Have you ever jumped into a car on a chilly morning, shivering while the heater blasts cold air right through you? You know it’s only a matter of time until the engine heats up turning the arctic blast into warm air, yet it seems like an eternity. Now, fast forward to the invention of heated seats. Gone are the teeth chattering minutes. Instead, radiant heat rises from the seat and virtually hugs you with warmth in just moments. You can employ the same principal to heating your greenhouse with bottom heat. For the rest of the article, bottom heat refers to warming the soil and plant’s roots from the underside rather than warming one’s backside. Either way, what is pleasant for people is often good for plants too.
Commercial growers have long employed the use of bottom heat to effectively and inexpensively heat their greenhouses. Growers more closely control growing conditions with bottom heat, saving on heating costs by directing heat energy directly to the soil and plant’s roots rather than heating ambient air. Adding heat to the root zone is a proven method for increasing growth, and may be the only way to root cuttings from some species.
Heating a large greenhouse environment without bottom heat would be cost prohibitive in many cases. Many seeds require minimum temperatures of 70 degrees for germination. Many cuttings benefit from temperatures of 70-90 degrees. Rather than heating an entire greenhouse, growers use various systems to deliver heat to the plants roots. As the heat rises, the rest of the plant receives some heating benefit as well. Additionally, most greenhouse plants are grown on benches since the floor is the coolest place in the greenhouse. Elevating plants is a simple way to ensure they receive more heat, or at least less cold.
Most plants in greenhouses are planted in containers. Some containers are for propagation purposes. Others are decorative containers housing plants waiting out the winter in the protection of a greenhouse. In nature, soil surrounds plants giving them added protection from the elements. Soil collects heat from the sun during the day and slowly transfers that heat to plants roots. Container plants have limited space for root growth. Often roots are touching or nearly touching container walls, making them much more susceptible to cold. Placing containers in a greenhouse gives them quite a bit of protection from the elements, but in some climates, additional heat may be necessary to protect the root zone of sensitive plants.
Adding heat to container bottoms warms the soil which in turn retains heat longer than air. As the heat rises in the soil it warms the plants roots, stimulating growth. In a cold frame or cool greenhouse, where air temperature are significantly cooler than the root zone (not cooler than 40 degrees), healthy compact growth occurs. The top of the plant will generally respond to the cooler temperatures by limiting foliage growth, while the stimulated roots grow vigorously. This is especially true for rooted cuttings.
Bottom heat is most commonly used for seed starting. Using bottom heat gives gardeners a significant jump on the season. Some seeds may be started as much as six to eight weeks early with a much higher success rate than without the additional heat. Many seeds are prone to damping off and botrytis, both of which are much less likely to occur in a warm environment. Giving seeds a head start means when the danger of frost is over, you have strong plants more likely to survive early spring weather and reach maturity faster. In the Pacific Northwest, for example, it is possible to harvest ripe tomatoes in May from seeds started in January or February using bottom heat. This early harvest date is significant, especially given the difficulty ripening late season crops.
Propagation is also more successful with the use of bottom heat. Some conifers that will not root without bottom heat may form roots in just a few days when heated. Many other crops show significant rooting results with bottom heat versus without. Plant’s rooting requirements vary, but few plants are harmed by the use of bottom heat and most plants benefit from it.
A third use of bottom heat is supplemental heat for dormant plants in a cool greenhouse. Containers do not provide as much protection as soil; they may need a little extra protection in cold climates. If you are wintering over tender perennials, then you will want to ensure the roots do not freeze and are not damaged by cold. Again, different plants react differently to cold. In many cases heat is not needed, but in the colder climates, a slight amount of bottom heat may be all that’s necessary to maintain the plant or to keep it from freezing. Other tender perennials will maintain their leaves or even continue to grow as long as the temperature does not dip below 40 degrees. The goal is to keep plants alive at this stage but not actively growing. Even the slightest amount of heat may accomplish this goal. Quite a few gardeners succeed by wrapping Christmas lights around specific plants to provide a small amount of consistent warmth.
Lastly, bottom up heat may be the only option for some greenhouses. Greenhouse structures built without bottom vents are difficult to heat. Heated greenhouses need a constant supply of cool fresh air, sucked in from the bottom of the greenhouse. This fresh air will keep the greenhouse from overheating, supply fresh air, and help create circulation. Because there is no risk of overheating the greenhouse with a heat mat, bottom heat is a safe alternative heat source.
There are several ways to add bottom heat to your greenhouse. The easiest method for applying bottom heat is from a purchased seedling heat mat. Seedling heat mats come in a variety of sizes that will hold between one and four flats of seeds. The design you select will depend on your requirements, budget and your plan. Up to four seedling heat mats are controlled by one thermostat. This is particularly useful if you want to precisely control the temperature, or want to make sure the mat does not produce too much heat for your purpose. The water proof mats are safe in a wet greenhouse environment and require little or no maintenance. Storage in spring and summer months requires little space.
Soil cables are another effective means of adding bottom heat. Quite a few plans for building hot boxes are available on the internet, but all use the same basic premise. Heating cables are spread over moist sand at the prescribed intervals (see manufacturer’s instructions for specific brands of cables). Moist sand is then layered over the top of the cable. Plants in their containers are placed directly on the sand. Most cables have an automated thermostat that will come on as the temperature falls below 70 degrees. This method requires more set up than heat mats and does not provide the same level of control; however, it can be an inexpensive solution.
Once you decide on your heating method, if you do not already have them, build or purchase benches to get your plants off the ground into the zone of warmer air. The heated area should be placed on a level surface. If the mats are not placed on a raised bench, it is a good idea to put them on an insulated surface like Styrofoam. This added insulation results in better temperate control and minimal heat loss.
When you are using bottom heat, you will want to cut back your winter watering. Excess water creates unwanted humidity in the cool air. Don’t stop watering all together though. Bottom heat will dry containers out quickly, so you will need to keep an eye on them. Plants stored in a completely dry container are also more susceptible to cold damage because some moisture actually reduces the chance of freezing.
When you determine the soil temperature you want to maintain, set your thermostat to roughly five degrees warmer. It is a good idea purchase a small soil thermometer so you can monitor individual containers. A low-tech instant read thermometer with about a four inch shaft is all you will need. Lastly, make sure the plants you bring into your greenhouse are pest free. Insects that normally die in the cold weather will live happily in your greenhouse if given a chance. The best way to avoid problems is to only put the healthiest plants inside. As an extra security measure, if you have special plants, it is a good idea to take a few cuttings as insurance.
With any method you choose, follow these few easy steps and you will not only enjoy your greenhouse; you’ll have spring in your life-even in the dead of winter.
Michelle Moore is the General Manager of the Greenhouse Catalog where she has worked with greenhouses for 20 years. Michelle writes and lectures about greenhouse gardening as an Oregon State University Master Gardener and member of Garden Writers Association. Michelle is a Fulbright Scholar and earned an International MBA from Thunderbird, School of Global Management.
