Warming Up Soil - How to Start Seeds Earlier in Your Garden

, written by Benedict Vanheems gb flag

Using horticultural fleece to help warm soil in raised beds

Depending on where in the world you are gardening, how the season is progressing and your local microclimate and soil conditions, the second half of winter is an excellent time to start thinking about stealing a march on the growing season. Previously I talked about how to protect crops from the cold. In this article I'm going to delve a little deeper into how to warm the soil up in the first place.

Many hardy types can be sown as soon as the soil is consistently at or above 7°C (44.5°F). This includes the likes of radish, spinach, peas, early carrots, most salad leaves including lettuce, and beetroot. Getting these first-past-the-post staples into the ground as soon as possible means they're out of the ground in a correspondingly quick time, allowing for repeat sowings or freeing up space for summer-planted veg like leeks and kale.

Warming soil in preparation for planting using organic matter

Soil Moisture vs Drainage

Water stores heat in much the same way as a night storage heater does because, just like the breezeblocks in a storage heater, water has a good degree of thermal mass. Translated to ground level this essentially means that soil that's moist as opposed to dust dry will hold onto daytime warmth and carry it through into the night. But, of course, it's never that simple! Soil that has frozen hard courtesy of its moisture content will then take longer to warm up in the morning. Soil moisture effectively acts a lag, slowing down the rate at which it responds to air temperature.

The best soils, therefore, are not those that are overly wet or saturated (which also serves to rot your seeds) but those that are well-drained while hanging on to at least a reasonable level of soil moisture. I'm sure you don't need me to point out the obvious but I will for the sake of completeness: to smooth out peaks and troughs of soil moisture it's important to add plenty of organic matter. Not only does organic matter improve the drainage of heavy, clayey soils by opening up the soil structure, it helps sandy soils to hang onto moisture for longer, meaning less watering in the summer.

Organic matter is usually darker that the soil itself. Basic physics dictates that dark colours absorb more heat, so organic matter is good news for those trying to get a few weeks ahead. Furthermore, as the organic matter continues to decompose it releases heat. Of course, we're not talking lots of heat – but at this time of year every little helps.

Mulching soil to help it warm up

Soils that are raised up – in a bed or ridge and furrow-type system – will also drain better to warm up quicker in spring. Align beds so that they are slightly angled towards the midday sun and you'll have a double whammy of drainage teamed up with maximum solar gain.

Using Plastic to Warm Garden Beds

You may heap on the organic matter and raise your soil into elevated beds but the only sure-fire way of warming up the soil in late winter is to cover it with plastic to help it dry out a little – invaluable if your soil is particularly wet after winter rains and snow. This is certainly the case on my allotment, which like most parts of the UK has endured a particularly wet winter with plenty more on the way!

It's hard to make a call as to whether black or clear plastic is best. The former absorbs plenty of heat, while the latter allows the sun's rays to pass through, acting as a ground-level greenhouse. I've tried both types of plastic and haven't noticed any difference between the two, so I'd say use whatever you have to hand. What I would recommend is that you pin or weigh down the sheets securely and at regular intervals around the perimeter to stop it flapping about and getting airborne in windy conditions.

Warming up soil with polythene sheeting

Plastic retains the heat of the day better at night because it is relatively thick and impermeable. That said, if you have reams of horticultural fleece then this will also work wonders to warm the soil, just not quite as effectively. Fleece will let the rain through, which isn't ideal if you are trying to dry out your soil in preparation for sowing. If you do use it, double up the layers to retain the day's heat longer into the night and offer extra warming potential. Lay your plastic sheeting or fleece at least two weeks before sowing so that the warmth can penetrate deeper into the soil rather than sit at the surface.

Get a Head Start on Your Weeding

One happy side effect of forcing the soil to warm up like this is that it will stimulate weed seeds to germinate. After a two-week period of covering your earthy goodness you'll have a rash of tiny seedlings that can be hoed off to give a ‘stale' seedbed that's ready for sowing and ahead of the competition.

When you come to sowing or planting out greenhouse-raised seedlings you'll want to keep the hard-won soil warmth locked in until the weather catches up. Cover your youngsters with fleece or row cover tunnels of your choice to help them establish.

By Benedict Vanheems.

