7 Ways to Protect Tender Annuals From Spring Frosts

As we move indoor seedlings outside this season, transplant our winter sowings, or direct sow seeds, chilly spring nights and cool mornings may impact tender annuals. 

In late winter and early spring, we’re likely installing cold-hardy annuals to get a jump on warm-weather blooms. With spring frosts or bounceback freezes, these are worth a little extra attention to protect young roots, stems, leaves, and buds. Tender or marginal selections vary depending on climate; even hardy selections like pansies and snapdragons benefit from protection against freezes when newly planted.

Whether the annuals are young and tender or seasonal conditions bring a drop in temperatures, added protection goes a long way in securing the season’s blooms. For good protection, we’ve got simple measures to ease climate fluctuations for a lasting, colorful show.

Frost vs. Freeze

There’s a difference between frost and freeze, and both merit extra insulation for new seedlings and sensitive selections in cold climates. Frost happens when ice crystals form on the surface of leaves, petals, and stems. Temperatures don’t have to reach the freezing point for frost to occur. The National Weather Service issues frost advisories when temperatures fall to 33-36°F (1-2°C) in the right conditions.

A heavy freeze can set in when the air temperature drops below 32°F (0°C). As plant tissues freeze, cellular damage occurs. You’ll see brown, mushy parts post-freeze, indicating the damaged portions are no longer functioning or viable.

In both cases, the upper growth may recover if the roots are intact. Growth will slow, delaying budding and blooming. Some annuals are more susceptible to damage than others, and extra insulation during dips promotes continued development. As warm days lead to a flush of annual growth, an unseasonable cold snap can cause damage.

Tender vs. Cold Hardy Annuals

Tender annuals are frost-sensitive and vulnerable to low temperatures. They bring a burst of single-season color to flower and seed before dying back in extreme temperatures. Spring annuals include cold-hardy ones that tolerate frost and thrive in transitional seasons, as well as those that prefer mild to warm weather.

Both tender and cold-hardy annuals lengthen the bloom display for weeks or months. In late winter and early spring, they bring an early show to welcome the season before heat-loving plants emerge. They supply pollen and nectar sources for bees, butterflies, and birds when many other plants aren’t in flower.

Cold-hardy annual flowers include violas, pansies, snapdragons, calendula, bachelor buttons, sweet alyssum, and sweet William. Hardy annual vegetables include arugula, leeks, parsnips, spinach, and kale. Depending on your climate and stage of the plant (seedling versus established specimen with more extensive roots), they’ll warrant extra coverage in conditions other than a light frost. 

Cold-hardy annuals are more durable and resilient to fluctuations than tender annuals (petunias, impatiens, tomatoes, peppers, and other warmth-loving species). Frost-sensitive annuals this time of year need the most protection.

Water Well

Watering before frosty conditions is beneficial. It creates water-filled stems and roots and provides ambient heat. When water freezes, it produces heat, adding insulation to the plant. Dry soil freezes more quickly than moist soil and can damage roots.

Moisture also supports turgidity (water-filled roots, stems, and leaves) and strengthens tissues against frost damage. It allows natural processes to continue stress-free for regular growing processes. Aim for evenly moist, but not overly saturated or soggy soils (which have an adverse effect).

Water Early

A good rule of thumb is to water early to mid-day to avoid dropping or freezing nighttime temperatures. During cold snaps, daytime watering allows moisture absorption without damaging roots in freezing soils. In warm conditions, it helps with water retention and absorption before evaporation as the day warms. It also prevents damp conditions that promote fungal diseases.

Let water penetrate the soils with a thorough session. Frequent, shallow water doesn’t support strong root development. Young, tender annuals won’t have extensive roots yet, but water enough to soften surrounding soils as they develop. Watering deeply encourages roots to dig deeper into the soil. Deep roots hold water longer, boosting drought resistance, physical stability, and ability to withstand cold and hot temperature extremes. 

The Value of Mulch

Mulch is one of the most protective measures against spring frosts. It insulates roots by regulating temperature in cool and warm seasons and aids in moisture retention. In late winter, it defends against extremes and icy situations. Mulching helps protect roots during frost heaving when soils naturally freeze and thaw due to temperature and moisture changes.

For the best protection, add a two-to-three-inch layer at planting. Leaf mulch, leaf mold, weed-free straw, compost, bark, and aged wood chips are good options.

In the fall, take advantage of the natural leaf drop for a valuable resource across the garden bed. In addition to protection, leaves support soil health and provide nutrition as they decompose, among other benefits.

Employ Cloches

To protect individual annuals against cold, drying winds and temperature dips, a simple cloche forms a mini-greenhouse. A plastic container like a milk jug or two-liter bottle becomes an insulative cover. Placed atop the leafy upper growth, it captures radiant heat to warm the ambient air, and the layer keeps ice crystals from forming on the foliage.

This method works for individual plants but is less effective for a mass planting, where frost cloth is more convenient. You can purchase frost-protective plant covers or make your own. To use a milk jug or plastic bottle, cut off the bottom of the container and slide it over the tender annual for protection. Remove it during the day when temperatures warm above freezing.

Easy Frost Cloth

Frost cloth is a handy way to protect tender annuals. It covers a group or mass of plantings and makes for easy placement and removal on warm days. The thick fabric insulates while allowing light and moisture infiltration, and it folds back easily. 

Different fabric weights correspond to varying temperatures, so you can adjust as is best for your area’s microclimate. Thicker fabric holds more radiant warmth but allows less light; something to keep in mind regarding growth and development during the day.

A floating row cover, tunnel, or hoop system are options for both hardy and tender annuals, herbs, and vegetables in cold climates. You can drape the cloth on top of the annuals, weighting it against winds with anchors like bricks or stones. As it touches leaves, cold moisture penetration is a concern. In this case, lift the cloth with wire hoops. Remove the covering on sunny, warm days, but keep the hoops in place for easy coverage at night until cold snaps pass.

Simple Hoop Tunnels

Protect tender annuals in the ground or raised beds with insulating hoop systems that shield them from frost. Depending on your setup, these can be small or larger scale. Wire hoops or pliable PVC pipe placed onto rebar anchors create a simple hoop. Place frost cloth or greenhouse polyethylene plastic over a series of hoops for a ready-made cold frame. 

The plastic covering creates insulation and allows natural light to penetrate. Vapor barrier plastic from the hardware store works well, too; just make sure the opacity enables light to infiltrate. Since the mini-greenhouse blocks external moisture, keep an eye on the soil moisture for watering needs. Lift the plastic or frost cloth on above-freezing days.

Pots and Containers

Protecting tender annuals in containers is beneficial as they are most susceptible to frost damage. Potted specimens lack the benefit of soil mass, which provides insulation to in-ground plantings.

The annuals’ above-ground situation and thin layer of protection from the pot expose them to the surrounding air temperature. The soil is prone to freeze-thaw cycles, which can lead to root disturbance and damage. Fortunately, there are many ways to get creative in quick insulation for pots and containers. 

Cover and Shelter

A simple thermal blanket, frost cloth, or burlap cover can provide enough insulation to protect the potted annual arrangement from spring frosts and winds. Move them to a wall or near a foundation for added protection. You can also cluster or huddle pots with the most tender and smaller pots in the group’s center. Surround them with the hardier specimens and larger containers.

Wrap

If a heavy freeze shows up in the forecast, extra insulation protects tender annuals from frost, reducing the risk of flower, foliar, and root damage. Wrap the pots themselves—in addition to exposed plant parts—in fleece, thermal, or frost blankets. Just as with in-ground plantings, remove the plant coverage portion on warm days.