From the Field-Agronomy Notes: Vol. 2, Num. 2

— Written By

February 16th, 2017-

With Valentine’s Day now behind us, many producers are ramping up to seed greenhouses. Many started last week thanks to some really nice weather, but generally speaking February 14th tends to ring in the official start of another tobacco season. As we begin the greenhouse season there are some things that should have already been addressed (sanitation, weed removal, tray cleaning, etc.). It is now time to think about getting seeds in a tray and properly managing temperature in a way that grows plants as fast and as cost effective as possible. So, how do we do that???

First, the general recommendation is that the internal temperature of a tobacco greenhouse should range from a low of 68°F at night to a daytime high of 86°F during the germination phase (Figure 1). Most importantly, it’s extremely critical to remember that temperature fluctuation is required for seed germination. Tobacco is an extremely small seeded plant, which is why they are grown as transplants and why seed are coated. Due to the small seed size it lacks the required energy reserves needed to 1.) germinate and then 2.) push upward through the soil profile. This is why tobacco seed are not covered or buried by soilless media after seeding. When greenhouse temperatures do not fluctuate, a tobacco seed is tricked into “thinking” that it is buried too deep in the ground to germinate and reach the soil surface. Crazy concept? You bet, but it’s pure seed physiology and something that can have a huge impact on germination, uniform growth, and plant usability.

Figure 1. Germination response of two flue-cured tobacco varieties (K 326 and NC 71) to differing temperature management schemes.

Second, once maximum plant stand has been achieved (usually 9-12 days after seeding), the minimum greenhouse temperature can be lowered to around 55°F. The primary benefit from this reduction is that fuel use decreases by 15% for every five-degree reduction in temperature. Fuel is fairly cheap right now, but it is one more way to save money. Typically, as we begin to discuss the concept of lowering greenhouse temperature, many growers become concerned about cold injury to young seedlings (Figure 2). It’s important to realize that cold injury is not simply a result of exposure to cold or cooler temperature, but rather the exposure of plants to a rapid decline in temperature. Cold injury, or really cold shock, is usually observed during periods of warm, sunny days (where greenhouse curtains must be lowered) that are matched with cold nights. It is during these events that greenhouse temperatures might reach 80-95 degrees during the day and curtains lower. As sunset approaches, the ambient air temperature outside of the greenhouse falls very rapidly (by 30+ degrees in as little as 45 minutes), and the greenhouse system cannot respond fast enough to appropriately heat the house. These are the normal conditions that result in cold injury; however, cool spots in greenhouses are not uncommon and can be addressed with better air circulation. Regardless of how cold shock occurred, exposure to warm growing conditions over a short period will correct the issue.

Figure 2. Cold injury, or shock, to young tobacco seedlings. Note that plants will recover from this after exposure to warm temperatures for a few days.

Lastly, it is better to maintain greenhouse temperatures that are on the slightly cool side than the warm side. During germination, a 95 degree daytime temperature can delay germination by one to two days. While this may not seem like a large difference, it ultimately delays plant growth and can make seedlings more susceptible to salt injury and disease. Once germination occurs, temperatures above 100 degrees in the greenhouse can severely injure (Figure 3) or even kill transplants (Figures 4a-b). Temperatures this high are usually a result of a failure to lower greenhouse curtains, either manually or mechanically, once temperatures begin to approach the upper 70’s or low 80’s. Excessive heat can completely destroy an entire greenhouse in a very short period of time and, should plants recover, sucker growth is likely to become problematic due to the death of the plant bud (Figure 4b).

Figure 3. Severe heat injury of tobacco transplants (photo provided by Wayne Batten, retired NCCES)

Figure 4a (left): Extreme heat injury after curtains failed to lower. Note that plans along the curtains are not injured, thus indicating that temperatures were moderated along the greenhouse wall. 4b (right): Seedling recovery from heat injury in the same house, green tissue growth is from suckers. All plants were unusable in this greenhouse.

More information specific to transplant production and temperature management can be found in Chapter 4 of the 2017 Tobacco Production Guide, so be sure to give it look. You should also give your local Cooperative Extension Agent a call if you have other questions or production issues.

Be sure to stop by next week as we begin to address greenhouse fertility programs for conventional and organic tobacco producers!!