In Iowa, the weather in spring is often erratic. Below-freezing temperatures in April and May can follow unseasonably warm weather in late March and early April. The cold spring temperatures can affect plants in gardens and home landscapes. The good news is that cold temperatures shouldn't affect most plants long-term.
How to manage cold temperatures and the damage it can cause to trees, shrubs, fruit trees, spring bulbs, perennials, vegetables, and other plants is provided below.
Freezing temperatures don’t harm dormant plants. However, freezing temperatures can damage new spring growth, especially flowers. As flower buds begin to swell, they become increasingly vulnerable to cold temperatures. They are most vulnerable just before, during, and after bloom. The extent of damage will be determined by the plant species, stage of plant development, and temperature.
The colder the temperature and the further developed the blooms are, the more likely they will be damaged or destroyed by frost or freeze events. If flowers are damaged, this year's fruit crop may be smaller than normal. However, the trees themselves should not be seriously harmed.
Apricots and peaches are more prone to damage from a spring freeze as they bloom earlier than apples, pears, and tart cherries. Colder temperatures will cause more extensive damage. For example, at petal drop on apples, a temperature of 28°F will kill approximately 10% of the developing fruit while a temperature of 25°F will kill approximately 90% of the developing fruit.
For best yields, strawberry plantings are mulched in fall to protect the plants from cold temperatures. If strawberry plantings are prematurely uncovered in March, they will likely have flower buds damaged or destroyed leading to smaller yields in summer. Flower bud loss on plantings covered with a protective straw mulch should be far less.
Yields of summer-bearing red, purple, and black raspberries may be reduced if exposed to below freezing temperatures in April or May. The yields of fall-bearing raspberries that are pruned back to the ground in late winter will not be affected as the canes produce fruit on the current year's growth.
If vines begin to bud or shoots begin to form, grapes are at risk of damage from a late frost or freeze in spring. Often the shoot or bud can recover. If temperatures are low enough, the bud or shoot may die. If the entire shoot is dead, secondary shoots should emerge at the base of the dead shoots in about 10 to 14 days. Flowers for grapes form on new growth so fruit production is rarely impacted by freeze events, although fruit set may be later in the season if new shoots have to form to replace damaged ones.
Blueberry, Gooseberry, & Currants
Freezing temperatures can damage new emerging growth on gooseberry, currant , and blueberry stems. After a frost or freeze event, inspect stems for injury. After shrubs leaf out, remove any dead stems.
The extent of plant damage is dependent on several factors. These factors include temperature, plant species, exposure, and stage of plant growth.
Damage to Flowers & Flower Buds
The most noticeable effect of cold temperatures on trees and shrubs is on the early-blooming plants, such as magnolias and forsythias. Cold temperatures can destroy the open flowers on early-blooming woody plants causing them to turn brown, limp, and mushy. The cold temperatures in early spring may also damage or destroy unopened flower buds on other woody plants that bloom later in spring, such as crabapples and lilacs.
When flowers or flower buds are damaged or destroyed plants may not bloom as well as normal this growing season but will have no long-term damage to the overall health of the tree or shrub.The loss of flowers for the season is unfortunate but provided unseasonably cold temperatures don't come again next year, you will be able to enjoy the blooms next year.
Because the blooms open early in the spring, species like magnolia, forsythia, rhododendrons, and azaleas often lose flowers in late-season cold snaps. When temperatures are extremely low or flower buds are close to opening, the flower buds of crabapple, ornamental cherry, and lilac can be damaged by late spring frosts or freezes.
Some species, like cornelian cherry dogwood and vernal witch hazel, are very hardy and even flowers with snow or ice on them frequently still survive.
Cold Temperatures Can Damage Newly Emerged Leaves
Newly emerged foliage is also susceptible to damage from below freezing temperatures.
Symptoms of freeze damage include shriveling and browning or blackening of leaf tissue. Damaged growth often becomes limp. Eventually, damaged or destroyed leaves may drop from the tree or shrub.
Fortunately, trees and shrubs have the ability to leaf out again if the initial growth is damaged or destroyed. Healthy, well-established trees and shrubs should not be greatly harmed and will produce additional growth within a few weeks. Good care during the remainder of the year, such as watering during dry periods, should aid the recovery of woody plants planted within the past 3 to 5 years. In rare cases, late cold snaps can damage branch tips. Dead branches can be removed, but be sure they are actually dead (i.e., dry and brittle) before making any pruning decisions. Fertilizer is not recommended to aid in recovery from cold damage.
Damage appears to be most severe on Japanese maple (Acer palmatum), northern catalpa (Catalpa speciosa), hackberry (Celtis occidentalis), katsuratree (Cercidiphyllum japonicum), yellowwood (Cladrastis kentukea), smokebush (Cotinus spp.) ginkgo (Ginkgo biloba), hydrangeas (Hydrangea spp.), magnolias (Magnolia spp.), oaks (Quercus spp.), and black locust (Robinia pseudoacacia).
