 Lettuce varieties will cross with each other , making spacing of 5 to 20 feet ( 1 @.@ 5 to 6 @.@ 1 m ) between varieties necessary to prevent contamination when saving seeds . Lettuce will also cross with Lactuca serriola ( wild lettuce ) , with the resulting seeds often producing a plant with tough , bitter leaves . <unk> , a lettuce variety grown primarily in Asia for its stems , crosses easily with lettuces grown for their leaves . This propensity for crossing , however , has led to breeding programs using closely related species in Lactuca , such as L. serriola , L. saligna , and L. virosa , to broaden the available gene pool . Starting in the 1990s , such programs began to include more distantly related species such as L. tatarica . Seeds keep best when stored in cool conditions , and , unless stored cryogenically , remain viable the longest when stored at − 4 ° F ( − 20 ° C ) ; they are relatively short lived in storage . At room temperature , lettuce seeds remain viable for only a few months . However , when newly harvested lettuce seed is stored cryogenically , this life increases to a half @-@ life of 500 years for vaporized nitrogen and 3 @,@ 400 years for liquid nitrogen ; this advantage is lost if seeds are not frozen promptly after harvesting . 
