The following link is a good gauge to plant seeds by. Notice that on most of the plants the seed germination is optimum at the higher end of the spectrum. If one doesn't want large transplant seedlings just count back from the date of last frost, but, IMO large transplants will produce earlier and longer than the smaller plants due to being older and more mature. If one has room to plant seeds earlier than the recommended 4-6 weeks before last frost, IMO he should do so. Soil temperatures for setting out transplants is different than for seed germination but still very important. Transplanting into a too cool soil will greatly slow down growth on most garden vegetables, but especially on peppers. The warmer the soil is, the better and faster plant growth is. I have found that tomato transplants do OK when transplanted when the soil is 65F but peppers do not do as well. I have found peppers need a temperature of at least 70F and preferably a soil temperature close to 80F for seedlings to maintain their rate of growth.
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