The transition of a potted plant from its container and its growing environment into your garden is what we call transplanting. The future success of any plant depends on a smooth transition. I have seen many plants never reach their full potential, or even die due to improper transplanting. To make a smooth transition, it is important to consider the conditions before and after the transition.
Plants (particularly annuals, and vegetable starts and sometimes perennials) grown for future transplanting are usually grown indoors under lights, on a windowsill or in a greenhouse. The environment is controlled. The temperature of the soil and air is warm, the air is humid, there is no wind and the sun is either not present or is filtered through glass or plastic. In the garden the temperatures are cooler (or warmer), the air can be dryer and there is wind and direct sunlight.
Before we transplant a plant grown in a controlled environment we slowly expose the plant to the conditions it will encounter in the garden. This process is called hardening-off. Sometimes structures such as cold frames are used to start the process. It usually takes about 2 weeks to harden-off a transplant. This means that you should start the process 2 weeks before it is safe to transplant. If it is done in a cold frame, the plants are placed in the cold frame that is kept closed at night and open during the day. Each day the cold frame window is opened longer and longer until the final 3-4 days it is kept open day and night. Or, if a cold frame is not available, the transplants are brought outside for increasing amounts of time starting at about 8 hours during the warmest part of the day. After 1 week the transplants can stay outside for up to 12 hours. During the last week they should be out all daylight hours and finally in the last 3 days they should be out day and night.
The containers in which transplants are grown are relatively small. The sides of the containers change the growth pattern of the root system. When plants grow naturally in the earth (except for say alpine plants that live in pockets of soil between rocks) the root system of a plant radiates outward in all directions from the base of the plant. In a container the roots hit the sides and bottom which causes them to go into a spiral growth form. So when you are transplanting container-grown plants lightly tease the roots before transplanting. By loosening them up, you promote outward growth of the roots into the surrounding soil.
Next put the transplant into the garden. When you remove the transplant from the container gently press in the sides of the container to loosen it. Then being very careful remove the transplant without pulling on the main stem since this could crush it and damage the plant. Use the leaves to pull the transplant out (grabbing as many as possible). Then tease the roots as mentioned earlier. With a trowel or another appropriate tool, remove enough soil so that the roots of the transplant fit into the hole. Make sure that the stem is at the same level in the garden as it was in the container. There are notable exceptions like tomatoes and peppers that send out roots along the stem. Plants like this can be planted up to their first set of leaves. Fill in the space around the transplant and tamp (press) the soil around the root ball making sure you leave no air pockets.
Finally water the plants thoroughly. Here is a trick that I found really works. I water transplants and spray the leaves for the first week with a combination of fish emulsion and seaweed extract. I used to blend it myself but Gardens Alive! has Sea Rich. They call it an anti-stress formula. I have found that it provides the nutrients the transplant needs to grow quickly and adjust to its new environment.
Peter Punzi Horticulture Guy http://horticultureguy.com