Answer: Planting to early can cause transplant shock and stunting, or it can risk them freezing to death or suffering extensive frost damage. Usually it is a good idea to wait until a week or so after you think you have had the last spring frost before you plant tomatoes. They do not tolerate any frost and also suffer a setback if planted into cold soil. If you plant them out and a cold night threatens or late frost is predicted you will have to try to cover them. You can use a frost blanket (sold at nurseries) or you can improvise with an overturned cardboard box or bucket or whatever you have handy. Make sure to remove the cover in the morning before the sun causes it to overheat inside.
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