Sowing Seeds: Absolutely Everything You Need to Know

Spring for many of us is tantalisingly close – just a few short weeks to go! At this time of year thoughts inevitably turn to what we’re going to be sowing and growing over the coming growing season. Plans are being made and the very first seeds are ready to be sown; the excitement is palpable! Let’s start the new growing season as we mean to go on – and it all begins with those precious seeds...

In this video and article we’re going to explore some simple tips for sowing with success under cover. Let’s start the new growing season as we mean to go on – and it all begins with those precious seeds!

Why Sow Under Cover

Starting seedlings off under the protection of a greenhouse, polytunnel or cold frame enables eager gardeners to get a head start on the growing season. Sowing under cover while conditions outside are still less than ideal means you can raise strong, healthy seedlings safe from chilly winds and pests.

Greenhouses and similar structures trap the sun’s warmth and hold onto it for longer, making nights less cold, while significantly raising the temperature on sunny days – effectively bringing the start of the growing season forward by up to a month.

Sowing Seeds Indoors

For the very earliest start, however, you can’t beat the convenience of an indoor windowsill. Sow into pots or smaller seed flats – anything that will fit on your windowsill. Tender crops like peppers and tomatoes may benefit from the added warmth and humidity of a propagator, or mimic these conditions by securing clear plastic over the top of pots with an elastic band.

Trays of cool-season crops – everything from onions to celery to cabbage – don’t need quite so much mollycoddling. You can even stack trays up after sowing to save on space. After two or three days start checking daily for signs of germination then move them out to the greenhouse or cold frame to continue growing. Or you can continue to grow seedlings on indoors, using grow lights to ensure strong, even growth.

Know When to Sow

Sow too early on in the season and you’ll end up having to transplant seedlings into bigger containers more often because conditions outside still aren’t suitable. This is a waste of potting mix and puts pressure on valuable space under cover.

Our Garden Planner can help. Use the Plant List that accompanies your garden plan to judge the perfect time to sow and plant in your location. The blue lines show when sowing can begin under cover, while the green lines indicate the range of dates for planting or sowing outdoors. In the example above, lettuce may be sown from the second half of February, to plant outside a month later, while onions can be sown from mid January. But for quick-growing pumpkins it would be best to wait until the beginning of May in this garden so plants aren’t too big before they can go outside.

Seed Tray vs Module Trays

Seed trays are useful for sowing very tiny seeds like basil, or easy-to-transplant flower seeds, which can then be moved on to their own pots or module trays when big enough to handle. Their compact size makes very efficient use of space during this first stage of growth.

Module trays on the other hand minimise root disturbance and save time, because often seedlings can go straight from the tray to their final growing positions. Module trays come in all shapes and sizes. Those with larger plugs are great for sowing chunkier seeds such as beans, while smaller plugs are just the job for sowing crops like lettuce, radish and onions. Well-made trays of rigid plastic can potentially last for many years, but if you want to avoid plastic look for alternatives made of biodegradable fibre or make your own pots out of newspaper.

How to Sow into a Plug Tray

Use seed compost or a quality all-purpose potting mix. If it’s a bit lumpy, screen or sieve it to give a finer, more even texture suitable for sowing. Fill the trays with the potting mix, pushing it down into the plugs with your fingers so it’s nice and firm, then add a little more of the potting mix. Now make shallow depressions with your fingertips. Most seeds don’t need to be sown particularly deep, with a covering of around a quarter of an inch (0.5cm) about right for most crops.

Drop seeds into the depressions you’ve made. Many vegetables including common crops such as salads, onions, beetroot, peas and radish may be sown in pinches of three to five seeds per plug for planting out as a cluster of seedlings. Larger seeds like beans are sown individually into deeper holes made with a finger, pencil or dibber.

Once you’re done sowing, sieve a little more potting mix over the top. Gently skim over the surface of the tray with your hands to ensure all the seeds are buried. Water trays carefully using a watering can or hosepipe fitted with a fine rose. Go over the trays a couple of times so that the potting mix is completely moistened through. Label trays with the variety and date of sowing.

You can now leave trays as they are or cover them over with a sheet of glass, wood or cardboard, which will slow evaporation and reduce interest from rodents such as mice, which have a habit of digging up and eating bigger seeds.

Check trays and pots regularly for moisture. Lifting them up is a good way to judge how much moisture there is in the potting mix. If it’s light, water. One way to achieve a thorough watering is to pop trays into a reservoir to soak up water through their drainage holes. Remove them once you can see it’s moist at the surface.

You can’t beat the satisfaction that comes with making the first sowings of the season. You know spring’s almost here when the first shoots push through!