Forcing Hyacinths Indoors on a Glass or in a Bowl

The cold dark period is the whole trick

A hyacinth bulb already contains next spring's flower, fully formed inside it. Forcing does not make the flower; it convinces the bulb that winter has been and gone so that it pushes what it already has. That signal is cold, and the bulb will not accept a short version of it. Thirteen weeks at 40 to 48F (4 to 9C) in complete darkness is the working figure, and ten weeks is the absolute floor.

Cut the cold short and you get the single most common failure: a squat flower spike that opens while still buried among the leaves, sometimes barely clearing the neck of the bulb. People blame the variety, the water or the window. It is almost always weeks of chilling that were never given. The stem elongates using cold-triggered growth, so a bulb chilled for eight weeks physically cannot make a long stem afterwards, no matter how good the conditions become.

Darkness matters as much as temperature. Not dim, not a shaded corner, but genuinely dark, because any light during the cold period tells the bulb to start making leaves early and you end up with tall foliage and a short flower. A cardboard box over the pots, a black bag with a couple of air holes, or an unlit cellar all work.

Some bulbs are sold already chilled, described as prepared or pre-cooled. Those have had the cold given to them in a controlled store and can go straight into their forcing position for around three weeks of cool dark rooting, then into the light. Unprepared bulbs from the general bulb bins need the full period yourself. If the packet does not say prepared, treat it as unprepared.

On a forcing glass, in water alone

A hyacinth glass is the hourglass-shaped vase with a narrow waist that holds the bulb above a reservoir. The rule that makes or breaks it is that the water must not touch the bulb. Leave a gap of about a quarter of an inch (5 mm) between the water surface and the basal plate. The roots will find the water and grow down into it; the bulb itself, sitting in a humid pocket of air, stays dry and firm.

Get this wrong and the base of the bulb sits wet for three months and turns to soft brown mush. This is the second great cause of indoor hyacinth failure after insufficient chilling, and it is entirely avoidable. Check the level weekly and top up, because evaporation in a heated house is faster than people expect.

Put the filled glass somewhere cold and completely dark for the full chilling period. An unheated garage, a shed, a cool cellar or the back of a refrigerator all work. If you use a refrigerator, keep ripening fruit out of it: apples and pears give off ethylene, which aborts the flower inside the bulb and leaves you with leaves and nothing else. That one detail sinks a lot of fridge-chilled bulbs.

You will know the cold period is done when the glass is full of thick white roots and a pale shoot of one to two inches (3 to 5 cm) has pushed out of the nose. That shoot is the signal, not the calendar. Roots but no shoot means give it another fortnight.

A drop of charcoal or a small piece of horticultural charcoal in the reservoir keeps the water from going stale. Change the water outright if it turns cloudy or smells, and rinse the roots gently under a cool tap when you do.

In a bowl of compost or on pebbles

A bowl of three or five bulbs is easier to keep alive than a single glass and gives a better display. Use bulb fiber or any free-draining compost, and set the bulbs so the noses sit just proud of the surface with an inch (3 cm) between them, not touching each other or the sides of the bowl. Bulbs that touch pass rot straight along the line.

1. Half fill the bowl Put in enough compost that the bulbs will sit with their tops level with the rim. Bulb fiber suits bowls with no drainage hole; ordinary compost with added grit suits anything that drains.
2. Set the bulbs, noses clear Space them an inch (3 cm) apart and press lightly. The shoulders can be buried but the noses stay above the surface so water runs off rather than sitting in the growing point.
3. Water once, then keep barely damp Moisten the compost through and let it drain. Through the whole cold period it should stay just damp, never wet. In a bowl without a drainage hole, err firmly on the dry side; you cannot pour excess away.
4. Into the dark and cold Cover with a box or black bag and put the bowl somewhere at 40 to 48F (4 to 9C) for thirteen weeks. Check monthly for moisture and for the shoot emerging, and put the cover straight back.
5. Bring out in two stages When the shoots are one to two inches (3 to 5 cm), move to a cool bright room at around 50F (10C) for a week before anything warmer. Going straight from a cold shed to a warm windowsill shocks the bulb into opening the flower short.

Keeping the spike upright and long

The two things that ruin an otherwise good forced hyacinth are heat and one-sided light. Warmth over 65F (18C) makes the flower open faster than the stem can extend, so the truss opens fat and low. A cool room, ideally 55 to 60F (13 to 16C), doubles how long the flowers last and lets the stem finish its growth. Move the pot off a windowsill above a radiator at night.

Turn the bowl a quarter turn every day from the moment it comes into the light. Hyacinth stems lean toward a window hard, and once a heavy truss is leaning it will not straighten. Daily turning is the whole fix.

If a spike does start to topple, a slim cane pushed in beside the bulb and one loose tie under the flower head holds it without looking staked. Some growers slip a paper cone over the emerging shoot for the first week in the light, which draws the stem up an extra inch (3 cm) before the flowers open.

Note the date you started chilling, the date the shoot appeared and the date the first floret opened. Three numbers, and next autumn you can count backwards from the date you want flowers on the table and start the cold period exactly right. Most people who force well are simply people who wrote last year's dates down.