Using shade cloth to grow lettuce in summer
We live in northern York County, PA (New Cumberland). Lettuce and spinach we planted several weeks ago has gotten bitter and gone to seed. We understand shade cloth can extend the growing season. We've seen 30% and 40% (and higher) shade cloth in different colors for sale on the Internet. Any advice about which product to buy will be appreciated. Thank you!
York County Pennsylvania
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Bolting in plants occurs when an internal signal turns on the production of flowers and fruits. In lettuce and spinach the process of flowering also triggers the production of chemicals that produce the bitter taste. It was long thought that these plants bolted in the heat of summer. That is, they were triggered to flower by high temperatures. It has been shown experimentally that lettuce and spinach actually take their cues for flowering from the length of the hours of darkness at night. They are triggered to flower when the length of the night gets short. Since temperatures also go up as the days get longer most gardeners tend to blame the bolting on increasing temperatures.
Lettuce and spinach can be kept from bolting by completely excluding light each night for a long enough period to prevent the release of the flowering triggering substances. That is difficult to do in the home garden. Shade cloth that reduces the quantity of light during the day does not increase the critical length of night, and will have little effect on bolting.
The best strategy to prevent bolting is to plant as early in the spring as possible ,at least 6 to 8 weeks before the night length gets shorter at the end of May or beginning of June. You can also choose varieties of lettuce and spinach are genetically programmed to bolt later in the spring. Buttercrunch, Salad Bowl, Ruby, and Romaine tolerate short nights more than other varieties and remain sweet and tender longer. Check your seed catalogs for more information. Lettuce and spinach planted in the fall do not bolt because the nights are getting longer in the fall.