Forcing bulbs indoors

Forcing bulbs indoors is one of the best things to do in December to liven those monochromatic winter months.  The bleak December winds and days of incessant rain drive even the hardiest gardener inside.

Like making jam, planting indoor bulbs can feel like a chore, but you'll be glad you made the effort. It isn't difficult, but there are certain questions to consider.

  • What are the best bulbs to force?
  • Should you buy prepared forcing bulbs?
  • Can you plant outdoor bulbs indoors?
  • What regime of cold and dark will they require?
  • How much food and water?

Preparing bulbs

I ask myself these questions every year. This year, I've decided to abandon educated guesswork and find out for certain. With many bulbs, ordinary garden varieties are fine. All that the bulbs grown inside need is protection against the worst of the wet and cold. This is true for crocus bulbs, fritillaries, anemones, tulips and narcissi bulbs.

Prepared bulbs have been pre-chilled to force them to flower at Christmas or early in the New Year. This is commonly done to hyacinths. If you can get hold of ordinary garden hyacinths early enough in autumn, you can prepare them yourself. Put them in a paper bag in the bottom of the fridge for four to six weeks.

Most other bulbs for forcing indoors need 10 to 15 weeks in the cold to flower well. Ideally, keep them at a temperature between 1.5C and 10C (at a pinch, 12.5C) in a cupboard, shed or garage. This cold period makes the bulb think it is winter, stimulating a biochemical response to make it start flowering. Amaryllis are the exception and do not require a cold spell.

Once they've had their cool period, you can bring them in somewhere warmer, above 15C, and they will quickly sprout, then bloom. As far as they are concerned, spring has arrived.

Most bulbs also need a period in the dark, to give the root time to develop before the light pulls the flower and leaves from the bulb. Again, amaryllis is an exception, as is freesia, 'Paperwhite Ziva' narcissus and all Tazetta narcissi, but for most, the dark spell is as essential as the cold one. This is often what goes wrong with cheap indoor hyacinths: they have had a cold spell, so they begin to bloom when brought inside, but the flower is tightly enclosed in the bottom of the leaves. If you knock them out of their pot, you will see that the root has hardly developed, so it can't push out the flowers and leaves properly from the bulb.

Planting

All bulbs need to be grown in a well drained but moist medium with a soil structure strong enough to anchor them. Again, that's often a problem with cheap bought bulbs: they are planted in peat-based compost, which becomes as light as dust when it dries and can't begin to hold the elongating stem and top-heavy flower-head of chunky bulbs such as hyacinths and amaryllis.

A loam-based soil lightened with some grit is ideal. Because it is a mineral soil, it also has a good combination of drainage and water retention.

I use the perfect, crumbly, fine soil from the molehills scattered annoyingly over the lawn. This is the only time of year that I feel triumphant when a few more emerge each morning. Most bulbs hate sitting in the wet, so for each bucket of molehill soil, I add just under half a bucket of grit. But my soil is quite heavy: use half as much grit if you have a freer-draining loam.

Lay an inch of pure grit in the bottom of each pot, then a shallow layer of the soil-and-grit mix. Place the bulb with its pointy tip just below the soil surface, and fill in around it.

If your container has no holes in the bottom, use bulb fibre. This is expensive, but contains charcoal and grit in the right proportions. The charcoal is porous and will help keep the soil damp but not too heavy, preventing rot and disease.

If you like the idea of planting bulbs in swathes in your best salad bowls, but don't want to ruin them, there's an easy solution. Find a plastic bowl or pot that just fits inside your bowl and plant bulbs in those. Don't just go for one variety of bulb, but plant as many as you can find, so you will have a succession of flowers.

It is easy to find a plastic pot to fit inside most shapes of terracotta, but it can be tricky to get one to go in a large, round bowl. I've just bought some cheap, flimsy colanders. The holes aren't a problem: they save you having to pierce the bottom of a solid container with a hammer and nail. I've lined them with newspaper, which will let the water through but keep the soil in place. I cut off the bases of the colanders, and the match of sizes isn't exact, but I can fill the gap with a bit of moss or cover the whole surface with turf.

I cut turfs from an area of rough grass and clipped them with scissors to fit. I did this last year and the bulbs looked great.

Watering and feeding

Anemones

Anemone corms look like crinkly lumps of dried soil. When my first lot arrived in the post, I thought the supplier had sent a pile of earth and forgotten the anemones.

