How to: Keeping the greenhouse warm and running during the winter

Updated Nov 12, 2021

snowy greenhouseAs edible gardening continues to become more mainstream, some of your clients may have opted to use a greenhouse to grow their veggies instead of raised beds.

If this is the case, they may be under the impression they won’t be able to grow anything during the winter due to the long, cold nights causing temperatures to drop. However, this sheltered spot for plants doesn’t have to go dormant if they follow these tips from you.

Let the sunshine in

If the contractor was smart about the site and space planning of the greenhouse when it was built, then the structure should already be in a location that is free from shading from trees or other buildings. But when the sun’s trajectory is lower in the sky, other obstacles could create shadows, so make sure the greenhouse is getting as much sunlight as possible.

Add thermal mass

One of the easiest and least expensive options for warming greenhouses in the winter is to create a thermal mass or heat sink. These are objects that absorb heat during the day and release it during the chilly nighttime hours. It will raise the temperature by a degree or two and it can make all the difference.

A popular method of creating a thermal mass is placing containers of water in the greenhouse. For smaller greenhouses, capped one-gallon plastic jugs filled three-quarters full of water can be placed throughout the greenhouse among the plants. The jugs can be painted black or black food coloring can be added to the water to increase heat absorption.

For larger spaces, 55-gallon barrels painted black and filled with water can be placed in areas of direct sun. They will release the absorbed energy over the night as well.

Use a germination mat

If your client is mostly wanting to use their greenhouse over the winter to get a head start on their spring plants, a germination mat can serve their needs nicely. These mats help decrease production time by maintaining optimal root-zone temperature and increase the growth rates in the early stages of a plant’s life cycle.

Cover up

Customers can go as elaborate or as simple as they want when it comes to covering plants during particularly cold nights to provide extra degrees of warmth. Tarps, horticultural fleece, row covers or sheets can all help hold moisture in, but remind clients to keep the covers off the plants themselves and to remember to remove them during the day, as the humidity level can get too high.

Create some compost

For customers who already have a pile of compost, moving it in to the middle of the greenhouse can be a win-win, as it protects the compost from the elements and is warmer. This allows the process to speed up, while raising the temperature of the greenhouse as it breaks materials down. The optimal location for the pile is in the center of the greenhouse, but if this isn’t feasible, it can be placed elsewhere and still provide warmth. Also, come springtime the homeowner will have a pile of black gold readily available.

This alternative heating method may not be ideal if your customer doesn’t know much about composting, or doesn’t like the idea of having a compost pile in the middle of the greenhouse. This option should be skipped if the greenhouse is one that is connected to the house, as they can attract mice and rats in the winter.

Add insulation

Believe it or not, bubble wrap has another purpose other than protecting fragile packages and providing entertainment for the easily amused. Attaching a layer of bubble wrap to the interior walls of the greenhouse can reduce heat loss and block winter drafts. Horticultural bubble wrap insulation can be found at garden centers and is UV stabilized and has larger bubbles, but traditional bubble wrap can suffice in a pinch. 

Install a heater

Probably the most obvious option when it comes to warming a greenhouse is simply installing some heaters, but these can be expensive to run and the heat can quickly dissipate when the heater is turned off. There are space heaters designed specifically for greenhouses, and propane heaters are a good option if your client doesn’t want to have to run extension cords to the greenhouse. It is important to have fans as well to distribute the warm air throughout the space.

If using an electric heater, be sure to check the cords and connections to make sure they aren’t frayed or worn. Ventilation is very important for controlling carbon monoxide levels and to prevent overheating the space, which is rare in the winter but still possible in warmer climates.

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