Late spring freezes are a problem in northern locations, especially after the plants have leafed out. If your trees have already started to grow and you expect a late freeze, then you should make every effort to protect them, such as using Grow Tubes and other protective measures. Some areas even in Zone 5 can have killing frosts as late as Memorial Day! Dunstan Chestnut trees can tolerate temps as low as -20 degrees when fully dormant and established trees. Chestnuts need at least 100 frost free days from flowering until harvest to fully ripen the nuts.