I don’t like the word “forcing”. Although, in a horticultural sense, it simply means stimulating a plant to bloom ahead of its usual season, it somehow sounds like you’re using some sort of nasty or painful process to get them to bloom. But you aren’t. And forcing is so easy to do.
You can force perennials and shrubs to bloom early indoors, but mostly forcing is used on hardy bulbs. These are the same bulbs you normally plant outdoors in the fall for bloom next spring, but instead of planting them in the ground, you pot them up and grow them indoors.
Fall is the season for forcing bulbs. You can force bulbs left over from your fall plantings or make a special trip to the garden center for a few more. Note that bulbs may be on sale in the late fall: a good excuse for trying your hand at forcing.
Forcing is pretty simple. Take a good-sized pot (at least 8 inches/20 cm in diameter) and fill it to ¾ of its height with your favorite potting soil (tip: potting soil will be easier to handle if you moisten it ahead of time). Place the bulbs on the mix, flat side down, pointed side up. Don’t be stingy: stuff as many bulbs in the pot as you can. And yes, in spite of what most references say, the bulbs can touch. The more bulbs you fit into the pot, the better the effect will be!
Next, cover the bulbs with mix. It doesn’t matter in the slightest if the tip of the bulb pokes through, but the bulbs will be sturdier if the lower, rounded part is well covered. Finally, water well.
You can also force bulbs over water alone if you want to, using a hyacinth vase or a decorative cache-pot with no drainage hole and stones or marbles. Just make sure the base of the bulb barely touches the water below.
Now comes the important part: the bulbs have to undergo a prolonged period of cold temperatures in order to bloom. That’s because they come from climates where there is a long, cold winter and, without a pronounced cold period, they won’t know when to flower. So pack your pot of bulbs into a plastic bag (so it doesn’t dry out too quickly) and place it in a refrigerator, a barely heated garage, a cold room or other location where the temperature will remain above freezing, but below 50˚F (10˚C).
In milder climates, there is another option. You won’t need a plastic bag this time: just place the pots of bulbs in a trench 12 to 15 inches (30-40 cm) deep, filling it with mulch and mounding up a further 6 inches (15 cm) of mulch on top. Thus, you can easily dig them up whenever you need to. This could theoretically work in cold climates too, but it’s very awkward when the mulch is frozen and the ground covered with snow. Also, in any climate, beware that voles and other rodents may discover bulbs buried in a trench. That’s why I prefer keeping mine safely indoors.
Bulbs need a long period of cold. 14 weeks is the norm (mark it on your agenda), although some bulbs, such as crocuses and grape hyacinths, only need 10 weeks and a few of the later tulips prefer 15 to 16 weeks. Of course, you can prolong that if necessary, pretty much as long as you want to (I once forgot a pot in my fridge and pulled in out in July: it bloomed beautifully!)
During this time, water as needed: the bulbs are not actually dormant, but growing in preparation for their upcoming bloom. So you may need to give them a good watering after a month or so. Do let them drain well, though.
When new pale yellow shoots appear and the pot is filled with roots, your bulbs are probably ready, especially if you have patiently waited the required 14 weeks. If so, remove the bag and put the pot in a cool but sunny spot: an east-facing window, for example. Leaves and flower stems emerge very quickly and the bulbs will soon be in bloom. Give the pot a quarter turn every three or four days to keep the stems growing straight and keep on watering them as needed. Cool temperatures are not absolutely essential and if you place the bulbs in a warm room, they’ll bloom anyway, but the stems may etiolate (stretch) and will then need staking.
Afterwards, you can simply compost the bulbs if you want, but you can also try planting them in the ground once the soil has thawed. They will bloom again in coming coming years, although they may need a year or two to adjust. Tulips are an exception: they don’t recuperate well from forcing and it is rarely worth doing anything but composting them.
It’s not worthwhile forcing the same bulbs a second time: forcing exhausts them. Instead, use fresh bulbs every year. Grape hyacinths (Muscari) are an exception: you can force them year after year.
Which Bulbs to Force?
You can force any spring flowering bulb, but in a given category, early-flowering varieties usually give the best results. That’s because they tend to be shorter and therefore, even if they become a bit etiolated (which is often the case when you force bulbs at room temperatures), it doesn’t show as much.
Here are some examples of bulbs that are easy to force:
- Anemone blanda
- Chionodoxa: any variety
- Crocus: ‘Yellow Giant’, ‘Joan of Arc’, ‘Remembrance’
- Iris reticulata ‘Harmony’, ‘J. S. Dijt ‘
- Hyacinth: ‘Blue Jacket’, ‘Delft Blue’, ‘Innoncence’, ‘Queen of the Pinks’
- Muscari: any variety
- Narcissus: ‘Carlton’, ‘February Gold’, ‘Peeping Tom’, ‘Tête à Tête’, ‘Unsurpassable’
- Snowdrop: any variety
- Squill: any variety
- Tulip: single early, double early and triumph tulips are best, including ‘Brilliant Star’, ‘Christmas Marvel’, ‘Couleur Cardinal’, and ‘Diana’.
Paperwhite narcissus don’t need a cold treatment to bloom. Plant them in early November and you’ll have flowers for Christmas. There are also “prepared hyacinths” whose bulbs were specially treated so they can bloom without a cold period, but plant them fairly early (certainly before Christmas), as the treatment “wears off”. Note they cost more than regular hyacinths.