Author: Michelle Moore

Garden & Greenhouse Magazine

Even Your Greenhouse Can Have Seasons

It's time to think about saving your plants for fall. If you are like me, you are still in denial that summer is ending, or perhaps where you live the signs have been clear for quite some time. Either way, October is a good time to get serious about staving off the winter months. Your goals may be to winter over some tender perennials or you may want to extend your growing season as long as you can. Perhaps you plan to start and grow cool weather crops, or maybe you want to recreate the tropics. Regardless of your goals, there are ways you can get more out of your greenhouse, you plants, and your enjoyment. Just a few tricks of the trade may be all you need.

Make your greenhouse more productive and efficient by creating chambers within your greenhouse. A greenhouse provides quite a bit of heat just by harnessing the energy of the sun. Many of your plants do well in a greenhouse and don?t need added heat, just the added protection a greenhouse provides. By grouping your plants and containers according to their heating requirements, you can easily configure your winter greenhouse so it works best for you. Heating small sections saves energy and heating costs.

Sometimes just a few degrees can make a world of difference to plants. Warming chambers can range from a simple grouping of like plants in the sunniest spots to creating a mini-greenhouse inside a greenhouse. Grouping plants together requires only a bit of observation and a few minutes to move the plants to the sunny side of the greenhouse. Greenhouse materials that do not diffuse the light (clear coverings such as glass or those you can see through) will have some areas with much more light than others. Watch the light patterns throughout the day and notice where the sun hits. Mark the sunniest areas with string and make sure the tallest plants are placed in the back (along the North side) so they don?t shade the smaller plants. Save the shadier spots along the North wall for the heartier specimens. If you would like to learn more about what your plants need, Shane Smith's book, The Greenhouse Gardener's Companion is an excellent resource. One of the best on-line references can be found on the Oregon State University Master Gardener's website. The plant database provides pictures as well as descriptions and requirements. The list by common name is found at this link: http://oregonstate.edu/dept/ldplants/2plants.htm#hyca

In most of the country insulation is the primary component behind creating a warm zone. If your greenhouse doesn?t have enough insulation or doesn?t get enough sunlight exposure during the day to stay as warm as you would like at night, you will need to add insulation. If your covering is single walled, it would be a good idea to add insulation to the entire structure unless you live in a very moderate climate. Bubble wrap will store solar heat where your single wall greenhouse covering will not. If your covering is already insulated, it may be less expensive and easier to insulate just the containers. If night-time temperatures are warm enough (this will depend on what you grow but a 45° night time temperature is sufficient for a large number of plants), then insulating the container may be all that is needed to give the roots a bit more protection. If you insulate the greenhouse, the translucent bubble wrap will work best and for containers, the darker the color the better. Wrapping the container with a layer of bubble wrap then flat black plastic will absorb the most heat.

If the ambient night time temperature is not warm enough in your greenhouse, you may want to create a mini-greenhouse inside the greenhouse. The easiest type is virtually a tent type structure made from bubble wrap. Bubble wrap allows light into the chamber and the small bubbles trap air, providing and easy to install effective insulation. This small tent type structure alone may be enough for your purposes but that will depend on your zone and the type of greenhouse covering you have. This solution should be used only for the coldest of nights. A small tent would not provide the proper environment long-term but would suffice for a short time.

If you require slightly more heat at night, add simple electric heating if you are equipped with electricity in your greenhouse. A small light bulb may be all that's needed to add a few degrees during a cold night. In some areas of the country a string of Christmas lights may keep the chill at bay. If you have a special container plant you can wrap the string of lights around the ceramic pot to ensure the roots are getting a bit of extra warmth. Seedling heat mats are also an excellent source of bottom heat. If the heating surface is too warm, a thermometer can connect up to four heat mats to deliver the right amount of heat for the type and size container. Grouping sensitive plants is an easy way to concentrate the heat. If you don't have electricity, you can still add extra heat to a small area by adding capped buckets of hot water to a small area at night. A small tent over the area will help retrain the heat. This solution is quite manual, but works well if there are only a few nights where the extra protection is needed.

The options listed above are great for occasional heating requirements and for small spaces. If you have a small commercial size greenhouse, you may want to create a separate room to heat. By constructing a temporary wall you limit the amount of heat output you need. You can temporarily wall off part of your greenhouse to create a propagation chamber. A translucent double wall material works best, however, plywood works as well, providing it's not blocking light to the rest of the greenhouse. If you do use wood, painting the interior surface white will improve the light quality.

A simple hot box can be used to add extra warmth to your plants roots. To create a hot box you will first determine the size you need. Create a very simple frame and plywood bottom. Cover the entire inside with thick plastic to keep water off the wood. Fill the bottom with several inches of sand then run heating cables through the sand in an “s” pattern. The cables should run 2-3 inches apart. Outdoor soil heating cables work well since they have a built in thermometer. If the sand temperature falls below 74° the heating cables turn on automatically. Cover the cables with 2-3 inches more of moist sand and place your containers directly on top of the sand.

If you plan to operate a warming chamber regularly, it's good to consider how you will handle humidity, ventilation and protect against overheating. In a commercial type greenhouse your warming chamber may be large enough to have its own heat and ventilation. If this is the case, you would operate the chamber like you would an independent greenhouse. If your warming chamber is little more than a small tent (less than 8' x 8') you will want to pay careful attention to ventilation. Ventilation will help with both humidity and overheating. Small areas tend to heat up very rapidly, so you will need to have a strategy for expelling excess heat, or you will want to use the chamber only during very cold times. Humidity will be another challenge in the small space. The rest of your greenhouse will be cooler, so as the warm air in the chamber is cooled, excess moisture will appear. This will not be an issue if the chamber is only used temporarily. However, carefully monitor the humidity if you plan to actively grow during cold months. A cool damp greenhouse is the perfect environment for disease. Proper ventilation will remove the warm air before the moisture forms condensation and it will ensure you are not overheating your area.

Whether you need the simple bubble wrap insulation for a few containers, or a complete mini greenhouse chamber inside your larger greenhouse, these ideas will keep you growing through the winter.

Michelle Moore is the general manager of The Greenhouse Catalog. She is an Oregon State University Master Gardener and has nearly 20 years experience working with greenhouses. She lives in Oregon with her husband where they are gardening outside of a greenhouse for the first time. You can contact Michelle at mmoore@solexx.com or you can visit their website at www.greenhousecatalog.com/garden .

This article was originally published in Garden & Greenhouse Magazine. Garden & Greenhouse is written for small commercial growers, hobby greenhouse owners and indoor/outdoor gardeners and is free to qualified readers. For more information visit http://www.GardenAndGreenhouse.net.

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