The rule of thumb has always been to never plant in summer, but rather plant in spring and fall when the weather is cooler. As this is a good rule, it is not set in stone. Actually, the plants root very quickly with the warm soil temperatures when planting in the summer months. It is not a good idea to plant new plantings in the beating sun, but most of the hostas are being placed in shadier area of the garden so that is not as much of an issue.

Two Rules About Planting In Summer

▪️You need to keep everything you plant well watered, especially during dry periods. So, if you are going on vacation for a week after you planted some new hostas, your results may not be gratifying.
▪️It is also not a good idea to plant bare root plants in the heat of summer as the roots have been disturbed and it takes time for them to redevelop.

The most important ingredient to any garden is water.  Extreme dry periods can stress the hostas and they will actually become smaller the following year.

Early morning is the best time to water as there is less evaporation and the plants dry during the day which helps against fungus and rot. A sprinkler left on for an hour once or twice a week will help the hostas through the dry periods.

SOURCE: NH Hostas & Companion Plants