There are guidelines for plant care before they go in the ground. You are no doubt shopping at box stores and local garden centers (please do, support the home-owned shop keeper). Most retailers have plants on shelves inside and outside for sale. For the most part, plants that have been growing outside are fine to take home and plant right away, in other words, no waiting. These plants have been acclimated or the term gardeners say is "hardened off." Because of their time outdoors, they are used to the sun (or shade, depending on the plant type), winds and nights.
Plants that you purchase from inside the store need some special handling at your house. They need to toughened up to handle the elements just like the plants that were outside. Once home, put them outside for a few days in a protected, shady site for a couple of hours, then move them indoors or in to a garage or shed at night. Increase the time and sun exposure each day. After about 5 to 7 days they are ready to be planted. After day 4 or so, you can leave them outside all night, provided the temperatures are in the mid-50s.
BUT, (there's always more), wait and plant most everything until the nights are consistently above 55 degrees. This is especially important for warm-season vegetables like peppers, eggplant and tomatoes. Wait on okra until nights are closer to 60 degrees. This ensures that they won't be cold affected so growth stops then extra weeks are needed for the plant to catch up (some never do). If you purchased cool loving annuals like violas or pansies or cool veggie starts like lettuce or broccoli, they can be planted with nights in the high 40s and above.
BUT, you can always cheat cold temperatures by using row covers or walls of water to keep warm-season plants toasty at night. It's a good idea to keep covers close on hand in case we have some more cold-weather events, through at least mid-June.
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