Garden frost protection: A practical survival guide

I just planted my garden and now frost is on the forecast!?! With the frost-free season quickly approaching, it is time to consider options to protect your tender vegetable plants for brief periods of near freezing temperatures. In many parts of Idaho, spring can feel like an early summer before chilly weather comes for several days or even weeks. Whether you are just starting your garden or have already begun planting, these early season protection techniques will help keep your warm-season vegetables thriving despite frost and freezing conditions. For more information on additional methods, refer to the other IN THE GARDEN articles on extending the growing season.

Understanding Cold Events

Understanding the difference between frost and freeze helps you choose how much protection your plants will need to survive the chilly weather.

Frost

Frost occurs when air temperatures remain slightly above freezing, but plant surfaces cool enough to form ice crystals on the leaves. A frost alert is issued when temperatures are forecasted between 33-36⁰F. While frosts typically cause only minor damage, they can harm newly broken buds and sensitive warm-season vegetables. These events while important to manage are not as significant compared to freezing conditions. Frosts become more common than freezes, as we get closer to Mid-May through the beginning of June.

Freeze

Freezing will significantly impact plants if they survive at all. A plant is considered frozen when the plant’s internal temperature drops below 32⁰F, water inside cells freezes and expands, rupturing and damaging cell walls. Although some plants may recover once temperatures rise, many warm season tender vegetables will not rebound or be productive. Freezing temperatures are more likely to occur before May although occasionally freezing temperatures do occur in May.

Timing and Planning

To minimize risk of freezing and frost, begin planning by checking your local first frost dates, a good resource available online by the National Gardening Association is available at, https://garden.org/apps/frost-dates/ another resource would be to contact your local Extension office. Keep in mind these frost dates are averaged and can vary by about three weeks, so monitor the temperature forecasts closely and even set an alert on your weather app if possible. In South and Eastern Idaho, mid-May to early-June is the beginning of the planting window with the lowest risk of frost and freezing conditions for warm season vegetables.

Primary Protection Strategy

Planting after the frost-free date. Timing your planting for after the last expected frost is the most effective way to prevent frost damage although it can limit the time you can produce and harvest vegetables.

Early-Season Protection Measures

Planting before the last frost free date gives the plants more time to establish their root systems and get a head start on the growing season as well as test the plants’ hardiness for the area it is planted in. If you get excited about gardening and want to begin planting before the last frost date protecting your young plants will be required and here are some measures to protect them.

Harden off Transplants/Seedlings

Hardening off gradually acclimates greenhouse- or indoor-grown seedlings to outdoor conditions- wind, sun, and temperature. This process takes about 2 weeks. These three steps outline the hardening off process.

• Days 1–4: Place plants outdoors for 1–2 hours each day, increasing time each day up to 6-8 hours.
• Days 5–9: Introduce 1–2 hours of direct sunlight daily, then extend sunlight exposure each day up to 5-6 hours.
• Day 10-14: Expose plants to 5–6 hours of direct sun in their permanent location, gradually increasing time of sunlight exposure to 6-8 hours or based on plant’s needs.

If the plants respond by wilting or leaves begin to discolor, reduce exposure time, and extend the process for several days for each step. It is important to ensure the plant’s soil remains moist during this process to ensure good acclimation.

Frost Cloth Covers

This strategy can be used on seedlings, plants already planted in the ground, and container plants. Frost cloth is an inexpensive, breathable fabric that traps warm air while allowing light and moisture to pass through the material. Group plants together and drape a layer of frost cloth over the top and around plants on frost-risk nights to maintain a higher temperature around the foliage. Putting a stake or cinder blocks larger than your plants in the middle of the frost cloth will keep the fabric from smashing the plants. Using stakes or rocks to keep the frost cloth anchored to the ground will help prevent wind and cold damage. Unlike plastic, frost cloth will not create a damaging greenhouse effect when the sun returns the next day. These cloths can be taken off and reused many times in a season and for several years.

Pre-Frost Soil Moisture Management

Another method used on plants in containers or already planted in the ground is to water the plants well before the expected frost or freeze. It may sound counterintuitive, but watering and focusing on the soil around the plants can keep temperatures above the freezing point. The wet soil retains more heat than dry soil because the air pockets become filled with water and water holds onto heat better than air. This method will provide a buffer for heat loss throughout the night and help keep the root zone and plants warm longer. Used in combination with frost cloth, this extra warmth can prevent severe damage even during freezing conditions.

Additional Resources

For more in-depth details on season-extension techniques not covered here explore other “In The Garden” Articles on extending the growing season or contact your local Extension office.