Cold Hardy Annuals – Choosing Annual Plants For Cold Climates

By: Liz Baessler

Cold hardy annuals are a great way to extend the color in your garden into the cool months of spring and fall. In warmer climates, they’ll even last through winter. Keep reading to learn more about good annual plants for cold climates.

Cold Tolerant Annuals

It’s important to understand the difference between cold tolerant annuals and perennials. Annuals get their name because their natural life cycle lasts for just one growing season. They won’t live through winter like cold hardy perennials will. That being said, they will last much longer into the cold season than tender annuals, and may actually thrive in cool weather.

If you’re growing cold hardy annual flowers, you can’t go wrong with these annuals that tolerate cold:

  • Calendula
  • Dianthus
  • English Daisy
  • Forget Me Not
  • Clarkia
  • Pansy
  • Snapdragon
  • Stock
  • Sweet Alyssum
  • Sweet Pea
  • Viola
  • Wallflower

These cold tolerant annuals can be planted outside in early spring or late summer to provide bright colors at a time when more tender annuals can’t survive. Some other cold tolerant annuals can be sown directly in the ground as seed before the last frost of the spring. These flowering plants include:

  • Marigold
  • Bachelor’s Button
  • Larkspur
  • Sunflower
  • Sweet Pea
  • Black Eyed Susan

Additional Annuals That Tolerate Cold

When selecting cold hardy annuals, nothing says you have to draw the line at flowers. Some vegetables are very tolerant of the cold and provide welcome, intense color. These vegetables can be started early in the spring before the last frost, or in late summer to last through several frosts well into the fall. Some good choices include:

  • Swiss Chard
  • Kale
  • Cabbage
  • Kohlrabi
  • Mustard

If you live in a climate that experiences light to no winter frosts, these plants will do best planted in the fall to grow through the cool months of winter.

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Read more about General Flower Garden Care

Understanding the Difference Between Annual and Perennial Plants

Supertunia® Royal Velvet® petunia (annual). Photo by: Proven Winners.

All flowering plants follow the same basic steps in their life cycle. Annuals complete that cycle in one growing season, whereas perennials live on for three years or longer. But, if you begin studying the labels on your new plant or seed packet purchases, you’ll discover many twists on this basic definition. You’ll come across terms such as “hardy" and “half-hardy" annual, or tender perennial. Plus there’s a third plant category, biennials, that combines some of the characteristics of both plant types.

What is clear when comparing annuals and perennials is that neither is superior to the other. Integrating both types into your garden designs (along with shrubs and trees) gives you the best of both worlds and unlimited options in color, texture, form, and bloom time.

Hardy Annuals

Hardy annuals are typically planted in the fall in their U.S. Department of Agriculture plant hardiness zones and die when summer temperatures become too hot. You can plant these hardy annuals when there are 40 degree temperatures as long as your plants are not brand-new seedlings. Instead, plant annuals from a garden store or nursery that have been growing for a while and are more established with large root systems. Hardy annuals include:

  • Pansies (Viola × wittrockiana). Pansies grow in USDA zones 6 through 10. Planted in the fall, the plants bloom in late winter to early spring and die back in hot summer temperatures.
  • Ornamental cabbage and kale (Brassica oleracea). Typically planted in the fall in containers by a front door, these annuals grow in USDA zones 2 through 11. The plants may survive in areas with mild winters and die back in the spring.
  • Edible, cool-season vegetables. These include cabbage, broccoli and spinach.

Fall Annuals: Extend the Gardening Season

With the first fall frost, tomato, basil and pepper plants shrivel and turn mushy. Most perennials have expended their energy for the season and are brown and dry. The garden can be a downright melancholy place, especially for the avid gardener. By carefully incorporating annuals into your gardening repertoire, though, you can extend the season by several weeks.

Many annuals are frost-tolerant some tolerate a bit of frost, while others bloom cheerfully through a layer of snow and ice. Pansies are probably the most well-known frost-tolerant annual, and they do look beautiful paired with flowering kale, but there are many other options that you may not have explored.

