65-80 Days to Maturity. Lactuca sativa. Summertime Lettuce Vegetable Seeds. Non-GMO, annual open-pollinated, heirloom lettuce seeds. Summertime lettuce strain is known for its heat tolerance and resistance to bolting making it a good choice for summer gardens. It is often difficult to grow crisphead lettuce in areas with cold winters and hot summers as lettuce performs best with a long cool season. This strain may be a better choice if lettuce bolts easily in your area. ~23,270 seeds per ounce.