Posted by Brent Wilson on 8/1/2017 to Design Tips
Leaf burn, or what some call leaf scorch, on evergreen plants is primarily caused by cold winter temperatures.
There are two different types of leaf burn and causes of it:
1. Winter Leaf Burn
Winter leaf burn usually happens on mature (hardened off) leaves of otherwise cold hardy plants when direct sunlight warms the leaves of plants when the ground is frozen and the roots of plants can't drink enough water to prevent tissue damage in their leaves. This often occurs when the soil around plants is dry before winter conditions set in.
You can do your best to prevent winter burn by watering well and making sure the soil has adequate moisture during the fall before winter arrives and the ground freezes. If the ground has yet to freeze in fall or winter, and forecasters are calling for a deep freeze, make sure to deep soak the soil around your plants prior to the freeze.
Also, when the soil is moist around the roots of plants the roots are less likely to suffer from cold damage. When freezing temperatures arrive, moisture in the soil will form an insulating sheath of ice around plant roots protecting them at 32 degrees F.
When spring arrives, you can use bypass hand pruners thin or mow damaged plants in the spring to encourage new growth. That said, you might want to wait to prune spring flowering shrubs and other plants, such as azaleas, until after their spring flowering cycle.
2. Frost Damage
Winter leaf burn can also occur on tender new growth that emerges on plants in late winter or early spring, or in fall to early winter. At our nursery we call this "frost damage,' which usually occurs during early to late winter, when an early warm-up can get the juices flowing on some types of plants, causing them to produce tender new growth too early. It can also occur in fall and early winter when plants have produced a late flush of growth and there comes a early frost.
Caution: Healthy plants that have had proper nutrition will be more tolerant of cold temperatures and recover from injury more rapidly. That said, as with late pruning, late feeding can stimulate new growth that can be damaged by an early frost. Therefore, I suggest ceasing fertilization of outdoor plants two months prior to the first average frost date in your area.
Frost damage to new growth usually doesn't cause permanent damage or death to otherwise healthy plants, but it's unsightly, requires removal of burned foliage, and can rob vital life energy from the plant by it to produce new growth all over again. When frost damage occurs on flowering plants they might not have enough energy left to produce flowers in spring or summer.
If there's an excessive amount of new growth on a plant when a late frost and/or severe cold temperatures hit in spring, the cold can work it's way through the soft, tender leaves into the branches and stems of the plant. If and when this occurs it can cause the plant to go into shock, sometimes resulting in death of part or all of the plant.
So, if some of your outdoor plants are producing new growth in spring, and the weather forecasters are calling for a frost, it's a good idea to take measures to try and prevent frost damage from occurring on your plants...especially prized plants such as Japanese maples.
Frost forms when dew freezes on new, tender leaves of plants, so the goal in preventing frost damage is to prevent frost from forming. The best way to prevent frost from forming on the foliage of plants is with a protective cover.
At our nursery we use a sheet fabric called "frost cloth," which is designed specifically for use by plant nurseries to cover large sections of plants that are not under plastic covered greenhouses or cold frames.
To protect plants in home landscapes, a cloth sheet or blanket or plastic film can be used to cover plants. Simply cover the plant with the sheet or plastic film and make sure it is somehow secured to prevent wind from blowing it off the plant. The sheet or plastic film can be secured by tying the ends to the base/trunk of the plant, or the fabric can be draped over the plant and bricks, stones or other heavy object can be laid atop it on the ground.
Caution: While cloth sheets can be left on plants for several days, to protect foliage from sunburn clear plastic film should be removed by mid-morning.
In the event new growth is burned by frost you'll need to remove the damaged growth.
When new growth is burnt by frost in spring, wait for a week or two after the frost has occurred to assess damage and then remove any dead or dying plant parts using your bypass hand pruners. Make your cuts at a point just beyond or below the point where the brown or black dead plant part meets green.
When new growth is burnt by an early frost in fall wait until the plant has gone dormant to remove dead plant parts. Reason being, pruning before the plant goes dormant might force more new growth.
Caution: To avoid stimulating new growth that could be damaged by an early fall frost, cease pruning and fertilization of all shrubs and trees 45 days before the average first-frost date in your area.
Hope this information was helpful.
Plant Long & Prosper!
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