Frost Hardy Annual Varieties

Consider the following frost hardy annuals for your garden and planters:

  • Bachelor’s button
  • Marigold
  • Larkspur
  • Black-eyed Susan
  • Sweet pea
  • Sunflower
  • California poppy
  • Sweet alyssum
  • Snapdragon
  • Bells of Ireland
  • Mignonette

Plant these annuals in the garden in the spring and they’ll continue blooming until the first hard frost, or even after, depending on the variety and care. You can buy most of these plants at a well-stocked nursery, but starting annuals from seed is a fairly easy task. When you plant annuals from seed you save money, have a greater variety of plants to choose from and have direct control over the health of the plants.

Starting Annuals from Seed Outdoors

Most seeds can be sown directly in the ground, and some, such as poppies and sweet peas, don’t transplant well and are best sown this way. To sow seeds directly in the garden, remove weeds and debris in the spring. Spread 1 to 2 inches of compost or manure over the soil. Till the soil and rake it smooth so the surface is very fine. A hard, crusty surface may inhibit germination. Place the seeds 6-12 inches apart and sprinkle them lightly with soil. Most annual seeds are very small and don’t need to be planted deeply, but consult the seed packet directions for planting depth. Water the soil with a light mist sprinkler as needed, to keep it evenly moist.

Sowing annual seeds among perennial plants is a bit tricky, especially if the perennial plants aren’t visible in early spring. Mark the location of perennials with a craft stick in the fall so you know exactly where they are. Sow annual seeds 2 to 4 inches apart between the perennials. Make sure to plant tall annuals at the back of the garden and smaller ones at the front. Select annuals that complement the color scheme of your existing perennials. As the annuals grow, thin them out if necessary.

Starting Annual Seeds Indoors

Some annual seeds, such as snapdragon, phlox and pansy, germinate well in cold soils, but most seeds germinate better when soil temperatures are at least 60 degrees Fahrenheit. Sow these seeds directly in the garden after the soil has warmed up, or start them indoors for a head start.

To start annuals indoors, fill a seed starting tray with a high-quality perlite or vermiculite starting mix. Do not use ordinary garden soil, which is too heavy and may contain pathogens. Poke holes in the starting mix with a pencil or a chopstick and drop the seeds in the holes, spacing them 3 inches apart. Mist the starting mix with a spray bottle filled with water to moisten it. You want it to be evenly moist, but not soggy. Cover the seed tray with a sheet of plastic wrap, which will create a greenhouse effect, conserving moisture and heat. Set the seed tray in a warm location, such as the top of your refrigerator. Check it every day or two and water as needed.

Once the seeds germinate, remove the plastic wrap. Move the seed tray to a sunny location and water the young plants as needed. When the plants stand 2 inches high, thin them to 4 inches apart. Move the plants outdoors when they stand 4 inches high.

Fall Annual Maintenance

Because annuals produce flowers prolifically for a short period of time, they take a bit more maintenance than perennials. Fertilize them every 3 to 4 weeks with a water-soluble fertilizer otherwise, they may not bloom through fall. Another important task is to deadhead annuals. Deadhead, or remove faded flowers, to promote more blooms. Impatiens and sunflowers do not need deadheading, but most annuals will benefit from this practice.

Want to learn more about extending the gardening season with fall annuals?

Winter Annuals for Fall Color [PDF] – University of Florida Extension
Cold Tolerant Annuals [PDF] – University of Minnesota Extension

Frost hardy plants and gardening tips

What is frost?

Most frost damage occurs when sunlight hits the still-frozen plant tissue the following morning. It is the sudden change in temperature, and not the cold itself, which causes the damage, so plants shaded from early morning sunlight will be far less vulnerable. Another effective, albeit more labour-intensive method, is to go outside on frosty mornings with a watering can and use the water to melt the ice before the sunlight hits.

