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Seed Propagation, Sowing & Planting Out


Which crops you decide to grow determines how the seeds are sown. Some can be direct sown where they are to grow, others are sown in a seed bed and then transplanted to their final growing position, others need to be sown indoors in pots of compost with heat to germinate.

Direct sowing- Outdoors

The majority of vegetables are grown by sowing the seed directly into the soil where they are to grow. To do this you make a drill (a shallow trench) ideally with a bamboo cane for straightness or hand trowel will do.  The seed is sown directly into the drill and covered with soil that is lightly firmed down. Always sow evenly and thinly to avoid the job of thinning out the crop afterwards. The soil is then watered using a fine rose watering can.

Some crops, especially those that are in the ground for a long time such as leafy brassicas (cabbage family), are sown in a seed bed to save space. The resulting young plants are then transferred to their final growing position once they have developed a few true leaves and are large enough to handle. These must be carefully lifted with a hand fork, firmly replanted and then thoroughly watered in.

Protected Outdoor Sowing

Most seeds won't germinate below a soil temperature of about 7°C (45°F), so early sowings must be delayed until the soil has at least reached this temperature, this will vary depending on where you live in the country and the exposure of your garden. 

One way to speed this up is to warm the soil by putting cloches or clear plastic sheeting over the soil a few weeks before sowing or planting out. The protection should be kept in place over the seeds and plants for a few weeks or until the weather warms up or the sudden shock of lower temperatures can seriously affect their growth.

Many vegetables, annuals, biennials and herbaceous plants can be grown from seed sown outdoors. The secret to success is to prepare a good seedbed, free of weeds and with a crumble-like soil-surface texture often referred to as a fine tilth.

Indoor sowing

To provide earlier crops or for tender crops that can't cope with early spring temperatures, seed is sown in a greenhouse with heat.  Thermostatically controlled propagators are excellent for this although germinating on a warm windowsill or in a warm conservatory can be achieved too.

Once the seeds have germinated the seedlings will need to be kept at a cooler temperature, but a cold greenhouse will normally be too cold - especially at night - for the seedlings to grow on without a check to their growth.

The seedlings are then pricked out and transplanted individually into small 7.5-9cm (3-3.5in) pots or cell trays for growing on.

[Pricking out - Transferring very young seedlings from the site where they have germinated to other containers or beds, in order to provide more space for growth. Great care should be taken to handle only the seed leaves, to ensure that the roots are mostly intact, and to gently settle them in with a dibber (a pen-sized instrument made of wood or plastic, with a blunt, tapered end), as well as by subsequent watering.]

Before planting outside they will need to be hardened off so that they acclimatise to the temperature. Move the plants to a warm, sheltered position outside on a warm, still day and then bring back in at night. Then leave them outside all day - either in an open cold frame or covered with fleece. This hardening off process should take at least 2 weeks longer if needed.

When to sow seed

As long as the soil is warm and moist, seed can be sown and it will germinate quickly. In practice, this usually means either mid-spring to early summer (April-June), or late summer (September). A good indication of the soil warming up is when the weeds start to grow.

If you can provide the crop with protection, such as cloches or fleece, sowing can begin in early spring. Likewise, regular watering will make it possible to raise rows of seedlings in the height of summer.

Don’t be tempted to sow early - always refer to the seed packet for the best time to sow, as it does vary with plant type.

Succession Sowing

Fast maturing crops should be sown little and often to prevent the glut/famine cycle that occurs when long rows are sown in one go. By sowing shorter rows at 10 to 14 day intervals you will harvest just the amount you need over a much longer period.

