How to Warm Soil and Protect Seedlings from Frost

, written by Benedict Vanheems gb flag

Using a polystyrene box to insulate seedlings from frost

Spring is without a doubt the most exciting time of year for us gardeners, and now’s the time to get sowing in earnest! But before you so much as tear open a seed packet you’ll need to make sure your soil is warm enough and that late frosts won’t hamper your efforts.

Warming the Soil

After a long, cold winter it can take a while for soil to become warm enough for sowing. Raised beds will warm up more quickly thanks to the free-draining conditions within them, so if you have raised beds, start your first sowings here.

Any soil can be warmed up by covering it over with black plastic, row covers or garden fleece. This technique is particularly useful for heavy or clay soils that retain a lot of moisture. Black plastic works best because dark colours absorb more sunlight, creating warmer conditions beneath. Lay the plastic over the ground at least one week before sowing and soil temperatures will rise by a couple of degrees, making all the difference for early sowings.

Row covers or garden fleece can also be used to help create a warmer environment beneath them. You’ll need to secure any cover firmly into place to stop it from blowing away. Either peg it down at regular intervals – U-shaped pegs are best – or weigh down the edges with rocks or bricks.


Protecting Seedlings

Seedlings and young plants benefit enormously from some initial protection, particularly when tender plants such as tomatoes have just been planted outside, or where nights are still quite cold.

Drape clear plastic or garden fleece over recently sown blocks or rows of young seedlings. Individual plants can be protected with squares of plastic or fleece cut to size, or by using purpose-sold cloches.

Alternatively, make your own from clear plastic drinks bottles. Simply cut a bottle in half using sharp scissors, then place the top half over your plant. Keep the lid off on sunny days, or screw it on when cold weather is forecast. Don’t discard the bottoms – these can be used too, though you might want to cut a hole into the base for ventilation. Keep your bottle cloches from blowing away by pushing them into the soil or by holding them in place with a cane.


Using Water Bottles to Radiate Heat

You can also use bottles to protect young plants by filling them with water then surrounding plants. The water in the bottles absorbs heat during the day then releases it at night, warming the air around your plants. This technique is especially effective within a greenhouse, tunnel or cold frame, where the additional warmth can help tender plants like tomatoes to quickly establish after planting.

You can also fill plastic bottles with hot water on cold nights to protect vulnerable seedlings. Cluster your seedlings into a confined place such as a plant house or cold frame. Now fill gallon-sized bottles with hot water and place these into the cold frame with your seedlings. Leave enough space around the bottle for the heat to escape and to avoid overheating nearby seedlings. Tightly shut the door or lid. The radiated heat from the bottle will lift the temperature inside by a few degrees.


Polystyrene Boxes

Polystyrene boxes, such as those used in fish markets, make excellent seedling containers. The white walls bounce light back into the box, encouraging strong, even growth, while the insulating properties of polystyrene shield your seedlings from extreme temperature fluctuations. On a really cold night you can simply pop the lid on, or lay a sheet of glass or a doubled-up layer of fleece over the top.

Better still, create an instant, portable cold frame by slotting lengths of plastic pipe into the corners of the box. Then simply pull your row cover plastic of fleece over the top to create an easy-on, easy-off roof.

If you have other useful tips, please share them in the comments section below – we’d love to hear them!

Plants Related to this Article

Bugs, Beneficial Insects and Plant Diseases

< All Guides

Garden Planning Apps

If you need help designing your vegetable garden, try our Vegetable Garden Planner.
Garden Planning Apps and Software

Vegetable Garden Pest Warnings

Want to Receive Alerts When Pests are Heading Your Way?

If you've seen any pests or beneficial insects in your garden in the past few days please report them to The Big Bug Hunt and help create a warning system to alert you when bugs are heading your way.

Show Comments


"Thank you "
Patricia on Sunday 10 April 2016
"Clear plastic warms the soil better than black plastic because it warms the black soil under it. At a depth of 2" it can be up to 10 degrees warmer with clear plastic than with black plastic per a study at Penn State University."
Maura on Friday 30 March 2018
"Thanks for sharing Maura."
Ben Vanheems on Tuesday 3 April 2018
"Excellent and novel suggestions! "
Sue Guyant Coughlin on Friday 2 April 2021
"I live in Phoenix and have a very limited area to garden and only shaded areas to plant. "
Phyllis Bella on Sunday 6 November 2022
"Hi Phyllis. You might find some of our other articles on growing in the shade helpful. Just search 'shade' in the search bar above."
Ben Vanheems on Wednesday 9 November 2022

Add a Comment

Add your own thoughts on the subject of this article:
(If you have difficulty using this form, please use our Contact Form to send us your comment, along with the title of this article.)

(We won't display this on the website or use it for marketing)


(Please enter the code above to help prevent spam on this article)

By clicking 'Add Comment' you agree to our Terms and Conditions