That depends on the type of Fertilizer you are using. Early Spring, before new growth starts is an ideal time.  That is one of the main things we like about the Nutri-Pak Fertilizer System; Nutri-Pak fertilizer is released slowly when soil temperatures are moderate -generally in Spring & Fall & when moisture /water is present in the soil; Root growth & development which ultimately leads to top growth takes place when soil temperatures are moderate -not hot, not cold;

 Install your Nutri-Pak packets now or when the soil thaws, water your plants in and the rest is up to Mother Nature; if the soil temperatures remain moderate your plants' roots will start growing which leads to healthier, stronger plants this summer!