When should you transplant? There are a number of factors. Firstly, your climate/region. For example, the average last frost date for the Upstate of South Carolina is much earlier than New York. Find out what planting zone you live in and start there. Secondly, you should pay attention to the weather forecast. If you know that a cold snap might come through, you certainly don’t want to transplant a warm-weather plant. Thirdly, know when to plant certain types of plants. Some plants like your brassicas (cabbage, broccoli, etc.) or lettuce do fine with a light frost, but warm-weather-loving plants like tomatoes and peppers really like warm temperatures with warm soil and will do much better if you wait until well after the last frost (in this area, these plants do better when transplanted after May 1). There are a number of planting charts online at your disposal (at the end of this post I will include links for our local extension office and the online Farmers’ Almanac).
In case you are wondering what we do here at Harp & Shamrock Croft, we transplanted our broccoli and lettuce in the middle of March (and covered them up during those couple of very cold nights at the end of March); we will transplant our squash, zucchini, and cucumbers anytime now; and we are planning to begin transplanting our tomatoes and peppers on May 2. Herbs can also go into the ground at any point now. Any other crops either have been started or will be started from seed (peas, beans, winter squash, okra, etc.).
Before you transplant anything, you need to harden it off. Seedlings that have spent their little lives indoors need to become acclimated to the elements in order to avoid shock during transplanting. You do this by exposing them to sunlight gradually over the span of a few days or a week. Set your plants outside (an overcast day for the first day is ideal, but, of course, we can’t control the weather) for a few hours the first day and then lengthen that until they have spent a full 24 hours outdoors. At this point they should be ready to go into the ground.
A tomato plant that has not been properly hardened off may develop sunscald (which is basically sunburn for a plant). Here is an image of a tomato plant with sunscald:
The plant can recover from sunscald, but it will probably not bear as much fruit as a healthy plant.
Once your plants are hardened off and ready to go into the ground, it’s a good idea to water them well and plan to give them a little fertilizer either a few hours before transplanting or even right at transplanting. Here we give each tomato a little bit of bloodmeal in the hole before placing the plant in the ground. Don’t overdo the blood meal, though. Too much nitrogen is not good for a plant, and it also may encourage foliage growth but not a heavy yield of fruit.
At this point you are going to want to learn what depth to plant, and that depends on what you are planting. Tomatoes, peppers, eggplant (and possibly others) like to be planted very deep. You may also need to stake these at this point, depending on their size. Other plants like squash, cucumbers, and zucchini don’t need to be planted quite as deep.
Dig your hole first, then carefully remove your plant from its container, carefully press the roots slightly to get them ready to take in their new home, place your plant, and cover up the hole. Be careful not to pile too much mulch (if you have mulch in your bed or garden) around the stem at this point. You need to be sure water can easily reach the roots and also keep the base of the stem free to breathe.
After you have transplanted, give your plants some water. Watch them closely for a few days to ensure they have taken well. We have some personal experience in this area where one year we nearly lost our tomato crop due to a nitrogen deficiency. After watching our transplants for about a week, we finally discovered the issue (with help from experts!) and were able to correct it, and the plants all bounced back and gave us a tremendous harvest. Plants are resilient and will recover, but you need to be in tune with your garden.
Probably the best advice I can give you at this point is to start familiarizing yourself with your available resources. Learn to use the Farmers’ Almanac, your local extension office (Clemson for those of us in SC), and anything else you find in a good reliable book or on the Internet. There is so much information out there and so many more experienced farmers from whom to learn. I’m merely giving you some basics to get started, but gardening is an ongoing learning experience.