When to Plant Spring Bulbs

Those who grab a spade in September to plant all their spring bulbs at once make an expensive mistake: tulips don’t belong in the ground in September, but in November or even December. The reason is tulip fire (Botrytis tulipae), a fungal disease that thrives in tulips in warm, moist soil and which is effectively controlled by late planting in cold ground. Daffodils, crocuses and snowdrops, on the other hand, can and should be planted from September onwards.

Why tulips wait

Tulip fire is a fungus that develops on tulip bulbs and leaves, particularly in warm, moist conditions. The disease causes brown spots on leaves and flowers, distorted plants and, in severe cases, complete rotting of the bulb. Those who plant tulips in September place them in soil that is still relatively warm and in which the fungus can establish itself undisturbed. By waiting until November or even early December, when soil temperature has fallen structurally, the fungus gets less chance.

Cold soil inhibits the development of Botrytis tulipae without harming the rooting of the tulip bulb. Tulips need a cold period to flower, but it doesn’t need to be long: a few weeks suffice. December is not too late, as long as the ground is not frozen and the spade can still get through. This exception does not apply to all other autumn bulbs: daffodils, crocuses, hyacinths and ornamental onions go into the ground from September onwards and are not affected by a longer waiting period in warm soil.

Those who see a batch of tulips on offer halfway through autumn can therefore buy with confidence and store them at home until it’s cooler. Bulbs are living plants and can dry out if kept waiting too long, but a few weeks in a cool, dry place do no harm and considerably reduce the risk of tulip fire.

How deep

The rule of thumb for flower bulbs is two to three times the height of the bulb, with a minimum of about 5 cm. A tulip bulb 5 cm high therefore goes 10 to 15 cm deep. This depth protects against frost and gives the plant stability. Planting too shallow leads to two problems: frost damage to the bulb and stems toppling over when in flower, because the anchor is too weak. Planting too deep costs energy: the shoot has to struggle further upward, resulting in late or even absent flowering.

The point of the bulb faces upward, the flat side with the root primordia faces downward. When in doubt: the flat side is always the bottom. Those who plant incorrectly will see the shoot take a detour, which delays growth but is rarely fatal. In naturalised bulb groups where the soil is no longer dug over, bulbs actually regulate their own depth: they slowly sink to an optimal position or pull themselves up slightly, depending on annual root work.

Larger bulbs such as those of alliums go deeper, smaller ones such as those of crocuses can do with less. The planting hole doesn’t need to be perfect: a few centimetres’ difference falls within the margin the plant can handle itself.

Where it goes wrong: waterlogged conditions

Bulbs are storage organs full of starch and sugars, and rot in the absence of drainage. Soil that holds standing water in winter is not a bulb spot, however sunny or beautifully situated. On heavy clay, a layer of sand or gravel beneath the bulb helps more than any amount of care. Even a thin layer can make all the difference between a bulb that survives and a bulb that rots away.

This same drainage principle applies to plants such as /plant/lavendel/, which naturally prefer dry, free-draining soil. Where soil holds too much moisture, improving the structure solves more than adjusting watering. With /plant/taxus/, drainage is also crucial: waterlogged conditions lead to root rot, even though yew is otherwise a tolerant plant.

Those who have no sand or gravel to hand can plant on a slight rise or choose a spot where water naturally drains away. In a border with perennials, this might be away from the hydrangeas and astilbes and towards the sunnier side, where the soil dries out faster. Planting bulbs in pots or containers is an alternative, provided the pot has a drainage hole and doesn’t sit on a saucer that catches the water.

Naturalising or one-off

Not all flower bulbs return year after year with the same vigour. Daffodils, crocuses and snowdrops naturalise well: they self-seed, the bulbs divide and after a few years there’s a field where you planted a handful. Modern tulip cultivars rarely do this. They’re bred for large flowers and vivid colours, and that comes at the expense of vitality. The first year they flower spectacularly, the second year more weakly, and after that they often disappear.

Those who want the same tulip display every spring plant additional tulips annually or replace them entirely. Those who want a self-expanding field choose daffodils, crocuses or wild tulip species such as Tulipa sylvestris, which do naturalise. This difference determines what you buy, how much you spend and whether you’re satisfied with the result in five years’ time. It’s not a mistake to choose annual splendour, but it must be a conscious choice.

Naturalising bulbs require little: plant them in a spot where they’ll be left undisturbed, let the foliage die back after flowering and enjoy. Annual bulbs require the same, but then every season anew.

After flowering: the foliage must remain

The most common mistake after a successful planting is cutting off the foliage as soon as the flower has finished. That foliage refills the bulb for next year. Six weeks after flowering, the foliage has usually yellowed sufficiently and comes away with a gentle tug. Those who cut earlier rob the bulb of its energy source and get weak or no flowering in the second year.

That yellowed foliage is ugly, there’s little to be said against that. Those who don’t want to see it plant bulbs among perennials that emerge whilst the bulbs are flowering and which later camouflage the yellowed foliage. Cranesbill, lady’s mantle and geraniums do that job well. Another solution is to plant bulbs in strips or groups at the edge of view, where you consciously look at them during flowering and ignore them during die-back.

Braiding the foliage or tying it together with an elastic band doesn’t help: it reduces the leaf surface that captures light, which has the same effect as cutting too early. The bulb doesn’t fill sufficiently, and next year the flowering disappoints. Leave the foliage alone, however ugly it becomes.

Soil, climate and cultivar differ from garden to garden, and what is described here is a general guide. When in doubt about toxicity for children or pets: consult the National Poisons Information Centre or a veterinarian rather than relying on an article.