Indoor gardening is all about control—control over photoperiod, control over temperature, control over plant nutrition, etc. By controlling everything from the photoperiod to the specific nutrition a plant receives, we effectively remove all barriers that could hinder our plants. Optimally, that control will allow them to reach their maximum genetic potential.

An often overlooked environmental factor that can greatly impact your plants is the DIF, or the day/night differential. DIF is the difference in the highest daytime (lights on) temperature and the lowest nighttime (lights off) temperature. Control over your DIF will give you control over your plant’s height and internodal spacing without the use of dangerous or untested chemicals or growth regulators.

¬Research about DIF is not new to science. Back in 1944, Went made detailed observations about the effect of the nighttime temperature (Tn) on the stem growth rates of tomato plants. He originally proposed the term “thermoperiodicity” to describe the apparently greater rate of plant growth and development in diurnally fluctuating temperatures compared to plants grown at constant temperatures. Although his research was disproven in 1990 by Ellis et al, Went’s research was the beginning of our attempts to understand the impact of temperature on plant growth.

In 1983, while studying the effects of temperature on the Easter lily (Lilium longiflorum), it was observed that there was an interaction between day and night temperatures that affected the length of the floral stem. This relationship, coined DIF (Erwin et al, 1989), was used to describe the elongation of the stem in response to diurnal thermoperiod and photoperiod interaction. They noted that the magnitude and nature of the internodal elongation varied between different species and also between different cultivars of the same species.

Plant height or stem length is simply the sum of the lengths of each of the internodes. Therefore, to control plant height one must manage internode number, internode length or both. The number of nodes and the length of each internode (the distance from one node to the next) are strongly influenced by temperature. As DIF increases, so does the internode length of most plants. It is important to understand that the effect of DIF on internode length is due to increased cell elongation and not an increased number of individual plant cells. Plants respond rapidly to changes in DIF, with altered growth rates that are often observable in as little as 24 hours.

Although managing your garden’s DIF will afford you some control over your plant’s internodal elongation, there are factors that influence or compound the DIF response. The average daily temperature influences internode length and thus the response to DIF in many plants. The quality of the light being received by your plants has been shown to influence the DIF response, presumably through effects related to phytochrome photoequalibria (Moe and Heins, 1990). While incandescent lighting used for photoperiod control can eliminate a plant’s response to DIF, fluorescent lighting has been shown to increase the response (Moe et al, 1991).

With the proven effects of DIF at controlling plant height, how do you exploit this information to grow a better garden? First, daytime and nighttime temperatures must be controlled independently and excess humidity must be removed from the air by using dehumidifiers. Watch for significant increases in your DIF; a large swing between your daytime and nighttime temperature will bring a marked increase in humidity. If the high nighttime humidity level is left unchecked, it can lead to mold and disease on your fruits and flowers.

During the vegetative light cycle (18 hours on, six hours off), your target DIF should be five degrees Fahrenheit. Try to maintain a daytime or “lights on” temperature of 85°F, and 80°F when the lights are off. For the blooming or fruiting phase of your plant’s life cycle (12 hours on, 12 hours off), your target DIF should still be 5 degrees Fahrenheit; however, the daytime maximum temperature should be limited to 80°F and your nighttime temperature should be 75°F.

By maintaining the DIF at 5 degrees, your plants will exhibit the tightest internodal growth, lowering the overall size of your plants while building a tight network of branches. Remember that the temperature and DIF recommendations above are starting points as different species and cultivars (or clones) will react differently to a controlled DIF. Still, controlling your DIF could make all the difference to your garden!