanted stem elongation is by using DIF.

DIF is the difference between the daytime and nighttime air temperatures. Mathematically, this would be DIF = daytime air temperature - nighttime air temperature. For example, if the air temperature during the day was 72° F and at night it was 65° F, the DIF would be +7° F. As the DIF becomes more positive, stem elongation is promoted. As DIF becomes less positive or negative (i.e., night temperatures warmer than the day temperatures), stem elongation is suppressed. Minimizing the positive DIF or using a negative DIF has been a successful and popular non-chemical growth control technique.

Although using a negative DIF may be a useful strategy to control plant size, it’s not as environmentally or economically friendly as we may like. In order to achieve a negative DIF, you must run warmer night temperatures. Of course, running a warmer night will take more energy compared to a cooler one, but this is exacerbated by the fact that heating at night is already more expensive — energetically and economically — than heating during the day. This is due to the fact the difference between temperatures inside and outside the greenhouse is greatest at night and, therefore, more fuel is required for heating to maintain target greenhouse air temperatures. Thankfully, there is an alternative option to a negative DIF for growers: DIP or DROP.

Instead of running a negative DIF, with warm night temperatures and cool day temperatures, DIP or DROP (synonyms for the same process) allows the traditional warm day and cool nights to be used — but with a “dip” or “drop” in air temperature occurring before sunrise and lasting for a few hours. Let’s use the air temperatures mentioned earlier in the article — a 72° F day and 65° F night. Again, this is resulting in a +7° F DIF that would promote stem elongation.

Now, 30 minutes before sunrise, drop the temperature down to 60° F and hold it there for three hours, after which time the air temperature is increased up to 72° F for the rest of the day. Since plants “register” the daytime air temperature that elicits a DIF response early in the morning around sunrise, the morning DIP or DROP to 60° F essentially creates a -5° F DIF since the nighttime air temperature was 65° F.

There are a few benefits to using a DIP or DROP instead of a negative DIF. As already mentioned, the biggest benefit to DIP/DROP is being able to run cooler nights and warmer days — and the energy savings that come with them — compared to the need for warmer nights and cooler days with traditional negative DIF. But there are other benefits as well. Depending on the temperatures used for achieving a negative DIF, warm temperatures at night may come with the risk of heat delay for flowering, especially for short-day plants. This problem is avoided by using DIP/DROP instead.

There are a few things to keep in mind when using a DIP or DROP. First, like with any other aspect of managing air temperatures, be sure that the coldest temperature used for the DIP/DROP is OK for the plant species in the greenhouse. Cold sensitivity (or, alternatively, cold tolerance) is determined by the base temperature of plants (the temperature at or below which growth ceases). Don’t drop down to air temperatures that could cause cold or chilling injury to the different crops you are growing. Also, although the DIP or DROP may only last two or three hours, it will lower your average daily temperature (ADT). Since the ADT controls the rate of crop development, you will want to bump your daytime air temperature up a bit to compensate for the drop. The magnitude of any effect of DIP/DROP on ADT — as well as the corrective bump to daytime air temperatures — will depend on the low temperature used for and duration of the DIP/DROP.

Using a morning DIP or DROP can be a great way to add some non-chemical growth control to your growing. And, compared to traditional negative DIF approaches, it conserves energy, a welcome choice with rising energy costs. Hopefully, the practices outlined in this article will help you utilize DIP or DROP to control not only plant growth, but also production costs.

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