Flower bulbs: planting depth, time, containers and forcing
Bulbs are plants packed with a built-in food store. Plant at the right depth, in the right season and in free-draining soil and most of them come back every year.
Depth and spacing
The standard rule from the New York Botanical Garden and most bulb suppliers is to plant at a depth of about three times the bulb's height, with the pointed end up. Large bulbs such as daffodils and tulips go deeper; small ones such as crocus and snowdrops shallower. Space bulbs a few bulb-widths apart.
When to plant
Spring-flowering bulbs (tulip, daffodil, hyacinth, crocus, allium) go in during autumn, once the soil has cooled. Summer-flowering and tender bulbs, corms and tubers (gladiolus, dahlia, canna, begonia, caladium, freesia, ranunculus, many lilies) are planted in spring after the last frost, or started indoors earlier. Autumn-flowering bulbs such as colchicum go in late summer. Winter-flowering types suit mild climates or pots indoors.
Containers
Use deep containers with drainage and a gritty mix. Bulbs can be packed closely in layers. Water when planted, then sparingly until growth starts, and keep pots cool through winter.
Forcing indoors
Tulips, daffodils and hyacinths flower indoors in winter if given a cold period first, commonly 12 to 16 weeks near refrigerator temperature; buy bulbs labelled for forcing. Plant in pots and keep them cool and dark until shoots appear.
Bulbs, corms and tubers
Daffodils, tulips, hyacinths and lilies grow from true bulbs. Gladiolus and freesia grow from corms. Dahlias, begonias, cannas and caladiums grow from tubers or rhizomes and must be lifted in cold climates. Ranunculus has tuberous roots, usually soaked before planting.
Arrangements and aftercare
Cut stems when buds show colour. After flowering leave the foliage to die back naturally so the bulb recharges; do not tie or cut it early.
Plants in this guide
| Plant | Germination temp | Depth | Days | Light | Before sowing |
|---|---|---|---|---|---|
| Allium (ornamental onion) Allium spp. | see page | 4 to 8 in (10 to 20 cm) for large types | varies | Treat seed | |
| Daffodil (Narcissus) Narcissus spp. | see page | 6 to 10 in (15 to 25 cm) | varies | Treat seed | |
| Gladiolus Gladiolus spp. | see page | about 2 to 3 times the corm's height | varies | Treat seed | |
| Hyacinth Hyacinthus orientalis | see page | 5 to 8 in (13 to 20 cm) | varies | Treat seed | |
| Iris Iris spp. | see page | Dutch iris 6 to 8 in (15 to 20 cm); dwarf 3 to 4 in | varies | Treat seed | |
| Lily Lilium spp. | see page | about 2 to 3 times the bulb's height | varies | Treat seed | |
| Tulip Tulipa spp. | see page | 6 to 10 in (15 to 25 cm) for garden tulips | varies | Treat seed |