A more insidious hidden frost known as Black frost occurs when the air is generally dry and the temperatures drop below freezing point, internally freezing the plant from the inside and blackening the leaves over a few days.

Hoar frost is far more common in cooler zones in Australia and is recognizable by the appearance of thick white ice crystals developing on top of the plants surface. These crystals form when surface temperature on an object drops below zero and the surrounding water vapour in the air freezes on contact.

Nutrition is everything

You can boost your plants’ natural resistance to frost by applying a diluted solution of SeaMax Fish & Kelp Liquid Fertiliser every 2 weeks, starting at least 3 weeks before the onset of severe cold. You can also help ‘toughen up’ plant tissue in preparation for frost by resisting overwatering or overfeeding from late autumn onward.

Get things covered

Young plants can be more susceptible to frost damage, so providing shelter by erecting shade cloth or wrapping hessian over the growing tips (until frosty conditions have passed) can get them through the most vulnerable stage. Even subtropical trees such as avocado, sapote and macadamia can be grown in areas with quite severe frosts if they are protected through the first two or three years of life.

How low can you go?

Frost tends to settle in lower-lying areas, which is a good thing to bear in mind when assessing which types of plants to put in different parts of the garden. Take note of which parts of the garden are most severely affected by frost, especially in undulating gardens. You don’t necessarily have to get outside on a cold morning and see the frost for yourself — just taking note of areas that have been frost damaged can be a good indication.

Mulching matters

Applying a thick blanket of mulch will help to insulate plant roots from temperature extremes and reduce the plant’s overall vulnerability to cold.

Once frost has bitten…

It may look unsightly, but it’s a good idea to leave any frost damaged plant tissue on the plant until spring has arrived. Dead foliage can shield the remaining plant from further frost damage.

A solution in a pot

Growing plants in pots is an excellent way to protect them from frost. In fact, doing this can allow gardeners, even in cold climates, to grow plants that require subtropical, or even tropical conditions. Allow your plants to reside happily in the garden through the warmer times of year but as soon as frosty temperatures threaten, move plants into a greenhouse or sunny indoor patio until the threat of frost has passed. Bring container-grown plants under cover at night during cold periods. If containers are too large to move, drape them with bubble-wrap or shade cloth at night.

Camellias, rhododendrons & hydrangeas

Many camellia varieties tolerate the cool frosty mornings but the most notable are the winter-spring flowering japonicas. Their glossy dark thick leaves make them suitable for frosty positions and are a gardener’s favourite for their big showy blooms and compact thick growing habit.

Rhododendrons grow best in cool and mountainous areas where frosts are typical. Their trusses of delicate flowers bloom in spring. There are over 800 different varieties including hybrids suitable for more warmer climates near the coast. Rhododendrons and azaleas are often grouped together and for very good reason. All flowering azaleas are classed under the genus of Rhododendrons umbrella. One common way to tell them apart is that Rhododendron flowers generally form on trusses.

Hydrangeas grow well in full sun in cooler climates, tolerating frosts well and semi-shade in warmer areas. Pink, blue or white flowers appear in summer (in alkaline soils flowers will be pink in acid soils flowers will be blue white flowers are always white). Hydrangeas hate drying out so keep them moist at all times. Trim off spent flower heads as they wither. Prune back to two plump buds in autumn and cut out dead wood at the base. Suited to mixed beds and borders around the house in containers on verandahs and patios or as an informal, low hedge. Hydrangeas make excellent, long-lasting cut flowers.

Frost happy plants

Bacon and eggs (Pultenaea villosa) – This evergreen shrub grows to 1.5m. Needs full sun and a light, well-drained soil. Red and yellow pea-shaped flowers appear in spring. Lightly prune after flowering. Drought tolerant. Well suited to native gardens.