On the other hand, most maples (Acer spp.), birches (Betula spp.), green ash (Fraxinus pennsylvanica), honeylocust (Gleditsia triacanthos), lilacs (Syringa spp.), lindens (Tilia spp.), and viburnums (Viburnum spp.) often experience little or no damage.
Cold damage on conifers and evergreens is rare. Typically the only time conifers see damage due to frost or freeze is when the cold temperatures come in late April through early May as the new growth is forming. If damage occurs on conifers from cold temperatures, it occurs on new growth.
Frost injury symptoms include dying and curling of the newly emerging shoot tips sometimes appearing only on the windward side of the tree. The damage typically appears right away. However, after significant cold temperatures, damage may show up later as wilting and browning of the new growth. Cold damage on evergreens and conifers is easy to mistake as herbicide drift. Herbicide injury will also damage other parts of the plant in addition to the new growth.
Often wilted and brown shoot tips will die. Dead shoot tips might not be evident until 2-3 weeks later. Susceptible plants can become stunted or bushy if injured by frost several years in a row. The plants will not be killed by the loss of some of their shoot tips but may look misshapen as laterals take the role of shoot tips. Live, crooked shoots may also be seen late in June or July. Some dead shoots might remain on trees until late fall or next spring. This can be corrected by pruning out the curled tips back to a live bud or side branch.
Spring Bulbs (Tulips, Daffodils, etc.)
Tulips, daffodils, and other spring-flowering bulbs normally begin emerging from the ground in March or early April in Iowa. However, mild winter weather can encourage premature growth. The early emergence of spring-flowering bulb foliage is most often seen on the south and west sides of homes and other buildings. These areas are usually warmer than the rest of the yard because sunlight is reflected off the building to the ground. In addition, heated basements keep the soil near homes relatively warm.
Early Emerging Foliage Needs No Protection
While the premature emergence of spring-flowering bulb foliage is undesirable, the danger is not as great as it may seem. The foliage of tulips, daffodils, and other spring bulbs can tolerate cold temperatures. If typical winter weather (cold temperatures and snow) returns after the foliage emerges, growth will be delayed. A blanket of snow is especially helpful in protecting leaves from extreme cold.
Damage is More LIkely When in Bud or Bloom
When freezing temperatures hit spring bulbs later in the spring while they are fully emerged and in bud or bloom, occasionally some damage may be seen. Overall even flowers can tolerate light freezes (low 30s to upper 20s °F) and even a little snow. Record cold temperatures (below mid to upper 20s) will damage or destroy many of the flowers of early blooming varieties. The foliage of fully emerged spring-flowering bulbs like tulips and daffodils can also be damaged. Portions of the leaves may turn white and the damaged leaves may collapse onto the ground.
No immediate action is needed to preserve cold-damaged spring bulbs. Despite its poor appearance, the foliage should not be cut back until it turns completely brown. The undamaged portions of the leaves need to be able to manufacture as much food as possible in order to bloom next spring. If foliage is damaged but flower buds were not impacted, plants may still bloom.
The newly emerged growth of most perennials can tolerate temperatures in the low 30s and upper 20s °F. This includes many early emerging perennials such as bleeding heart, daylily, catmint, and columbine. In most cases, early leaf growth on these plants will be just fine with cold temperatures. Freeze damage may occur if temperatures drop into the lower 20s or teens. This damage includes leaves that turn white, brown, and/or collapse.
The extent of plant damage is dependent on temperature, plant species, exposure, and stage of plant growth. Unseasonably warm weather in early spring can encourage lush growth on many perennials, such as hosta, ferns, astilbe, and pig squeak that is more prone to damage from low temperatures. Perennials planted within the last year are most at risk of serious damage.
Most well-established perennials that have been damaged by freezing temperatures should survive. Plants with light damage will continue to grow and the damage will be masked or can be trimmed out. The roots and crowns of severely damaged perennials will send up a second flush of growth in the weeks that follow but plants will be smaller than usual this summer. Good care (for example, watering weekly during dry weather) this spring and summer should help damaged perennials recover.
Asparagus spears that were damaged or destroyed by cold temperatures should be cut off and discarded. New spears will emerge to replace lost ones, but yields may be smaller this year.
Temperatures in the upper 20s or low 30s °F usually causes little or no damage. A hard freeze (temperatures in the mid-20s or lower) is usually required to cause serious damage. Rhubarb damaged by freezing temperatures will have black, shriveled leaves and soft, limp leaf stalks. After freezing temperatures, some gardeners express concerns about the edibility of rhubarb. It's safe to harvest rhubarb if the plants show no signs of damage 2 or 3 days after the freeze event. Damaged rhubarb (blackened foliage and limp stalks) should be pulled and discarded. New stalks that emerge after the freeze are safe to harvest.
