Customers frequently ask me when buying bedding plants when is the best time to plant? Should they plant in the evening? Is it better to wait and plant on a cool or cloudy day? While it is true that very hot days and very bright sun can cause newly planted stock to collapse into wilted ghosts that are often unredeemable, I always answer these questions the same way: you can plant just about any time if you shade your plants and water them well.
It is always best to plant as soon after buying as possible. When you remove plants from their containers for planting inspect the roots.Healthy roots are white so if you see black stringy roots cut them off with scissors. Then check to see if the entire root ball is hydrated all the way to the bottom. If not, set the plant in a shallow dish or tray of water for a several seconds until the planting medium appears completely wet. If the weather is hot or the soil where the plant is to be set is very dry, pour a little water into the hole then back fill soil around the roots. Then water well.
Then shade the plant for at least one whole day. Two or days is better if the weather is very sunny. This can be done by setting an inverted flower pot over the plant. Other items that work well to shade plants are paper bags, thin pieces of cloth, or a mulch of leaves or straw. Do not use plastic bags since they will suffocate the plant and create an environment similar to a small hothouse. If using inverted black flower pots, be sure the pot is big enough so that the plant’s foliage will not touch the pot since black can get quite hot and burn the foliage. Larger plants may need several days of shading first with an opaque cover, then later with a cover that allows some light through. Mulching will help keep the soil moist, so mulch whenever possible. Small bedding plants will need daily watering for at least a week after planting. If weather is very warm and you have not mulched they will likely need to be watered more than once a day until they are well established.
The reason that shading helps is that inside the leaves of the plant are cells that act as light receptors very much like solar panels, and like solar collectors they are flat. These cells can change their orientation to accommodate very bright or lower light conditions by changing their position so the broad, flat side is angled to reduce the effects of bright sun. However, they cannot reposition these receptors instantly: it takes a couple of days to make the adjustment. This is one of the main reasons experts recommend that plants be “hardened off” before planting outdoors by setting them in a sunny spot for just a couple hours their first day out of the greenhouse, and increasing the number of hours daily for a few days. If the plants have been in a greenhouse or a shaded environment their light receptors have oriented to a flatter position to gather maximum light.
I often do not have the time and the patience to acclimate bedding stock this way. So I simply plant well hydrated plants and shade them for at least one day. If I get lucky and there is a cloudy or rainy day on the following day, I remove the shade covers. If very hot weather returns the day after that, I usually replace the shade covers for another day. Another advantage to having plants covered in this way is that it reminds me that the plant needs to watered frequently while it establishes its roots into the new soil. Shading also helps insure success when dividing and transplanting small shrubs or perennials, and small trees although transplanting larger plants is always best done in the cool seasons of early spring and early fall. Since transplanting is always a stress to a plant, shading eliminates one of the stressors and ensures more success.
Since, I have started shading newly planted stock, I very rarely lose a plant.