Even though the average last spring frost is May 16th in Pocatello and May 22nd in Idaho Falls, you don’t have to wait until then to plant many of your flowers. Many annual flowers and vegetables are tolerant of frost and can be planted earlier in the season. Note: Most perennials are hardy and can be planted out beginning in early April.
The following list should help you determine which annual plants are most hardy. Remember, even hardy plants need to be hardened off for a few days if they have been growing in a warm greenhouse. Just put them outside during the day and bring them in at night if there is a chance of frost. After a few days they should be ready to plant outside.
Codes: T = Tender – Plants may be damaged with even a light frost. Wait until late May to plant these unless you are prepared to cover them in the event of frost.
HH = Half Hardy – Plants will take a light frost (probably down to the upper 20’s), maybe more if they are fully hardened off. These are usually safe to plant out in late April or early May.
H = Hardy – Plants will usually tolerate a hard frost (probably to the upper teens) without any permanent
damage. These can usually be planted out in early April.