This must certainly vary from region to region. I can only speak for mine.
Pepper seedlings are very sensitive to cold temperatures. Weather is highly variable People, (especially gardeners), are anxious to embrace any sign of warm weather as a sure sign that spring has arrived. Don’t get caught in this pitfall!
In my area (30 miles south of San Francisco, California) I don’t plant peppers in the ground until the nighttime temperature is consistently above 55 degrees fahrenheit all night long. For me that means a planting time from mid April to early May. I’m fortunate in that California grows a lot of peppers commercially, and I can always gauge my judgment against the planting times of commercial growers in the central valley (warmer than my area by several degrees). They plant around May 1, and even though I’m cooler by some and I plant a little earlier, I don’t suffer the risk of significant financial loss if the weather turns unseasonably cool.
Why is 55 degrees fahrenheit the critical parameter? I’m not sure that it is. It works well as a indicator in my area, though it may not necessarily be the actual parameter of importance. It could be soil temperature, for example, or something else I haven’t thought of. Bottom line is that peppers want warm temperatures and planting too early has negative effects..
Unfortunately, there are other popular indicators that are unmistakably wrong in regards to planting times. The worst indicator is the availability of pepper transplants at local stores. I start seeing these in early march (!) with the largest preponderance in late march. By the time the proper time arrives they are nowhere to be seen, apparently sold out to a bunch of people who are bound to be disappointed.
So what happens when you plant too early? Nothing! The plants just sit there if you’re lucky – they get eaten by bugs if you’re not. Meanwhile they plants still in pots and brought in at night continue to grow and get stronger.
Planting in the ground
When the weather is consistently above 55 degrees at night I start planting the seedlings into the ground. The ground has already been prepared by digging in a 2 inch layer of compost and a dos e of slow release vegetable fertilizer with my Smith and Hawkins digging fork. I dig down 10 inches making a raised bed of loosely packed soil. Once dug I never walk on the raised bed.
Some like to use the double dig method and dig down 20 inches. If you want to do this that fine – but for me it is just too much work for my 1300 square foot garden.
Each pepper is transplanted into the ground using a 12 to 18 inch spacing. I use smaller spacing for small plants, and a larger spacing for the large plants (such as c. Baccatums). Each plant gets a “tomato” cage for support and I take the plant label, bend it around the top rail, and staple it in place.
Disease and pest advice
In my area the only pests of major concern are aphids and slugs/snails. Both of these are only a significant threat in spring and then again in late fall. I use pyrethrins for aphids – snail bait for slugs/snails.
When leaving seedlings out all night (when the weather is warm) I put the flats on a pair of two by fours spanning two saw horses. This makes is hard for the snails to find. Also, I move the sawhorses regularly to avoid letting the snails mark a slime trail to my plants.