How to sow seed

Sowing seed is very easy – this happens all the time naturally by plants self seeding scattering their seeds and growing where they land as soon as it is moist and warm. However, for the best success, this is the best way to sow:

  • Ideally your beds should be dug over in the autumn/winter to add organic material such as well-rotted manure.  
  • Cover bed with plastic or a double layer of fleece to suppress weeds and, in early spring, to help warm up the soil
  • When you are ready to sow, uncover the bed. Use a rake to level the surface and create a fine tilth removing weeds as you go.
  • Mark a line with string or place a cane across the bed and lightly push it into the surface. This will create a straight drill the depth should be as directed on the seed packet along with spacing’s for sowing.
  • Water the row prior to sowing with a fine rose watering, watering afterwards can wash the seeds away and cool down the soil.
  • Thinly scatter the seed into the bottom of the drill.  Don’t be tempted to put extra seeds in as you will only have to thin this out at a later date. A finger width apart is usually right for small seeds.
  • Use a rake or your hand to gently cover the seeds with soil, filling the drill back in again.
  • Place a label in the soil at one end identifying where and what you have sown and the date.
  • Cover the patch with a single layer of fleece if sowing early in the season securing it firming.
  • If there is no rain water twice a week until the seedlings germinate, give enough water to wet the ground to a depth of a few centimetres but don’t drown the area.  After a couple of weeks reduce watering unless  there is a particular dry spell or if you are growing potatoes, salads and broad beans which will need plenty of water.

Hardening Off

Plants raised indoors or in a greenhouse need to be acclimatised to cooler temperatures, lower humidity and increased air movement for about two to three weeks before they are planted outdoors. This ‘toughening up’ process is known as hardening off.

Why harden off plants?

Hardening off allows plants to adapt from being in a protected, stable environment to changeable, harsher outdoor conditions. If suddenly placed outside, the shock can severely check a plant's growth. Although plants usually recover eventually hardening off is thought to be preferable to a sudden shock.

To be on the safe side, do not plant out tender plants before the date of the last frost which is usually late spring in the south of England, later in the north and Scotland.

How to harden plants off

All plants are hardened off in gradual stages.

Plants raised in heated glasshouses and on windowsills should go first into a cold glasshouse if available. When moving plants out of propagator it is best to do so on an overcast dull day as this will help reduce wilting. After two weeks in these cooler conditions, plants should be moved into a well-ventilated cold frame.

If you don't have a greenhouse, move plants into a cold frame, with the lid open slightly during the days of the first week and closed at night. Gradually raise the lid during the next fortnight until removing it entirely prior to planting. A cloche may be used but this does not give as much frost protection as a cold frame.  If you do not have access to these items place plants in a sheltered position in front of a south-facing wall or hedge and cover with two layers of fleece to prevent sun scorch and temperature shock.

For the first week, leave outside during the day, but bring in at night. In the second week reduce to one layer of fleece. Towards the end of the fortnight remove the fleece during the day. If the weather is suitable leave the plants outside at night but ensure they are covered. Towards the end of the third week leave uncovered before planting out.

Planting out

Plant out young plants after a hardening off period making sure the soil and compost is moist at planting time:

  1. Rake the soil level, removing any large clods or stones
  2. Gently loosen plants from their trays by pushing them up from the base. Knock out plants from pots by giving a sharp tap to the bottom with the handle of your trowel. Remember to keep your hand securely over the top of the plant when doing this so as not to drop it on the ground!
  3. Handle plants by leaves or the root ball not the stem.
  4. If you have a lot of planting to do working off a timber board to avoid disturbing and compacting the soil is a good idea.
  5. Plant so the top of the root ball is just below the soil surface
  6. Firm in your plant to secure it in place.
  7. Once planting is completed, water in using a watering can without a rose.
  8. Shallow-rooted plants dry out quickly, so water regularly when they are growing strongly.
  9. Deadhead spent flowers frequently to promote continual flowering.
Meet the Author: Jo Blackwell
Jo  Blackwell

Jo Blackwell is new on the Harrod Horticultural block and has recently taken over her post as Horticultural Advisor and Kitchen Gardener in Stephanie's Kitchen Garden. She caught the gardening bug when she bought her first home 18 years ago.  Her first greenhouse soon followed and she later gained an allotment, where she grows her own organic fruit and vegetables.

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