A primrose plant (Primula spp.) that has survived winter in the garden should have no trouble dealing with the frosts of spring, and primrose flowers reportedly can tolerate temperatures as low as the upper 20s Fahrenheit for brief periods. Greenhouse-raised primroses, however, which probably have never been exposed to freezing temperatures, often are sold in late winter in the north. Such plants should be hardened to the cold before they experience much of it.
Primroses' cold tolerance varies from U.S. Department of Agriculture plant hardiness zones 3 through 11. The hardy species most often sold in grocery or department stores is the polyanthus primrose (Primula x polyantha), which -- according to Cornell University -- is considered perennial in USDA zones zones 3 through 8.
The more tender fairy (Primula malacoides) and poison primroses (Primula obconica), both perennial only in USDA zones 8 through 11, sometimes are sold as decorative indoor plants as well. Considered frost-sensitive, they shouldn’t be transplanted outdoors in zones where they aren’t hardy.
About six weeks before the last spring frost date for your zone, begin taking a hardy primrose outdoors on days when the temperature remains above freezing. Set it in a shaded position and bring it back into the house at night. If you plan to site the plant in a location that is only partly shaded, expose it to a little more sunlight every day. After one week of this partial hardening, begin leaving the plant out at night too, when no freezing temperatures are predicted.
After a week of the nighttime hardening, transplant the primrose into the ground. Choose a location with fluffy soil, similar to woodland humus, where the plant will stay cool and at least partially shaded during the summer. Water it well and mulch it with dead leaves to help preserve moisture in the ground.
Garden columnist Calvin Finch, who writes for the San Antonio Express News, recommends covering blooming primroses whenever the temperature is forecast to drop below 30 degrees F. An easy way to provide such protection is to invert large, empty flower pots over the plants to shield their flowers, removing the pots once the temperature rises above freezing again the following day.