Zone hardy plants can be injured during unusually cold temperatures, when temperatures drop rapidly or when temperatures fluctuate from warm to below the upper 20 degrees F. Typically, late spring and early autumn frosts do the most damage to active plant tissues that have not hardened off to withstand the cold.

With below freezing temperatures, water in plant cells freezes forming ice crystals that expand as they freeze and rupture cell walls. The effect is similar to pricking a hole in a water filled balloon. The special ‘glue’ holding the plant cells together leaks out and the plant cells collapse.

Flower buds, vegetative buds, stems, crowns, or even whole plants may be injured. The damage starts with the softest, actively growing tissues like new leaves and tender shoots. Later plants become limp, blackened, and/or distorted. Damage to stems can remain hidden until active spring growth starts. Evergreen shoot tips curl downward and may turn red or brown 1 – 2 weeks after the damage.

Integrated Pest Management Strategies

1. Proper plant selection and placement. Use plants that are zone hardy for your planting area. Avoid placing sensitive plants in low areas where colder air collects.

2. Provide protection. Protect container plants over winter by sinking into the ground, heavily mulching them or moving them to protected areas.

3. Good culture. Avoid nitrogen fertilizer in late summer (after July). Plants respond by producing soft, sappy growth that is vulnerable to cold damage. Mulch after the first hard fall frost to insulate soil surfaces. Feed damaged plants a balanced fertilizer.

4. Don't prune too early. Wait until after the last spring frost date, when the plant is actively growing, to prune dead areas. It is an advantage to be able to see for sure the extent of the damage. Prune the affected areas to a healthy new bud directed outward from the center of the plant. Proper cutting helps the plant by directing its growth without the need for later fix-up cuts. Fewer, good cuts are better than repeated cutting that takes plant energy to heal. Leave old growth unpruned over the winter to protect/insulate the central crown of the plant.

5. Provide protection. Respond to spring hard freeze warnings by covering tender plants with cloth or paper. Plastic is not advised. Cover the plant from top to soil level to trap heat radiating up from the soil.

Organic Strategies

Strategies 1, 2, 4 and 5 are strictly organic approaches. Using an appropriate organic fertilizer would be a viable organic approach to Strategy 3.