How hot do you keep your greenhouse? After analyzing heating prices, it may be more appropriate to cool it down a bit. Depending on your location and the type of flowers you grow, it can be surprisingly beneficial to keep the temperature turned down during the growing-on stage of flower production. This will affect the time it takes your plants to flower by increasing your production time, but with the costs you can save on heating, it may be worth it.
A Little Background
The base temperature of a plant is the temperature at which a plant stops growing and developing, which varies greatly between plant species. According to the University of Massachusetts’ (UMass) Greenhouse Management Web site, when temperature increases above the base temperature, plants will grow faster and reach their maximum rate of development, called the optimum temperature. When temperatures are raised above the optimum, the rate of plant development will decrease. The Web site states that this varies across species and can cause problems when attempting to grow multiple species in a single environment. Imagine a bell-curve: if temperatures are in either extreme, plants will grow slowly or not grow at all, with the high point of growth in the center of the curve where the optimum temperature lies. As could be assumed, plants from warmer climates have a higher optimum temperature.
These factors must be taken into account when deciding whether your greenhouses can go cool. If temperatures are too low during seed germination, the process will be delayed and uniformity can decrease. The Greenhouse Management Web site recommends growers do not lower heat during this time. It is best to wait until plants have an established root system, probably 2-3 weeks into growing.
Slower but Healthier
During the growing-on stage, favorable results can occur at lower temperatures, yielding many reasons for a grower to choose the cooler route. Vicki Stamback, Bear Creek Farms, Stillwater, Okla., explained that growing cool saves energy and can help to grow a stronger, better flower. “I grow a little slower and cooler, but this helps to make [the flowers] more vibrant and prettier. They’re also healthy, really healthy,” she stated. The amount that production time will be slowed down depends on how cold you make the greenhouse and what types of flowers you are growing. If you’re worried about increasing your production time too much, you can choose to lower the temperature only slightly, perhaps from 60-65° F to 40° F at night.
Stamback says that this change has only increased production time for her by about a week. Lowering to 35° F at night can increase production time by a month or more. Stamback said she doesn’t mind the increased time, stating, “big producers try to hurry up the process, get it in, get it out, get it going, but on some plants, like snapdragons and freesias, [they are worse if you are] rushing along.” At her location, and taking into account the quick temperature changes in colder seasons, growing completely without heat is not an option, yet growing cooler, and thus slower, will allow the plants more time to develop and grow stronger.
The Greenhouse Management Web site states “branching, flower size and flower number often increases when plants are grown cool.” This was evident in a study undertaken by the University of Minnesota, in which “lateral branching, number of flowers and flower size increased for fuchsia ‘Dollar Princess’ when temperature decreased from 77 to 59° F.” However, the same temperature reduction “almost eliminated flowering on New Guinea impatiens — clearly showing that growing cool isn’t for every crop.
In her greenhouse, Stamback grows a variety of flowers, including dahlias, delphinium, freesia, lupine, ranunculas, sweet peas, snapdragons, trachelium and tulips. She has also grown poinsettias cooler than is recommended and has had great results. Stamback knows of other growers who have successfully grown sunflowers cooler, though crop time is increased. Cooler temperatures will actually encourage longer-lasting blooms for the appropriate flowers, such as snapdragons and freesia.
Free From Fuel
Another grower who has experienced positive results from cool-growth greenhouses is Leah Cook, Wild Hare Farm, Piedmont, N.C. Cook explained that winters do not get extremely cold in Piedmont, but temperature does fall “enough that it can limit outdoor production.” The flowers that Cook grows in her cool greenhouses are ranunculus, freesia, delphinium and occasionally tulips and Dutch iris — plants that aren’t slowed down too much by the cooler temperatures. They are “crops that like cool days, cool nights and short days…they really make up the ground with the daytime temperatures,” she said. She uses unheated hoophouses with roll-up sides and greenhouse plastic for insulation. Solar heat naturally warms the houses during the day. To keep the houses from getting too hot, she generally rolls up the sides about six inches during the day to generate air-flow and keep the greenhouses from getting too humid.
Since her greenhouses are completely unheated, Cook needed to carefully select the crops she could grow during winters. “Because these houses only have one layer of plastic on them, the night-time temperature is pretty close to what it’s doing outside.” She uses Reemay, a brand of row cover, to cover up particular crops, such as ranunculus and freesia, if the temperature drops too low and they are ready to flower. There is nothing very technical involved, nor very expensive, yet Cook can grow these crops successfully in a cool environment. No money is spent on fuel and besides initial purchase cost, the hoophouses themselves are quite inexpensive.
When growing cooler, you will need less irrigation. UMass recommends irrigation be adjusted to begin at 10 a.m. and finish by noon, if possible. If plants are watered too early, the growing media will be further cooled. Cold-water sources should be used. Adjust fertilizer use according to the new, lower use of water and continue to check EC and pH levels, the University recommended.
Disease is still a concern when growing cool. Stamback stated that the foliar disease Botytris can be a problem, mainly due to poor air circulation. To prevent this disease, relative humidity levels should be managed through ventilation and heating; close monitoring will also help. Cook also spoke of the importance of good ventilation, which she accomplishes with the roll-up sides. This keeps the inside of the house dry. She has a rotation schedule in place for her houses, so that the same crops are not grown in the same place year after year. This can help break up disease cycles and, in general, contributes to disease prevention. There is also less residual build-up of salts and fertilizers in the houses because of this rotation schedule.
Stamback has noticed that pests are less of a problem when growing cool, as the temperature helps to keep them down. However, for the same reason, biologicals have not been a useful form of protection, in her experience. She thinks this may vary with the type of biological and specific greenhouses, but little research exists in this area. Both beneficial predatory insects and parasite insects are most comfortable when temperature ranges from 65º to 85º F, with humidity ranging from 60 to 90 percent. If night temperatures drop, chances are good that bugs will not be able to flourish in your greenhouse.
While it won’t be a cure-all, nor universally applicable, cooler growing is a viable option for many greenhouses. It’s definitely worth checking whether your crops can grow cooler as even lowering temperatures at night can save you money on fuel and, depending on your varieties, may not slow down production enough to impact sales. This option presents an environmentally friendly way to still produce beautiful, healthy crops. As Cook describes, “we’re not doing anything Herculean with these houses,” but it is enough to make a difference. And often that’s all that you need.