Frost Hardiness

Plants vary in their ability to tolerate cold temperatures and harsh conditions. Different areas in Colorado can have drastically different climates and varying weather conditions each year. All of these factors make it difficult to determine a “safe date” to plant.

We have complied a list of plants ranging from “Very Hardy” to “Tender”. We offer recommendations on planting dates in the Denver area for each group but caution that these dates are approximations. We strongly recommend hardening off your plants (especially annuals) before planting and keeping a close eye on the weather. If needed, covering plants during cold snaps or chilly nights can help protect them.

Very Hardy Plants

Very hardy plants can typically be planted 7 weeks before the average last frost date. In the Denver Metro Area, this is usually after April 1st.

Very Hardy Flowers

Alyssum
Anchusa
Centaurea
Dracaena
Dusty Miller
Larkspur
Pansy
Snapdragon
Sweet Pea
Viola

Very Hardy Vegetables

Asparagus
Fennel
Onions
Peas
Potato
Radish
Strawberries

Hardy Plants

Hardy plants can typically be planted 5 weeks before the average last frost date. In the Denver Metro Area, this is usually after April 20th.

Hardy Flowers

Arctotis
Artemesia
Baby Blue Eyes
Bellis
Calendula
Carnation
Cerastium
Columbine
Dianthus
Diascia
Erysimum
Flowering Kale
Heuchera
Iberis
Lavender
Lenten Rose
Lobelia
Lupine
Matricaria
Myosotis
Nemesia
Osteospermum
Phlox
Roses
Vinca Vine

Hardy Vegetables

Broccoli
Brussels Sprouts
Cabbage
Cauliflower
Kohlrabi
Lettuce
Pak Choi
Perennial Herbs
Radicchio
Rhubarb
Spinach

Half Hardy Plants

Half hardy plants can typically be planted 3 weeks before the average last frost date. In the Denver Metro Area, this is usually after May 1st.

Half Hardy Flowers

Anagallis
Angelonia
Bacopa
Bells of Ireland
Calibrachoa
California Poppy
Campanula
Coreopsis
Cosmos
Datura
Delphinium
Dichondra
Euphorbia
Felicia Daisy
Gazania
Gerbera
Gomphrena
Lotus Vine
Mexican Feather Grass
Ornamental Grasses
Pennisetum
Penstemon
Petunia
Phlox
Pincushion Flower
Regal Geranium
Ruby Grass
Rudbeckia
Sanvitalia
Stocks
Strawflower
Sutera
Sweet Pea
Trifolium
Verbena

Half Hardy Vegetables

Artichoke
Celery
Leek

Tender Plants

Tender plants can be planted when there is no longer danger of frost. In the Denver Metro Area, this is usually after May 20th.

Tender Flowers

Abutilon
Achimenes
Ageratum
Alternanthera
Alternaria
Amaranthus
Angelonia
Argyranthemum
Asparagus Fern
Aster
Banana
Balsam
Begonia
Bidens
Bouganvilla
Bower Vine
Brachycome
Browallia
Brunfelsia
Caladium
Calibrachoa
Calla Lily
Callopsis
Campanula
Canna
Cardinal Climber
Celosia
Chrysanthemum
Chrysocephalum
Cigar Plant
Clematis
Cleome
Coleus
Cordyline
Coreopsis
Crassula
Crossandra
Cuphea
Dahlberg Daisy
Dahlia
Dallas Fern
Daylily
Dipladenia
Elephant Ears
Evolvulus
Ferns
Fiber Optic Grass
Four O’Clock
Fuchsia
Geranium
Gloriosa Lily
Guara
Heliotrope
Hibiscus
Impatiens
Iresene
Iris
Jasmine
Lantana
Lisianthus
Livingstone Daisy
Lothospermum
Lysmachia
Mandevilla
Marguerite Daisy
Marigolds
Mecardonia
Melampodium
Millet
Mimulus
Moon Vine
Monopsis
Morning Glory
Napa Valley Fern
Nasturtium
Nemesia
Nicotiana
Nierembergia
Nolana
Oleander
Oxalis
Pampas Grass
Pentas
Periwinkle
Perilla
Plectranthus
Polka Dot Plant
Polygonum
Portulaca
Rudbeckia
Salpiglopssis
Salvia
Scarlet Runner Bean
Scaevola
Schizanthus
Sedum
Statice
Streptocarpella
Sunflower
Swan River Daisy
Sweet Potato Vine
Tithonia
Thumbergia
Torenia
Trachelium
Trailing Portulaca
Tropical Hibiscus
Tropical Water Plants
Veronica
Vinca
Zinnia

Tender Vegetables

Annual Herbs
Cantaloupe
Corn
Cucumber
Eggplant
Okra
Peanuts
Peppers
Pumpkin
Squash
Tomato
Watermelon

Hardening Off Annuals

HOW TO HARDEN OFF YOUR PLANTS

The gradual process of hardening off is a crucial one, like the slow but steady  way we brace our entry into a chilly lake in summer. Jumping in all at once is a shock to our system, but if we start by sticking a toe in and slowly working our way in, the total immersion is not nearly so stressful.

Plants need a period of time to get used to their new homes.  The adjustment period is called “hardening off.”  We recommend a hardening off period of about three to five days.  This will give the foliage tissue time to toughen up so the plants don’t go into shock.

COLD SPRING WEATHER

1.   Put new plants outside in a place that is protected from sun and wind.  Make sure they get watered as needed.

2.   If nights will be cooler than 38-40 degrees, bring into the garage or house.

3.   Move the plants out a littler farther each day toward the place in which they will be growing.

4.   Leave the plants out at night unless there will be low temperatures.  They still might need covering at night if there is a drastic change in the weather.

5.   Plant on a cloudy day and they will take right off.  Using a root stimulator when planting will help.

WARM SPRING WEATHER

The process above can be sped up, but it is basically the same.

HOT WEATHER

The process here is somewhat different since you are acclimating your plants to hot sun and drying winds.

1.   Put plants in a cool, protected place for a day or two.

2.   Gradually move plants towards their new location over a period of three or four days.

3.   Plant in the cool of the evening or on a cloudy day.

4.   Be prepared to shade with a shingle or board until plants are settled in.

5.   Water as needed.  Always check the soil first.