Cool Season Vegetables
Cool season vegetables will tolerate temperatures below freezing down to 26°F with little or no damage. Very hardy cool season vegetables like spinach, beets, collards, kale, carrots, and Brussels sprouts, typically only see damage when temperatures get below 26°F. Other cool season crops such as cabbage, cauliflower, broccoli, radish, lettuce, chard, and onion, will see minor damage when temperatures get below freezing but will readily recover over 7 to 14 days.
Any plants that see significant damage or death will need to be replanted.
Cold Temperatures Can Cause Poor Head Development on Cabbage, Broccoli, and Cauliflower
Cabbage, broccoli, and cauliflower that survive cold temperatures often need to be replaced even if the damage is not extensive because they likely will not form usable heads. Exposure to prolonged periods of cold temperatures cause cabbage, broccoli, and cauliflower plants to form heads prematurely (referred to as "buttoning"). Plants that button do not form usable heads.
Warm Season Vegetables
Freezing temperatures will damage potato shoots. Symptoms may vary from blackening of the leaf margins (minor damage) to death of all aboveground growth (severe damage). Fortunately, severely damaged potatoes will send up new growth within 10 to 14 days. There is no need to replant the potatoes.
Other Warm Season Vegetables
Transplants of warm season vegetables such as tomatoes and peppers or seeds of crops like green beans, should not be planted until the danger of frost has passed as they do not tolerate freezing temperatures (below 32°F) and may see damage when exposed to frost (wich can occur at temperatures below 36°F). Vegetables that have been properly hardened before planting are better acclimated to cooler temperatures and are more likely to survive a frost or light freeze with with only minor damage.
Plants with cold damage will have brown or black leaves or leaf edges. Plants may also wilt, become limp and collapse, or die. Plants that have suffered minor damage (minimal leaf damage) should recover within a few weeks. Replant those warm-season vegetables that have suffered major damage.
Be Sure to Harden Annuals Before Placing Them Outside
Any annual planted outdoors should be properly hardened to acclimate them to cooler temperatures. Those annuals that are hardened are more likely to survive a frost or light freeze with little to no damage than those that are not hardened.
Frost Damage & How to Manage It
Damage to annuals from cold temperatures includes brown or black leaves or leaf edges, portions of the leaves turning white, wilting, tip die-back, plant collapse, and death.
Annual transplants set out before before the danger of frost has passed in the first part of May are likely to see damage from frost or freezing temperatures. Impatiens, petunias, marigolds, wax begonias, and other warm season annuals that have suffered major damage will need to be replanted. Those with light damage (brown leaf edges) should recover in 7 to 14 days.
Cool Season Annuals are Less Likely to Be Damaged
Cool season annuals are annual plants that prefer cool temperatures, growing best in spring or fall. They include species like pansy, sweet alyssum, snapdragons, ornamental cabbage, and many others. These species are tolerant of a light frost often surviving down to 28°F or sometimes even 25°F with little damage to flowers or leaves. Most cool season annuals will see no damage during a late-season frost or freeze and those that do see light damage will recover over the next 7 to 14 days. Plants that suffer major damage will need to be replanted.
The cool season turf grown across Iowa, including Kentucky bluegrass, perennials ryegrass, and fescues, are very tolerant of cold temperatures. After a frost or freeze the turf may temporarily stop growing, but little or no damage will occur. Growth will resume with the return of warmer temperatures.
- A hard freeze in early spring damaged the newly emerged growth on several trees and shrubs. Will the plants recover?
- Will trees and shrubs be seriously harmed if they begin to leaf out and are then subjected to freezing temperatures?
- Will fruit trees be harmed if they begin to leaf out or flower and are then subjected to freezing temperatures?
- Our area experienced very cold temperatures in January. What affects will the cold temperatures have on my fruit trees?
- Should I cut off the foliage on my tulips and daffodils damaged by cold temperatures in early spring?
- The newly emerged growth on several of my perennials has been destroyed by a late spring freeze. Will they come back?
- What effect will freezing temperatures have on newly emerged perennials?
- Freezing temperatures in May damaged my potato plants. Will I need to replant?
- Is it safe to eat rhubarb after the plants have been exposed to freezing temperatures?
- What are cool-season annuals?
- Is snow beneficial to landscape plants?
- What shrubs bloom in early spring?
- How can I over-winter container grown trees and shrubs?
- How should I prepare woody trees and shrubs for winter?
- How can I over-winter unplanted perennials still in their containers?
- Frost Dates in Iowa
- Using Deicing Salts in the Home Landscape
- Trees and Shrubs with Colorful Fruit in Fall and Winter
- Overwintering Unplanted Trees, Shrubs and Perennials
- Protecting Trees and Shrubs in Winter
- How to Care for Newly Planted Trees through Winter
- Early Spring-Flowering Shrubs for the Landscape
- Winter Damage on Trees
- How to Prevent Ice and Snow Damage on Trees and Shrubs
- How to Protect Plants from Frost and Freeze