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Show Comments


"Soil Moisture is useful for plants.Soil Moisture is very beneficial for any season Winter or summer for green plants. It also reducing carbon dioxide emission and enhancing biodiversity. I like this ecofriendly idea."
Steemoberoy on Tuesday 11 February 2014
"When does the soil warm in North Central Florida, Lake City enough to plant seeds. OR what temperature should the ground garden soil be for planting seeds..."
Barbara on Thursday 11 February 2016
"Hi Barbara. The right time to start seeds in your area depends on which seeds you wish to grow and on your exact location. Our Garden Planner uses climate data from your nearest weather station to recommend ideal sowing, planting and harvesting times for your location (we find this to be more accurate than the more general 'hardiness zone' system). Click on Planner in the menu bar at the top of this page to find out more and to take out a free trial."
GrowVeg Customer Support on Thursday 11 February 2016
"I live in Western Canada where spring comes late but the sun is strong. I will go with the black sheets as I have raised beds and sometimes the soil dries out pretty quick if it is sunny and the wind blows. But the soil stays cold. I have had a terrible time with germination lately. "
Peter Faulkner on Tuesday 3 January 2017
"I live in Western Canada where spring comes late but the sun in March and early April is pretty strong. I will go with the black sheets early and seed after a couple weeks. Soil tends to dry out first because with that sun then the wind, its moisture content is gone quickly especially with the raised beds. But it stays cold. I have had hard time with germination. "
Peter Faulkner on Tuesday 3 January 2017
"Hi Peter. It may be worth starting some seedlings off under cover, in module trays or pots, then hardening them off to plant out once the soil has properly warmed up. You could also make extensive use of covers and cloches. But once the soil does warm up things will quickly catch up with the longer day length versus most of the United States, and the more temperate climate."
Ben Vanheems on Thursday 5 January 2017
"Hi Ben, This query is from the subtropics (Brisbane, Queensland)during a heatwave ( a couple of weeks or more of 30C plus. Bought some lettuce and silver beet seedlings for my new raised bed. Top layer is compost to plant in, but I think lower layers (straw, rotted manure, some soil, rinsed seaweed etc.) are still composting because despite mulching the soil is very warm, high 30's probably. The seedlings are struggling a bit despite careful covering with layers of white shade cloth above and sides. So, please, what is too hot in soils?!"
Marjorie Edwards on Friday 3 February 2017
"Hi Marjorie. Your soil may be too warm if the lower layers are still rotting down and generating heat. Lettuce is certainly a cool season crop and doesn't really thrive in very warm temperatures, so that could explain why it isn't thriving. I'd imagine the same is true of the silver beet. It may be worth waiting until the temperatures cool down a little and then you can plant autumn crops which prefer shortening day lengths. Of course, warmth loving crops such as cucumbers, tomatoes, peppers etc. will do well, but as it is already late summer I'm not sure you'd have enough time for a good crop."
Ben Vanheems on Friday 3 February 2017
"Regarding the black plastic, what are your thoughts about planting "into" the plastic so that it remains around the plant starts to continue with warming? Of course the opening around any given plant would need to be wide enough to allow proper watering... but I'm thinking about trying this for some of my hot pepper plants. Thoughts?"
Vertical Gardener on Thursday 28 February 2019
"Black plastic can be used as a mulch - known as a sheet mulch - around robust plants grown in cutouts as you suggest. Black plastic would help to warm the soil throughout the time they are on the ground, which would be great for pepper plants growing in more temperate regions, but perhaps not as good where summers reliably get very hot. I have also seen white plastic used as a mulch to reflect light back up into the foliage early on in the season. My inclination would be to remove the plastic and lay a normal organic mulch of something like compost. I fear that plastic may harbour slugs. But I'd be interested to know if anyone else has used black plastic successfully as a summer-long mulch."
Ben Vanheems on Thursday 28 February 2019
"For the last two years I have been growing melons in my polytunnel. I have two raised beds heated from the bottom via heating elements. First year was great, “mmmm we will grow more next year” I said, then second year (1999) was a disaster, all three types withered and died before fruiting ?? So, my question is...what fruit or exotic vegetables would benefit from the heated raised beds? Grateful for any suggestions you may have. Thanks"
TNorris on Saturday 1 February 2020
"Where can I buy dwarf sweet pea flower seeds"
SAlly kKolar on Tuesday 10 March 2020
"Hi TNorris. Melons should thrive from a warm bed, as well as any heat-loving crop such as aubergine/egg plant and okra. Perhaps the melons died for a reason that could be avoided this time round? It's certainly worth experimenting with heat-loving crops, though I'd make sure to switch the heat off once the warmer weather returns."
Ben Vanheems on Wednesday 11 March 2020
"Hi Sally. Dwarf sweet pea flower seeds are widely available, at least here in the UK. A simple web search for them returned lots of seed companies selling them. Let me know which country you are in and I can take a look for you if you like."
Ben Vanheems on Wednesday 11 March 2020
"About plastic to reduce rain moisture. I might try it myself, but first put a 2x2 across the middle of the raised bed, and staple the plastic over it, which would allow the rain to drain off rather than down the insides of the wood frame. Does that seem reasonable? I'm in 8b in Western Washington, so lots of rain to deal with."
Robert Sanford on Friday 28 January 2022

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