Chilling Tulip Bulbs: How Many Weeks of Cold They Need

What the cold is actually doing

The flower is already inside the bulb when you buy it, complete with petals and stamens in miniature. What is missing is the stem. A tulip bulb only extends the stem after it has accumulated a long spell of cold, and until that requirement is met the flower stays down at ground level or never opens at all.

The threshold is a sustained period below about 45F (7C). Twelve weeks is the minimum for most tulips and sixteen weeks gives a taller, more reliable stem. Warm spells during the period do not reset the clock, but they do not count towards it either, so a stop-start winter effectively lengthens the time needed.

This is why a tulip that flowers on a two-inch stem is not a poor bulb. It is a bulb that was brought into warmth too early, or planted in a garden that never delivered a long enough cold spell. The flower forms; the stem does not.

In a cold winter garden the soil does it for you

If your winter regularly holds ground temperatures below 45F for three months or more, you never think about chilling. Plant the bulbs in autumn at the right depth and the soil delivers the cold for free while the roots grow. Depth helps here too, because deeper soil is more stable and less likely to be warmed by a mild week in late winter.

The bulbs need to be in the ground for that to work. Bulbs left in a bag in a shed will get cold, but they will not root, and a chilled bulb with no root system produces a weak stem and a small flower. Cold and rooting need to happen together.

Pre-chilling in the fridge for mild climates

Where winters stay above 45F, buy bulbs in early autumn and chill them yourself for twelve to sixteen weeks before planting, then plant them into the coolest part of the garden at the very end of the cool season. Treat them as one-season plants; they will flower once beautifully and will not repeat, because the summer that follows is too warm for them to rebuild.

Use the main compartment of a domestic fridge at 40 to 45F (4 to 7C), in a paper bag or a mesh sack that lets air move. A sealed plastic bag traps moisture and the bulbs mold. Check them once a fortnight and throw out anything soft.

Keep the bulbs away from ripening fruit. Apples, pears, bananas and tomatoes give off ethylene, and ethylene in a closed fridge kills the flower bud inside a tulip bulb outright. The bulb still grows leaves in spring, which makes the cause hard to identify after the fact. If your fridge always has fruit in it, use a separate one, a cool cellar, or an unheated garage that reliably holds the right temperature.

1. Start the count backwards from when you want to plant Decide the planting date first, subtract fourteen weeks, and put the bulbs in the fridge on that day. Buying bulbs early and chilling them straight away is better than buying late and cutting the period short.
2. Bag them so air moves Paper bags, mesh onion sacks or open cardboard boxes all work. Label each bag with the date it went in, because after three months you will not remember.
3. Keep fruit out of the same space Ethylene from stored fruit destroys the embryonic flower. This is the single most common reason pre-chilled bulbs come up blind, and it is completely avoidable.
4. Plant straight from the fridge Do not let chilled bulbs sit at room temperature for days before planting. Take them out and get them into cold soil or a cold pot the same day.

Bulbs sold as pre-chilled or prepared have already had the treatment at the supplier and should be planted on arrival, not chilled again. Chilling them a second time is not harmful but wastes weeks they could have spent rooting.

Chilling in pots instead of in a bag

The better version of pre-chilling is to plant the bulbs into pots of compost, water them once, and stand the pots somewhere cold and dark for the chilling period: an unheated garage, a cold frame, a north-facing wall, or a fridge if the pots are small. The bulbs root while they chill, which is exactly what happens in a cold garden, and the resulting plants are noticeably stronger than bulbs chilled dry.

Keep the compost barely damp, not wet. A pot that dries out completely stalls the roots; a pot standing in water rots the bulbs. Check every couple of weeks and add a little water only when the surface is dry to a fingertip's depth.

Bring the pots into light and warmth once shoots are an inch or two (3 to 5 cm) tall and the chilling period is complete. Moving them into a warm room too early is the same mistake as insufficient chilling, and produces the same short stems.

Keep a note of the cold you gave them

Write down the date bulbs went into cold storage, the date they came out, and where they were kept. When some pots come up tall and some come up short, that record turns a mystery into an answer within seconds, and it tells you whether to lengthen the period next autumn.

It is also worth noting the coldest and mildest weeks of your winter, or simply whether the ground froze at all. After two or three seasons you will know whether your garden supplies the cold on its own or whether tulips here are a fridge job every year.