When a cold period follows a long period of unseasonably warm temperatures that initiates new growth, temporary leaf damage may occur if temperatures dip below freezing. The damage appears as white, bleached areas on leaves. Most plants outgrow this type of cold injury but if they do not the damaged portions should be pruned off.
Many perennials and bulbs send up shoots when warm weather occurs in early spring. This foliage is very cold tolerant and not usually damaged by frost.
Frost injury usually occurs on annuals planted outside before the last frost date or after the first hard frost in the fall. Diagnosis is easy because frozen tissue becomes limp, appears water-soaked and quickly turns blackish brown. Leaves may be entirely blighted or portions of the leaf may be spotted. If damage is limited to the top portion of the plant, simply prune off the damage. Severe damage may warrant replacement of the entire plant. After a hard frost in the fall, clean up and removal of dead foliage will help reduce disease and insect problems for the next season.
Several problems may occur when the temperatures are not freezing but dip below the minimum temperature preferred by many tender plants. Tender plant roots may be unable to absorb nutrients from cold wet soils, causing a temporary phosphorus deficiency, which appears as leaf purpling. Flower production is reduced or halted and plant growth may be stunted.
Problems associated with frost damage and cold injury can largely be avoided by waiting until after the last frost date in your area to plant warm season annuals. Plants will become established faster and perform better when planted after the frost-free date when soil temperatures are warmer. Reduce problems by allowing seedlings or newly purchased plants to become acclimated to your garden before planting. Place the plants outside for two to three hours per day and gradually increase the exposure over a one-week period
Injury to flower buds or blossoms during the late winter or early spring season can be from frost or freeze injury. Late spring frost kills young buds and tender new growth in early spring resulting in fewer flowers and later leaf development. Diagnosis is easy because frozen tissue turns blackish brown. Blasted or damaged blooms result from the freezing of flower buds in early spring before or during flowering. Although the flower display for the year may be damaged by freezing temperatures, there is no long-term damage to the plant.