I've been using these trays for years. My Aunt gave me her 10 year old used trays when they moved to town. I've finally worn them out after 15 years of use and forgetting them outside repeatedly. We live where it snows and I start my seeds in December-January. My seedlings have survived in an unheated cold frame with 3 inches on snow on top. I'm going to get more of these trays.