Rx for spring: pre-warmed soil

I don’t know about you, but after a long winter I’m raring to go in my garden. Since I’m a northern gardener, I don’t have the much time to waste. My growing season is short, so I need to get to it as soon as I can!

Gardeners are a tricky bunch. We’ve found ways to extend the growing season.
“One of the easiest ways to get the garden started earlier is by prewarming the soil,” says Kyla Dixon-Muir at the 2015 Toronto Master Gardeners’ Technical Update.

Kyla Dixon-Muir

It’s an intriguing thought. I haven’t used season extenders in the past because I like to work with what nature provides every year. But with this extra long snowy winter, I’m tempted to try it this year in my vegetable patch.


I also garden on clay soil, which is a double whammy. Clay soil takes ages to dry up and warm up. Far longer than sandy soil. Plus if I start gardening when the clay soil hasn’t dried out, I run the risk of destroying the soil structure.


After Kyla’s presentation at the Master Gardeners’ technical update, I did a bit of digging in the library, and on the Net. There are many ways to extend the gardening season in spring! Here are few examples:


  • Raised beds drain and dry faster than flat ground.


  • Stones warm up soil around it. Rocks gather and store heat during the day. At night, the rocks release the heat. Moderating soil temperatures also encourages better root growth.


  • Planting south of a rock or building wall generally speeds up plants up by as much as one month.


  • Top-dressing garden beds with dark organic matter, like compost and well-rotted manure. Just like dark shirts warm up faster than light ones in the sun, the dark mulch also warms up the soil.


  • Another quick and easy way to pre-warm soil is to cover the soil with plastic. Using a plastic mulch means you can plant two/three weeks before your neighbor.


  • Building cold frames, which provides protection from the wind and concentrate the sun’s warmth under glass.


Pre-warmed soil with plastic

I’m attracted to using the plastic cover idea because I can use it this spring. I don’t to have to time travel to last fall and build a raised bed or a cold frame. It also means I can start before the snow has completely melted.


Six weeks before my first planting date, I can cover the vegetable bed with clear plastic right over the snow. Ontario Ministry of Agriculture, Food and Rural Affairs (OMAFRA) has a handy guide on the planting dates for vegetables in Ontario.

In my neck the woods, I can plant asparagus, broccoli, Brussels sprouts, cabbages, lettuce, onions (set, seed, Spanish), radishes, rhubarb, peas, spinach, parsnips and early potatoes between April 15 – 25.


The next set of early crops include beets, carrots, cauliflower, celery, late potatoes and early sweet corn, which OMAFRA suggests starting between April 25 and May 10.


To get an April 25 start on my early vegetables (because face it, I have 3 ft snow drifts in my backyard), I need to place my plastic cover on the vegetable beds on March 14.


Specs? A 4 or 6 mil thick plastic is recommended, as it will stand the wear and tear of repeated use in the garden.


“Clear plastic can raise the soil temperature up to 15 degrees Fahrenheit,” says Kyla Dixon-Muir. Black plastic raises the soil temp 5 degrees Fahrenheit. Floating row cover, a light-colored lightweight fabric, warms up the soil 1 to 2 degrees Fahrenheit.


Clear plastic solarizes (warms the soil) by keeping the soil dry during the rainy early spring months, by magnifying the sun’s effect, and sheltering the soil from cold winds.


I’ve an extra wrinkle to deal because clay soil is so full of water. I need to raise the clear plastic off the ground a little to allow the soil to dry out. Once the clay soil has dried out, it starts to warm up.



I’ll need to check to see if the soil temperatures have warmed up to at least 40 degrees Fahrenheit (5 degrees Celsius), then I can plant my early crops. Hopefully by April 25! Once the seeds germinate under the plastic, I’ll need to remove the plastic cover, or at least make a hoop structure with it.



There’s always something new to try in gardening. I’ll let you know how it goes!

Caution: Don’t get lazy/efficient and leave the plastic cover on all winter. Soil web of beneficial organisms will be killed off. The lost web of beneficial organisms will take a long time to build up again.



Dixon-Muir, Kyla. Growing to Eat through all four seasons. Cloches, coldframes, hot beds, raised rows and more. Self published. Email Kyla to buy a copy.

OMAFRA Climate Zones and Planting Dates for Vegetables in Ontario
Rodale Publications. Garden Wisdom & Know-How. Everything you need to know to plant, grow, and harvest. Rodale Gardening books. 2010


Thorness, Bill. Cool Season Gardener. Extend the harvest, plan ahead, and grow vegetables year round. Skipstone. 2013




Written by Cristina da Silva
Thursday, March 5, 2015 in Plants & Soils

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  1. Many thanks for the posting, Cristina!

    I’m pleased to announce the release of my new DVDs on this subject.

    There will also be a tour of our ColdFrames In Action at the community garden, this Saturday 18 April 2015, 2-4pm. See mudpies.ca or riverdalemeadow.ca for more details.

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