Growing fennel can be a tightrope act

When something goes wrong in the garden, it's wise to ask why. When something goes unexpectedly right, the same question is in order. Take bulb fennel, also called Florence fennel. One is told that bulb fennel is a cool-weather plant best grown as a fall crop, because warm weather makes it bolt - that is, go to seed and fail to make bulbs. But since it's also frost-sensitive in fall, and the bulbs keep poorly in storage, that season is disappointingly short. So at our place we grow a spring crop as well.

Gardeners who've grown such bolt-prone annual crops as lettuce, broccoli, basil and cilantro know the accuracy of that term, as flower stems shoot up in warm weather with the speed of a runaway horse and stop producing their tasty leaves (or in broccoli's case, tasty heads). It's in their nature to bolt, and the best you can do is stall them by frequent picking, to get regrowth, or have a fresh crop coming along.

Less familiar is the practice of induced bolting, when you want a biennial plant to go to seed the first year, but its nature is not to make seed until it has experienced winter's chill. Left to its own, this wouldn't happen until Year 2. You induce bolting by giving the plant an artificial winter, a process called vernalization after the Latin word for spring. An early practitioner was Trofim Lysenko, who in 1928 showed you could get winter wheat and other biennial grains to bear early, after an induced cold spell, instead of after a Russian winter that so often killed them.

We vernalize artichokes, which survive winter in moderate climates and, as biennials, bear the second year. After a growth period in a warm greenhouse, we give them six weeks of early spring chill outdoors, and they set about producing those delicious edible buds when warm weather arrives, believing they are two years old.

Gardeners grow lots of biennial crops, such as carrots, parsnips, parsley, beets, Swiss chard, onions, cauliflower, Brussels sprouts, kale, cabbage, celery and, yes, bulb fennel. Biennials aren't supposed to go to seed the first year but sometimes do.

Several factors are involved, including day length, plant spacing and degree of soil moisture, but for many of them, unwanted bolting is triggered by a protracted cold spell outside after germination in a warm place inside.

Cauliflower, celery and fennel are especially susceptible. If a sudden, unseasonable cold snap hits the garden, we cover those crops with plastic on bent hoops. Otherwise they may be accidentally vernalized and go about making seed. It may seem as if the summer's heat is to blame, but more likely it's an early chill. The plants simply think they are two years old.

There is another strategy to help gardeners through this tricky business - choosing the right varieties. With artichokes we grow one called Imperial Star, bred to bolt. With fennel we choose ones bred not to. Try Zefa Fino from Johnny's Selected Seeds, Montovano from Adaptive Seeds and Seeds from Italy, Preludio from High Mowing, or Trieste from Peaceful Valley for a sweet, tender and well-mannered crop of early fennel.