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Tip of the month: March

Become a windowsill propagator

Raising your own plants on a well-lit and warm windowsill is a great way to get a head start if you don’t have a greenhouse. In fact, that’s how I first got interested in raising my own plants from scratch. The immediate thing you discover when you try windowsill propagation for the first time, is that getting seed to germinate is the easy bit. Finding sufficient space to keep the resulting seedlings and young plants healthy and growing well is far more challenging.

Tools for the job


How to make the most of your windowsill propagator

  • Sow cautiously
  • Choose new and unusual varieties
  • Invest in a propagator
  • Use modular trays and mini-seedtrays
  • Add shelves or buy a coldframe
  • Maintain the right temperature
  • Make a seedling reflector box

It is essential that delicate seedlings and young plants do not receive a check in growth or become weak and leggy. This is usually as a result of there being no room to prick them out, or because they remain overcrowded once you have. That’s why I always recommend that you use the growing-on ‘bottleneck’ as your starting point. Calculate how many seedlings and growing plants you can accommodate and then sow sufficient seeds to grow that number of plants - plus a few extras for luck! Of course, you don’t have to restrict yourself to one windowsill to grow-on the seedling and plants, and you can double or triple the area of growing space in a single propagation window by putting up shelves. Another great way to maximise your growing-on space is to invest in a coldframe. Then you can move established young plants into the coldframe to become acclimatised before planting out.

The most efficient windowsill propagators (that's the people, not the plastic boxes!) will factor in planting dates, starting with hardy vegetables that can be planted outside as soon as they are large enough. Try crops, such as broad beans, Brussels sprouts, calabrese, cauliflower, lettuce, early peas and turnips, that can be sown now and planted in the garden in six to eight weeks provided weather and soil conditions allow. With tender vegetables, such as courgettes, French beans, runner beans and sweetcorn, as well as many types of tender bedding, you’ll have to make sure you don’t sow them too early, because they cannot be planted out until after the last frost. Final frost dates vary from area to area and from year to year, but as a rule of thumb you can set out tender plants no earlier than the second week of May in the south, waiting until the second week of June in the far north. With this in mind, you can work backwards to determine the most suitable sowing date for tender plants in your area.

Unless you just want a small number of plants, you are unlikely to be able to raise sufficient for your needs – even with an efficiently run propagation windowsill. That's why it is a good idea to concentrate on newer and more unusual varieties that are difficult to buy as plants, as well as those that are expensive, such as tomatoes and pelargoniums. The main drawback of raising such long-season tender plants on your windowsill, is that they occupy valuable propagation space from February until they can be planted outside safely after the last frost.

Another experienced windowsill propagator’s trick is to provide the ideal germination temperatures, so that seeds germinate quickly without getting leggy. You can do this by using a thermostatically controlled electric propagator on the windowsill. This will help shorten propagation time, increasing productivity and efficiency. A heated propagator will also allow you to grow plants that require higher germination temperatures, such as aubergines, courgettes, impatiens, peppers, sweetcorn and tomatoes, that may otherwise struggle to germinate.

To make the most efficient use of space and time when pricking out, it is worth investing in modular seedtrays, rather than using lots of individual small pots. Modular trays fit together without gaps, are easier to handle and move around, and require less seed-sowing compost than individual pots.

Low light levels combined with high temperatures encourage weak, leggy seedlings and young plants. You can help prevent this by controlling the room temperature and spacing out young plants and turning them daily, so that they are not drawn towards the light. Alternatively, you can make the most of available light levels by standing the seedlings and young plants in a reflector box, covered with tin-foil. Cut down the front of an empty cardboard box and angle the sides before covering with foil – creating a wedge-shaped silver tray with a high back. The low front will allow the maximum amount of light in and the foil-covered high-back, sides and base will reflect the light in all directions - eliminating the need to turn plants and seedlings.


Happy gardening!