How to force bulbs

Forcing bulbs is a process used to make bulbs flower earlier than they naturally would. Hyancinths, for example, are often forced to flower over Christmas.

Some bulbs can irritate the skin so gloves should be worn when handling them.

Plant the bulbs in the container you want them to flower in using bulb fibre compost, or any other potting compost (you may wish to use houseplant compost to reduce the ‘composty’ smell in the house) so long as it is free-draining (you can improve draining by mixing in perlite or vermiculite). Put a layer of compost into the container and sit the bulbs on top if it, their tips should be about ½ cm below the rim of the container. The bulbs can be close together but must not be touching. Tulip bulbs should be planted with their ‘flat’ side facing out.

Fill the container with compost up to 1cm below the rim of the container (so the tips of the bulbs are poking just above the compost) and then water.

Put the pot in a cool (around 9°C) dark place to allow the root structure to develop. This can either be in a cellar or shed in a black bin liner (don’t tie the top too tightly so that air can still get in), in the shade against a north facing wall, covering the container with about 15cm depth of bark chippings to keep the light out, or in a cold frame (again covered with bark chippings) which should be opened on sunny days to ensure they do not get too warm. In all cases the container should be protected against rodents or worms; siting it on a couple of stacked paving slabs outside or putting it on a high shelf indoors. Make sure the compost stays moist but not waterlogged.

If you live in a hot climate then you can place the container in the refrigerator for the cool period, just take care to ensure that the compost doesn’t dry out.

When the shoots are about 5cm tall and and you can see roots through the drainage hole in the base of the pot, bring the pot indoors and put in a cool spot out of direct sunlight. Once the leaves are a strong green colour move the pot into a warmer and lighter situation. Avoid putting the pot in a draughty place or above a radiator. Keep the atmosphere moist by standing the container in a shallow tray of gravel which is kept topped up with water.

If, after a couple of weeks, you’re getting loads of foliage but no sign of flowers, return the pot to a cool, dark place for no more than 2 days, before gradually returning it back into the full light.

Once the bulbs have flowered they can be planted out in the garden, returning them to their natural flowering pattern, although they may not flower well for a couple of years as they recover from forcing. Do not try to force the same bulbs twice (except Hippeastrum bulbs which may respond to consecutive years’ forcing if fed and watered after flowering until August, when they should be allowed to dry out and then re-potted and forced as before).

If you are growing the bulbs in specialist bulb vases, or in jars on a bed of gravel/stones, so they are grown compost-free with just a reservoir of water to sustain them, follow the same process ensuring that the water level in the vase is just below the base of the bulb.

Paperwhite daffodils (Narcissus tazetta) bulbs do not require the cool, dark period and can simply be potted up and kept in light conditions indoors.

Forced bulbs flower for shorter periods than they would naturally do, therefore you may wish to prepare a pot every week or so to ensure you get a continuous display.

The following bulbs are suitable for forcing:

  • Crocus (15 weeks cooling followed by 2 to 3 weeks indoors)
  • Fritillaria (15 weeks cooling followed by 3 weeks indoors)
  • Galanthus (snowdrops) (15 weeks cooling followed by 2 weeks indoors)
  • Hippeastrum (amaryllis) (about 8 weeks to force, no cooling period required)
  • Hyacinthus orientalis (forcing timescales vary by cultivar, but as a generally rule they take 8 to 10 weeks cooling followed by 20 days indoors, ensure you purchase ‘prepared’ bulbs)
  • Iris (15 weeks cooling followed by 2 to 3 weeks indoors)
  • Lilium (lilies) (6 weeks to force, no cooling period required, must be bulbs which have been prepared for forcing)
  • Muscari armeniacum (13 to 15 weeks cooling followed by 2 to 3 weeks indoors)
  • Narcissus (daffodils) (15 to 17 weeks cooling followed by 2 to 3 weeks indoors, ensure you purchase bulbs which are for indoor flowering)
  • Narcissus tazetta (paperwhite daffodils) (6 to 10 weeks to force, no cooling period required, ensure you purchase bulbs which are for indoor flowering)
  • Tulipa (14 to 20 weeks cooling followed by 2 to 3 weeks indoors)