Veronica (Hebe spp.) – This evergreen, summer flowering shrub needs full sun and a light, well-drained soil. Grows to 2.5m Cultivars produce white, blue, lavender, purple, violet and pink flowers. Prune lightly in autumn to prevent plants from becoming leggy. Mulch lightly in early summer to keep roots cool. Great in shrub borders and mixed plantings. Veronicas are tolerant of moderately windy and dry conditions.

• The deciduous mulberry tree is cold hardy and produces delicious fruit in spring.

Conifers are renowned for their stamina in frosty conditions.

Hellebores – the winter rose flowers heavily in winter. Loves the winter sun, protect from humidity.

Deciduous trees and shrubs – acers, oaks, flowering cherries and dogwood.

• Many grevilleas and callistemons are frost tolerant. Try G. Deua Gold, G. Gold Cluster, G. Lady O and for heavily frosted areas try G. Fireworks, G.victoriae and G. rosemarinifolia.

Rhaphiolepis indica.

Lavender (Lavandula spp.) This small, evergreen shrub needs full sun and grows to 1.5m. Great in containers and can tolerate some wind in a preferably light, sandy, well-drained, slightly alkaline soil (pH 7.5). Flowers in spring, summer and intermittently throughout the year. Flowers in white and various shades of pinkish-mauve, purple and blue. Trim back lightly after the main flush of flowers has finished to prevent the bush from becoming straggly. Mulch around the roots in summer. Looks great in herb and cottage gardens. Attractive, pale grey foliage. Lavender flowers and foliage are superbly fragrant. The flowers are perfect for potpourri and dried arrangements. Lavender is easy to propagate from cuttings.

Frost hardy grasses

Many varieties of Liriopes are frost hardy and compliment gardens as border plants and mass planting situations. They are evergreen, compact and display dense clumps of strappy foliage and spikes of purple-pink bubbly flowers, depending on the variety, in summer and purple-blue berries in winter. Plant in a full sun to shaded locations in free draining soil.

Lomandra longifolia is very hardy with almost no maintenance. Suited to dry soils and dry climates, but will grow in almost any climate, soil, location or situation. Excellent plant choice to manage erosion control and weed suppression. Grows to about 1.2m.

‘Utopia’ Dianella features stunning contrasting purple and green strappy foliage which will add thrilling colours to any landscape or garden. Suitable for most soils and climates except for hottest parts in Australia. An elegant clumping, ornamental Australian flax growing 50cm wide, with graceful flower stems reaching over 1m in height.

Frost hardy tips

• Grow deciduous plants or perennials that die down completely in winter.

• Protect small plants grown outside with a shelter made of shade cloth, bracken or fern fronds.

• Taller plants prevent cold, frosty air from falling on lower plants, so grow a canopy or hedge near susceptible species.

• Grow vegetables in raised beds to lift them above cold, frosty air if you are not on water restrictions, set the timer on the sprinkler system to come on before dawn to prevent frost damage.

• Select plants from cooler climates such as China, Japan, northern Europe and the northern regions of North America.

Half Hardy Annual Flowers

Annual flowers provide so much instant color in our flower gardens, but it seems to be a matter of "easy come, easy go" when fall approaches: Many annuals turn brown, mushy, and hideous at the first whiff of frost on the lawn. However, not all annuals are created equally in this respect. A number of annuals, referred to as half-hardy annuals, can tolerate several light frosts in the garden, potentially extending the beauty in your landscape for an additional month or longer.

Why are some annual flowers more tolerant of frost and cold weather than others? Part of the answer lies in the plant's metabolism. The proteins, sugars, and moisture content of the plant affect its ability to withstand cold weather without demonstrating symptoms of cold injury, such as white or brown blistering or dead, water-soaked tissue. Eventually, most annuals succumb to damage caused by ice crystals, which form and puncture cell membranes at temperatures below freezing. Only truly cold-hardy annuals, like pansies and violas, can bounce back after a hard freeze. However, these eight annual flowers can form the foundation of your early spring and late fall flower garden, as they will tolerate light frosts without damage.

Watch the video: Growing a hardy annual cutting patch

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