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Ah lettuce; the most popular of the salad greens, offering a long season of sweet, crispy leaves. It’s one of my favourite veggies and I plant at least two dozen varieties each year, enjoying the variation of leaf textures and colours. For us, the lettuce harvest begins in our cold frames in early spring, followed by the open garden from late spring through early summer. And, just when I think the lettuce harvest will never end, the summer heat kicks in and I’m surrounded by beds of bolting lettuce.
Bolting, when the plants shift from leafy growth into flower production, is caused by a number of factors including high temperatures, long daylight hours, and less moisture – in essence – summer.
Lettuce does offer a few clues when it’s about to bolt. First, the center of the plant begins to elongate as the flower stalk is formed. Another obvious indiction is that the formerly sweet flavour is replaced by a bitter bite. The leaves also toughen up, so unless you plan on saving the seeds from your lettuce, it’s best to pull the plants, compost, and replace with a fresh planting of a heat-loving vegetable like bush beans.
3 ways to delay bolting lettuce:
1) Grow bolt tolerant cultivars. Certain varieties of lettuce, spinach, radicchio, cabbage, and other bolt-prone crops have been selected or bred to be more resistant to bolting. Steve Rodrigue, Trial Technician at Johnny’s Seeds, notes that some of the more bolt-resistant lettuces include cultivars like Sparx (romaine), Salvius (romaine), Starfighter (green leaf), and New Red Fire (red leaf). “The Summer Crisp or Batavia class of lettuce is the most heat tolerant” he says, and suggests trying Muir, Panisse, or Rouxai. However, because a number of conditions (heat, increased light, dry weather) cause bolting, merely choosing varieties that are said to be ‘slow to bolt’ or ‘heat-resistant’, may not be enough to prevent summer bolting lettuce. “Even heat-tolerant varieties have upper limits of temperatures they can withstand. In areas such as the South and Midwest, Johnny’s recommends avoiding planting lettuce during the warmest part of the season,” says Steve.
2) Give lettuce some shade. Less light means lower temperatures and often more moisture. Plant summer lettuce in pots on a shady deck or alongside taller, shade-producing crops like corn or pole beans. You can also erect a mini hoop tunnel over summer lettuce beds, topping it with a piece of shade cloth. Many garden centers sell shade cloth by the running foot or packaged in pre-cut sizes.
3) Water and mulch. Keeping lettuce well irrigated can delay bolting, but many of us are trying to conserve water or don’t have the time and inclination to baby thirsty lettuce. Preserve soil moisture and cut down on watering by mulching lettuce with straw or shredded leaves. Apply about three inches of mulch around young plants after a deep rain.
Do you have any tricks to share on growing lettuce or preventing bolting?