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

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 beets. 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.

Soil Moisture vs Drainage

Water stores heat in much the same way as a night storage heater does because, just like the cinder blocks 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 colors 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.

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 (community garden), 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.

Plastic retains the heat of the day better at night because it is relatively thick and impermeable. That said, if you have reams of garden 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.