At this time of year, it’s common for leafy greens to bloom or, as it’s known in garden-speak, bolt. Long hours of sunlight, combined with torrid temperatures induce flowering. In most cases, there’s nothing to be done. It’s simply time to pull the plants that haven’t yet been harvested and add them to the compost pile.
For example, spinach blooms when days last more than 14 to 16 hours. (Interestingly, spinach will only bloom when days are long.) Warm temperatures will accelerate this process. Dorothy Hinshaw Patent and Diane E. Bilderback explain why this happens in The Book of Garden Secrets:
Lettuce and spinach perceive light by way of a chemical, phytochrome, in their leaves. Phytochrome is activated by lights and triggers a signal to move from the leaves to the apical tip of the plant. When enough of the signal has been received by the apical tip, it starts to produce flower stalks and buds instead of just leaves.
Gardeners can use this knowledge to prolong their harvests. Since spinach needs long days to flower, crops sown to mature during the shorter days of autumn will not bolt. Also, the oldest leaves have been exposed to light the most. By choosing the old leaves to pick and eat, you can minimize the effect of the phytochrome. Doing this will extend your spinach harvest for several weeks.
Lettuce is a little different. Once mature, lettuce will eventually bloom no matter how long the days are. Moreover, old lettuce becomes bitter before it bolts. It’s hard to harvest immature plants, but in July, that may be the only lettuce we get. Then, some kinds of lettuce will bolt when the plant has received a certain number of daylight hours—either many short days, or fewer long days, no matter how big it is. It’s frustrating when, just as the tomatoes are finally becoming ripe, the lettuce becomes inedible!
You may notice that some lettuce varieties are labeled “slow to bolt.” Plant breeders created these cultivars by selecting seeds from the slowest-to-bolt plants, generation after generation. If you want to grow lettuce during the long, hot days of summer, these are the varieties to choose.
Unfortunately, even ‘slow to bolt” varieties aren’t foolproof, and it’s getting worse. Lettuce grown for seed is now chemically induced to bloom and set seed while the plants are still small, so there is no way to determine which plants would have lasted longest in the garden. Home gardeners can have an advantage here, in that you can grow and save your own seeds from the plants that stayed sweet the longest. Just remember that hybrids won’t breed true from seed; you’ll need to use open pollinated varieties.
Another way to stretch your lettuce harvest is to provide some shade to keep the plants a bit cooler. Either situate your crop in partial shade or use shade cloth draped over the beds. It won’t work miracles, but every bit helps.
When deciding where to plant lettuce, be aware that sunlight isn’t the only light source. Even a streetlight or security light can activate the phytochrome.
You would think that planting lettuce very early would allow time for a harvest before days get too long. While lettuce can be remarkably hardy (I’ve had Simpson Elite survive temperatures of 10°F and still produce a crop), exposing young plants to very cold temperatures will induce them to bloom earlier than they would otherwise. Still, if you want to harvest an early crop of small plants, go ahead and plant in late winter. You can always sow additional seed later in the season.
The best way to ensure summer salads is to plant more lettuce every few weeks. Choose bolt-resistant leaf lettuces and pick the leaves while the plants are quite young. When the flavor begins to deteriorate, harvest the entire plant.
At this time of year, you may find that you have to start the plants indoors. Lettuce seeds go dormant in temperatures over about 80°F. (A few weeks in the refrigerator will reverse this dormancy and allow them to germinate.) Be sure to harden off the seedlings before subjecting them to the mid-summer sun.
It takes a bit of effort to harvest lettuce in July and August, but a home-grown summer salad is ample reward.