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Covered sowing, why and how to protect seedlings in spring

Covered sowing

Covered sowing is for all the seedlings prepared before April. Indeed, it would be too early to sow outdoors and your plants would risk freezing…

so sowing under cover is the key!

Types of  cover include greenhouses, sheets of plastic, panes of glass, cold frames, garden cloches…

Technique for sowing under shelter

  • Choose a spot in the garden where the temperature is warmest. To clarify, this is usually where soil gets noon-to-afternoon sun. Bulb flowers bloom there first in spring.
  • Build a shelter in place, see below. It needn’t be very tall, ankle or knee-height is fine.
  • Prepare the soil. For this purpose, seedling soil mix with perlite added in will enhance drainage and air circulation. It’s one of the ways to prepare the soil against damping of seedlings.
  • After that, sow your seeds using the tray-sowing method.

A simple thermometer in the shaded corner will tell you if you need to insulate more at night or open and ventilate on hot days.

What type of cover is best?

The goal is to keep temperatures warm inside the covered area, while letting as much light in as can be.

Greenhouse-type shelters

These use the greenhouse effect to lock heat in from the sun’s rays.

  • Beds prepared to cover seedlings and sowingSides can be either glass, plastic, wood… Even hollow blocks or bricks are great.
  • Evidently, top must be see-through. Glass or plastic are perfect. Some types of roofing composites also let light through, such as that used for lean-ins or winter gardens.
  • An old window in its frame is a good option. It’s safe to assemble and attach to the sides.

Clear or translucent plastic boxes are also excellent. Their lid is already the perfect size and they’re lightweight and easy to move around.

Try to slant the top of the greenhouse so that the surface faces towards the midday sun. This maximizes light intake.

  • Of course, there are a number of full-size greenhouse designs that can serve the same purpose as this covered sowing.

Insulation for cold nights

Along the sides, insulation really helps keep heat in.

Even though it’s a hassle, on cold nights, keep a slab of insulation to cover the mini-greenhouse over. Pull it off in the morning. A tarpaulin, for instance, can do the trick, especially if it’s lined with bubble wrap.

Easy to open and close

The shelter should be easy to open and close. Indeed, on very warm days, temperatures might rise too much. Seedlings might get roasted! In that case, simply opening the shelter a crack can let excess heat float away.

How long to protected plants with covers?

Sowing and preparing a covered sowing bed.Sowing under cover lets plants start about a month before they would naturally if sown outdoors in the plot.

By the time non-covered seeds sprout naturally, it’s ok to open the cover all the time and start moving your seedlings out. At this time, here are a few notes on how to transplant them.

Smart tip about covered sowing

Covered seedlings, to grow perfectly healthy, need lots of light! If you can add supplemental lighting, give them 12 hours a day.


Images: 123RF: blessingscaptured, CC BY 2.0: Linda N., woodleywonderworks
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