The best time to transplant seedlings into the yard or garden is on a cloudy day, in the late afternoon or early evening. The transplanting process is hard enough on the plants without their having to cope with bright, hot sun on their first day in the ground. Do everything you can to minimize damage to the root system of each seedling, and to reduce any other unnecessary shock to the plants.
Trick - When the weather won't cooperate at planting-out time, seedlings may be stored for a few days in a cellar, porch or outbuilding kept at 35 deg to 40 deg.
Dig a hole big enough to contain the root ball and deep enough so the plant is settled in at about the same level it rested in the container.
Pour a little water into the hole before placing the seedling. After all the seedlings have been set, give them a thorough watering, without drowning them. A little seaweed extract in that water can help minimize transplant shock.
Another Trick – there is now on the market a new microbial soil amendment product that actually replaces soil microbes that are usually absent from most home landscape soils in this country. A half a teaspoon of this powder in each planting hole is going to give the plants a better chance for long term health.
One trick for protecting new transplants in danger of being hurt by frost is to give them a foliar spray of dilute seaweed extract just before a night of expected frost (1 tablespoon to the gallon of water). Seaweed applied just before frost helps stop cold damage by increasing the sugar content in plant cells, thereby lowering the freezing point of the sap.