Varieties Anemone coronaria are the best anemones to force. Plant them inside now and they should be flowering by March. If you plant them in September, you can force flowers by mid-February. Single flowers of a single colour are the loveliest: Anemone coronaria 'Mister Fokker' (deep purple-blue), Anemone coronaria 'Cristina' (purple-crimson), Anemone coronaria 'Sylphide' (deep pink) and Anemone coronaria 'The Bride' (pure white). Mixed-colour bulb bags often include muddy, greyish tones, without the clarity and velvety texture of these named forms.

Buying The corms should feel hard, almost like a stone.

Preparation for planting To get them off to a fast start, soak them in water for a couple of hours before planting. Leave them in just long enough so that you can make a dent in the skin of the bulb with your fingernail.

Planting medium A loam-based soil lightened with some grit is ideal.

How deep to plant About 2.5cm (1in) deep, leaving the tips just exposed.

Temperature and light/dark They don't need pre-chilling to flower, but many people think they grow more vigorously if you cover them after planting and chill them for six weeks, at about 5C. They will flower best with bright light, a cool location and consistently moist (not wet) soil.

Watering Regular watering is crucial. Don't let the compost dry out.

Planting to flowering 10-12 weeks Keep or plant out? Leave them where they are to flower next year, or dry them off for re-planting in the autumn.

Iris

A perfect miniature iris is as intricate as any Fabergé egg, and all the better when examined close-up. Irises make ideal plants for pots and containers. I now grow them only in pots inside.

Varieties Iris reticulata, Iris histroides and Iris danfordiae are all lovely miniature irises, ideal for forcing. There is also an iris relative, Hermodactylus tuberosus, which looks magnificent growing indoors: I first saw this green-and-black-velvet wonder growing wild on an olive terrace in Corfu when I was five, and I've loved it ever since. It thrives in a well drained, sunny, sheltered spot in the garden, if left undisturbed, but in a pot, it will be safe from the weather and the birds.

Buying The bulbs should be firm and not sprouting.

Planting medium A loam-based soil lightened with some grit is ideal.

Pots These are small bulbs, so shallow bulb trays are ideal, but the bigger the better.

How deep to plant Just below the soil surface.

Temperature and light/dark These irises need a spell in the cold to flower well, with a temperature below 10C for 10 to 15 weeks. They require 15 weeks in the dark.

Watering Keep compost moist.

Planting to flowering 17 weeks.

Keep or plant out? Leave them in their pots undisturbed - against a wall, perhaps - or plant them somewhere in the garden where you'll see them clearly next year. They are nice right beside a path.

Lily of the valley

These aren't, strictly speaking, bulbs, but you can treat them in a similar way. There are few lovelier smells, and they grow well inside. Don't keep them in pots from one year to the next.

Buying It doesn't matter if they have begun to sprout.

Preparation for planting Soak in warm water for a couple of hours.

Planting medium A loam-based soil is ideal.

Pot These are small bulbs, but they have an extensive root structure. Tall, narrow pots are perfect.

How deep to plant Just below the surface of the soil.

Temperature and light/dark They are often pre-cooled when you buy them, so there is no need for a cold period.

Watering Convallaria - Lily of the valley likes more moisture than most bulbs, so keep well watered. They don't like bright light.

Planting to flowering Three to four weeks.

Keep or plant out? Plant them in the garden in a shady, moist spot and they will thrive. Do not cut off the foliage before planting. They may take a year to recover from the forcing process, but once they have settled in, they will last for decades.

Narcissi

If you're forcing narcissi inside, go for powerfully scented forms with delicate flowers and leaves. These look and smell fantastic growing inside.

Varieties The classic ones to force are paperwhites such as narcissi 'Ziva' and  narcissi 'Tête-à- tête'. One of my favourites is narcissi 'Avalanche', but almost any of the highly scented Tazetta varieties do well. 'Ziva' and 'Avalanche' are good for early flowers, while narcissi 'Silver Chimes' and narcissi 'Double Pheasant's Eye' are good for later in the spring.

Buying These large bulbs need to be firm, but the shoots can be quite long and they'll still grow well - it's worth planting narcissi at almost any stage up to flowering.

Planting medium As with hyacinths, a loam-based soil lightened with some grit is ideal.

Pot Large, deep pots are ideal for their vast root structure.

How deep to plant Just below the soil surface.

Temperature and light/dark Narcissi need a spell in the cold to flower well, so keep below 10C for 10 to 15 weeks. They do not require time in the dark.

Watering Keep compost moist.

Planting to flowering Paperwhites take five to six weeks, others 16 to 18 weeks.

Keep or plant out? Paperwhites are not hardy, so leave them in their pots for next year, or dry them off and re-pot them again late in the summer. Other hardy varieties can be kept or planted out.