Question
Two things;what if any varieties of lettuce are heat tolerant and can they be planted herein the Washington DC area where the temperatures exceed 90 degrees? Also I planted chery Belle radishes in late March,I know radishes mature rapidly and are a cool weather crop also but we had a few weeks of cold weather where daytime highs seldom got above 50 then a week of unseasonably warm weather with daytime highs in the 80s and a day at 93! hardly typical April weather for this region ,normal is about 65 to 70 in the day,Will this sudden changes in temperatures cause stress on this type of crop?I HAVE MADE SURE TO KEEP THE SOIL MOIST AS MUCH AS I CAN,SINCE IT RAINED WHEN IT WAS COOL THEN DIDN'T RAIN WHEN IT GOT HOT. or is this Ok? Since the hot weather lasted for just a week ,now it is back in the low 70s where it should be.Would appreciate your help.

Answer

Dear Peter:

Lettuce is a somewhat hardy plant and is generally ideal for growing in spring and fall. Requiring a cool moist soil for good germination, it simply will not germinate in soils that are above 80 degrees. For summer growing, plant beneath shade cloth or in light shade. Sow seeds thinly, about 1/4" deep, in rows 12" apart, thinning to the strongest plant every foot. Harvest young leaves when needed then allow the rest of the plant to size up to nice full heads. Lettuce needs plenty of water so in the extra dry regions, plant within 4" of a drip irrigation system and make sure it is timed to come on daily for about 20 minutes. Shade netting can make all the difference in helping conserve soil moisture and is essential to prevent bolting and bitterness of all salad greens during the hot days of summer.  Recommended heat tolerant varieties include: Four Seasons Butterhead Lettuce and Anuenue
'Anuenue' is the Hawaiian name for "rainbow" . A valuable feature of this variety is the ability of the seed to germinate at higher soil temperatures (above 80 degrees F).

Regarding the radishes and temperature shifts during their growth.  Sudden and extended temperature shifts will cause stress on this vegetable since it is a cold weather crop.  Consider using shade cloth or planting in light shade as suggested above for the lettuce crop.

Regards,

Mack Jean
AllExperts
Master Gardener
Tennessee