While springtime may be the best time to plant ornamental perennials in our gardens—it’s not the only time. So don’t worry if you didn’t plant all the perennials you wanted many months ago. From mid-July through September, there’s plenty of time to set them in the garden, where their roots will have time to grow, the root hairs to make contact with the soil, and for them to get ready to sail through winter. And then, next spring, when they wake up and start growing again, they’ll be in every bit as good a shape as the perennials you planted in the previous spring, just not quite as advanced in their maturity.
There are some rules of thumb for planting in the hot, dry months of summer and early fall, though. Since we are no longer in the cool, moist throes of spring, our task now is to give newly-planted perennials conditions as close to spring as possible.
This means a thorough soaking of the soil where the perennials are to go, preferably with an overhead sprinkler to mimic a refreshing spring rain. Remember that by July in our parts, uncultivated soil is dry deep down, and so a gentle soaking over 8 to 10 hours will get water down to where the roots will plunge. The word “gentle” is important here, so that the water penetrates the soil and isn’t applied in such quantities that it runs off the surface.
The sun is also blazing hot in July and August. Once the soil is wet and the perennials are put in, I cover them with a cardboard box for at least the first day, if not the first two days, depending on whether we’re in a foggy-morning (cool) period or a heat wave. A day or two of complete shade won’t hurt the plants and will give them a chance to get set in the soil without having to respire so much water just to deal with the stark sunlight.
After that initial total shading, it’s good to continue to give them some shade. This may not be necessary if we’re in a cool and foggy stretch of weather, but if it’s hot and sunny, simply propping up a piece of plywood against a ladder, or other such homemade device to throw some shade, will help them get started without drying out. You can also buy garden shade cloth—a fine mesh cover that cuts sunlight by about 50 percent. This can be erected like a simple tent over the perennial patch or even just laid down over the plants with a few stones on the edges so it doesn’t blow off in afternoon winds—it hardly weighs anything.
Keep the soil moist but not sopping wet. After a week, you can expose the plants fully to the elements. The perennials should be acclimated to their new home by then, but keep an eye on them for any signs of stress, such as wilting, insect damage, or the predations of gophers and deer.
The easiest perennial to transplant is probably the herbaceous peony (Paeonia hybrids). They don’t mind their tuberous roots being moved any time of the year, although the best time to transplant them is in the fall after their foliage and stems die back. They may sulk a bit after being moved during the growing season, but they’ll recover.