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Ask Tom: Summer Cooling

July 27, 2012

Lettuce at Thatchmore Farm

Dear Tom –

How do you manage to grow lettuce in the middle of a summer like this one?

– Hank in Edneyville

 

 

 

 

 

 

 

 

Tom Elmore

Dear Hank –

This year was challenging with three weeks of drought and temperatures in the high nineties. The lettuce was not happy but we still managed to meet the demands of our two markets for salad mix.

Our main strategy in producing summer lettuce is to pick the right field. One of our two fields has a row of trees to the west and lettuce seems to prefer cool evenings in contrast to baking in western sun when the air and soil temperatures are already high. I have not tried the idea of planting a row of corn or tomatoes to the west of lettuce but it seem to me that approach will work too.

I also recommend avoiding dry soil, so we irrigate every other day when it has not rained recently. Summer lettuce is subject to foliar diseases so it is also important to time irrigation to avoid extending the period of time that the leaves are wet each day. I try to water when the leaves are already wet from dew or for a brief mid-day period after the leaves have dried and ending well before evening so the leaves will be fully dry before the dew happens again. Periods of leaf dryness help slow the spread of leaf fungi.

Intentional watering mid-day can lower the canopy temperature of the lettuce plant and may help it grow more quickly. Rapid growth gets your crop to market more quickly and helps win in the ‘race with rot” as Eliot Coleman puts it.

Fogging or misting heads for your irrigation system take this concept to the next level. Some growers mist with a timer when propagating cuttings. A similar approach is likely to help cool your crops in mid-day heat. A variety of suppliers for these products can be found here.

When plants over-heat they begin to wilt and shut down photosynthesis. The most intense sunlight and the highest potential for making plant sugars are in the middle of the day (10:00 – 2:00). If your plants are wilted during part of that period they are growing more slowly each day. Keeping them cooler and photosynthesizing will not only help avoid triggering a bolting response but will also generate more crop more quickly. Apple growers found this to be true when they sprayed a white clay spray (Surround) on their trees. The clay reflected some sunshine but it also kept the trees productive in mid-day by lowering the canopy temperature. Spraying clay on lettuce is probably not very practical since it is hard to remove at harvest but it might work on crops like squash and tomatoes where the leaves are not harvested.

With climate change we are told to expect more and more extreme weather events. From our experience this summer, longer dry periods and higher temperatures during those droughts are likely to be more common.

Stay cool.

–Tom

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