Tips and Tricks for Growing Tomatoes in Cool Climates

Growing Tomatoes in Cool Climates

Everyone wants to grow tomatoes, right? There is just something about homegrown tomatoes that is so.... delicious! Then why is it so difficult here! People spend more time and money trying to get tomatoes than any other crop, but they still don't always do very well! 

Tomatoes are finicky. They prefer temperatures between 70-85° during the day and 59-68° at night. Temperatures outside of this range can prevent blossoms from setting fruit, and can cause deformities in the fruit. Green tomatoes will not turn red unless they get enough daytime heat. The plant itself can survive in temperatures as low as 33°, but starts to show signs of stress below 50°.

The average high temperature in South-central Alaska in the summer is 65°. The average low temperature is about 49°. Some tomatoes are more adapted to cool weather and will set fruit in lower temperatures. Stupiče, Oregon Spring, Glacier, and Gold Nugget are a few examples of cool-climate tomatoes that can be grown outside in a warm microclimate. I have had great luck growing tomatoes on the south side of our house. 

If you have a greenhouse or tunnel, you might have temperatures too hot when the weather is at its best. Use a thermometer that registers minimum and maximum temperatures so you can make sure you are within the desired range. You may need to increase ventilation during the day and find some way to retain heat for the night.  

The easiest way to increase ventilation is to open it up when it gets too hot inside, and close it when the temperatures start to dip in the late afternoon. Don’t wait until it gets too late or you will have lost too much heat.

If you want to automate your ventilation to make life easier (highly recommended, and less prone to forgetfulness), you can use a fan on a thermostat. If you have a glass or hard plastic greenhouse, you can use a very handy invention called an automatic vent opener. This is a wax cylinder that expands when it gets too hot, opening the window, and contracts when it gets too cold, closing it again. You can buy them online and they last for many years.

There are several ways to store daytime heat for night. Rocks, concrete, solid bricks, soil, or water will absorb heat during the day and release it during the night, helping to moderate the temperature in your greenhouse or tunnel. Put these features somewhere the sun will shine on them directly for the most heat retention. Dark colors will absorb more than light colors.

Some people put a rock or brick wall in their greenhouse, or make a water wall with glass jugs. Other people use large trashcans full of water or even small ponds. You can use a fan to pump warm air through tubes buried beneath your beds, which will both store heat and help moderate it.

In tunnels I have used big jugs of water placed between the plants with great success. Some websites suggest black plastic mulch for warming the soil, but I prefer organic mulch. It adds organic matter to the soil, feeding the soil life, while protecting the soil from drying out.

Build a Tomato Tower

A tomato tower uses the heat of compost to provide that little bit of extra heat that tomatoes need in a cool summer to set fruit and ripen. The compost supplies nutrients to the tomatoes as well as warmth. It is handy to have a compost pile right in the garden and distributing it at the end of the season is also a snap.

To make a tomato ring, find a 4-5-foot section of your garden that is towards the north side where it won’t shade other veggies. Dig a pit approximately 3 feet wide and one-foot deep and bury some high nitrogen material such as chicken manure, food scraps, fish carcasses, etc.

Take a 10-foot section of 4-foot high chicken wire and use stakes to fasten it into a circle over the buried nitrogen material. Fill it 2 feet full with compostable material. Layer manure, food scraps, and coffee grounds with dried leaves. Water it well.

On the outside of the chicken-wire ring, plant tomatoes on the east, south, and west sides, about a 1-2 feet apart. For a 3-foot ring, you can plant 3-4 tomato plants, burying their stems as much as possible. Water them well. As an additional warming measure, you can wrap row cover around the whole tower.

As the plants grow, tie them to the chicken wire ring. Add more compostable materials to the inside of the ring in layers. The compost ring provides warmth, nutrients, and support for the tomatoes. 

At the end of the summer take the chicken wire out of the garden and spread the compost over the garden. Cover with mulch and let it finish decomposing over the winter.