The weather can be erratic as it transitions from autumn to winter and again from winter to spring. It is common to have below freezing temperatures in late September or early October followed by a stretch of a week or more of warmer temperatures. When this temperature dip happens, protecting plants can be beneficial, allowing to continue harvest and enjoy them in your garden longer.
It is also common to have a period of warmer temperatures in late March or early April that can bring plants out of dormancy followed by below freezing temperatures that can potentially damage the new growth or emerging flowers.
Protecting plants during the season's first frost or freeze event can allow you to enjoy the gardening season just a little bit longer. Below are tips on protecting garden plants from freezing temperatures in fall and spring.
Frost occurs when ice crystals form on the surface of the leaf. This can happen even when air temperatures are slightly above freezing, but the plant surface cools to the freezing point. The National Weather Service issues frost advisories when low temperatures are forecasted to be between 33-36°F. Frosts generally cause minor damage but can cause damage to sensitive species, especially flowers and flower buds that have broken dormancy.
A freeze is more damaging than a frost. Freezes occur when the air and interior of the plant drop below 32°F. When a freeze occurs, plant cells freeze damaging cellular tissue and causing plant tissue to turn brown or black, wilt, and/or collapse.
There are two types of freeze events; advection and radiation. An advection freeze occurs when a dry, cold air mass moves into a region and it remains relatively windy. A radiation freeze occurs during clear, calm nights. During a radiation freeze, cold air settles at ground level while warm air (and with it heat) “radiates” or escapes into the upper atmosphere.
Garden Plants to Protect
Vegetables, annuals, and tropical plants grown outdoors are the most practical plants to protect during an early frost or freeze event. Protecting perennials, woody shrubs, roses, small trees, or other woody plants is not necessary or beneficial. These plants need must experience cold temperatures to help them fully enter dormancy. Additionally, most perennial and woody plants will tolerate temperatures at or just below freezing with minimal damage.
Watch the Forecast
Starting in September, it is beneficial to watch the forecast and take note of forecasted low overnight temperatures. Once temperatures dip to the mid-30s °F, it will be necessary to take action by bringing plants indoors, covering plants, or utilizing cold frames and hoop tunnels.
Bringing plants indoors is the easiest way to protect them from frost or freeze damage. Potted tropical and annual containers can be pulled into a heated room, porch, or into a protected and warm structure, such as a garage, to protect them overnight.
If plants are not easily moved indoors, then they can be protected with coverings. Potential coverings include sheets, blankets, towels, tarps, frost fabric, or row cover material. These coverings help trap the radiant heat from the ground to keep frost from forming on the leaves and help reduce the risk of plants freezing. Use stakes, posts, PVC pipes, patio furniture, saw horses, wire loops, or other structures to elevate the covering so it does not touch the foliage. Remove the covering the next day when temperatures get above freezing.
Cold frames, grow tunnels, floating row covers, and similar structures can also trap radiant heat from the soil to help prevent frost from forming on vegetables or other plants. The coverings of these structures should be opened or removed during the day and pulled over or shut before temperatures drop below freezing.
More information about using cold frames can be found in this article: All About Cold Frames.
Garden Plants to Protect
Vegetables, annuals, and tropical plants planted outdoors early in the season are the most important plants to protect during a late frost or freeze event in spring.
Protecting emerging spring bulbs and perennials is not necessary. The newly emerged growth of spring blooming bulbs, like tulips and daffodils, and the early growth on 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.
Flowers, developing flower buds, and newly emerged foliage on fruit trees, shrubs, and trees can be damaged by a frost or freeze. This is especially true if the cold temperatures are preceded by a stretch of unseasonably warm days that promotes abundant growth and development of flowers and leaves. It is not practical, however, to protect these plants. These plants are not in danger of dying from cold temperatures but flowers can be damaged and fruit production or length of flower display will be negatively affected for that year. Learn more about managing cold damage on woody plants in this article: Cold and Freeze Damage to Garden Plants
Don't Plant too Early
An early warm-up in spring temperatures can make you excited to get outdoors and plant. Annuals, vegetables, and greenhouse-grown perennials planted before the typical frost free date are at much higher risk of being damaged by late frosts and freezes. Wait to plant warm season vegetables like tomatoes, peppers, and green beans, as well as warm season annuals like impatiens, petunias, marigolds, and wax begonias, until after the typical frost free date.
Find your frost free date in this article: Frost Dates in Iowa.
If you want to plant vegetables early in the season, plant cool season vegetables like spinach, beets, collards, kale, carrots, Brussels sprouts, radish, lettuce, chard, and onion. Cool season annuals like pansy, sweet alyssum, snapdragons, ornamental cabbage, and many others are good for early spring planting. These vegetables and annuals tolerate a light frost often surviving down to 28°F or sometimes even 26°F with little damage to flowers or leaves.
Harden Plants Before Planting in Spring
Any annual, vegetable transplant, perennial, or other plants grown in a greenhouse or inside the home should be properly hardened to acclimate them to cooler temperatures before planting them outdoors. Plants that have been hardened are more likely to survive a frost or light freeze with little to no damage than those that are not hardened. Learn more about how to harden plants in this article: Selecting, Hardening, and Planting Bedding Plants
Bring Indoors or Provide Cover
When late spring temperatures are forecasted to be below freezing, just like in the fall, spring plants can be protected by bringing them indoors, covering them with sheets or frost blankets, or utilizing cold frames or grow tunnels.
- Frost Dates in Iowa
- All About Cold Frames
- Cold and Freeze Damage to Garden Plants
- Selecting, Hardening, and Planting Bedding Plants
- Overwintering Unplanted Trees, Shrubs and Perennials
- Protecting Trees and Shrubs in Winter
- How to Care for Newly Planted Trees through Winter
- Winter Damage on Trees
- How to Prevent Ice and Snow Damage on Trees and Shrubs
- How to Protect Plants from Frost and Freeze