Quick Facts...
- Low humidity, fluctuating temperatures, a short growing season, poor
soil characteristics, watering restrictions (in some cases), wildlife,
wildfire, and drying winds make gardening in the mountains a challenge.
- Selecting plants that tolerate these conditions is key to Colorado
mountain gardening.
- Gardeners who are patient, know how to select plants that will do
well and manipulate the soil and microclimate, will be amply rewarded.
Gardeners new to mountain communities of Colorado often have trouble
getting plants to thrive or even survive. More often than not, they previously
gardened where you stick a plant in the ground and it grows. Many gardeners
try for a while and then give up.
There is no doubt that gardening in the mountains of Colorado can be
challenging, particularly above 7,500’. Sunlight is usually of high
intensity and the humidity generally is low. Combinations of cool nights,
a short growing season, drying winds, steepness of slopes, aspect, topography,
and soil all influence how well plants perform in this climate. Most of
these challenges can be overcome with proper site preparation and plant
choices.
Site Choice
To determine where to plant your garden, first evaluate your site. The
best place to grow flowers is in a site that already supports some grass,
wildflowers, or even weeds. This will usually be in a fairly sunny, open
area. If the area has weeds, control them before planting something new.
Aspen groves are an ideal environment for many plants, but other open
areas also work well.
If dense evergreen trees cover your desired garden area and there is
little vegetation growing underneath, most plants are unlikely to thrive.
It may be necessary to remove the conifers and add organic matter to make
these areas plantable. Likewise, if the soil is very rocky and there is
no existing vegetation, increasing the organic content of the soil is
critical.
Soils
There are two major types of soil found in the mountains. Light-colored
decomposed granite soils, are low in organic matter, dry out quickly,
and do not absorb heat well. They are usually high in most nutrients except
for nitrogen. Clay soils are also high in nutrients, but generally have
poor drainage.
Soil preparation is often the key to growing healthy plants in the mountains,
particularly for non-native plants. Native plants are often adapted to
leaner soils (lower in organic matter), and may ‘flop’ or have
a shorter life span in well-amended soils.
In general, it is beneficial to add organic matter to any type of mountain
soil, although in poorly drained soils it is best to add some each year,
rather than all at once, in order to avoid salt buildup. Incorporate 2
to 3 inches of organic matter (or 3 cubic yards per 1,000 square feet
of garden), such as alfalfa pellets, compost, or aged manure, to a depth
of 6 to 12 inches. Avoid using Colorado mountain peat, as it is a non-renewable
resource, has too fine of a texture, and is alkaline. It is best to work
organic matter into the entire area that will be planted. If this is not
feasible, dig a larger than necessary hole, and amend the backfill with
20 percent soil amendment. This also helps to prevent a ‘soil texture
interface’ when planting nursery-grown container plants. The soil
around the roots in a container is often high in organic matter, while
the native soil can be lower in organic matter and may be a different
texture. This soil texture interface may cause a zone beyond which the
roots will not grow. Test the soil after adding organic material for nutrient
deficiencies. Contact your Colorado State University Extension county
office for soil test information. For more information on amending soils,
see fact sheet 7.235,
Choosing a Soil Amendment.
Raised Beds
Raised beds can solve many problems for mountain gardeners. Raised beds
can be created with good, weed free soil, and are especially beneficial
if soils are poorly drained or are very rocky and hard to dig. They also
warm faster in the springtime and can help to protect the plants from
burrowing rodents if a wire mesh is tacked onto the bottom before the
soil is added.
Microclimates
The successful mountain gardener learns to exploit or create microclimates.
For example, gardens placed in full sun (southern exposures) will have
a longer, warmer growing season than other exposures. These warm or hot
microclimates are the places to experiment with plants that need more
heat during the growing season to come into flower before frost. If the
site is protected in the winter, this is also a place to experiment with
less hardy plants. Another good site for more tender plants is in front
of rock formations or walls (natural or created) where the thermal mass
can raise winter temperatures.
Because plant growth is slowed by cool mountain soil temperatures, creating
a perennial bed that slopes towards the south or using raised beds will
cause plants to grow faster and emerge earlier in the spring due to the
increased soil temperature. These plants may be vulnerable to late spring
frosts. Gardens on south-facing slopes are warmer and drier than gardens
on north-facing slopes of the same valley at the same elevation.
Some mountain areas have a reliable blanket of snow over much of the
winter. This acts as insulation and may allow less hardy plants (zone
5 or 6) to overwinter. Snow blankets can be encouraged in specific locations
by putting up a snow fence; this will cause snow build up on the lee (downwind)
side of the fence. This same snow blanket, however, may cause some xeric
plants to rot out during the winter, even if they are cold hardy.
Also consider the flow of air; at night, cool air drains down to low
spots. Valley floors may be over 10 degrees F cooler than surrounding
gardens on hillsides above the valley floor.
Strong winds can cause plants to dry out. Dessiciation can be reduced
by using fences, trees, or shrubs to create a wind barrier.
Plant Choice
Even though many mountain gardeners live in wooded areas, ‘woodland
plants’ are seldom good choices – this term in catalogues usually
refers to Eastern woodland conditions (moist, organic rich, acid soils,
and humid air). We have few to none of those conditions in our mountain
areas.
Plants with smaller leaves will often require less water, and will also
experience less damage from hail.
Be cautious with late-blooming plants or plants that are heat-lovers,
as they probably won’t bloom before frost. Flowering usually is delayed
four days for every 400 feet rise in elevation, and frost comes earlier.
Plants that bloom in spring to early summer (cool season plants) are more
reliable.
In general, choose plants that are hardy to zones 2 to 4. Low temperatures
are not the only factor in whether a plant will overwinter, however. Other
factors include day length, source of plant material, recent temperature
patterns, rapid temperature changes, soil moisture, wind exposure, sun
exposure, and carbohydrate reserves.
Native plants are some of the best plants for the mountains because they
are already adapted to the harsh conditions.
For further information on plant choices, see fact sheets:
- 7.406,
Flowers for Mountain Communities,
- 7.423,
Trees and Shrubs for Mountain Communities,
- 7.413,
Groundcovers and Rock Garden Plants for Mountain Communities,
- 7.421,
Native Trees for Colorado Landscapes,
- 7.422,
Native Shrubs for Colorado Landscapes,
- 7.242,
Native Herbaceous Perennials for Colorado Landscapes, and
- 7.233,
Wildflowers for Colorado.
Local nurseries and garden centers (rather than nationwide chain stores)
are also good resources, as they have more knowledge of local growing
conditions.
Planting
In mountain areas, the best time to plant flowers is either immediately
after the last frost or during the rainy season. Planting in late summer
or fall decreases the chance of survival, especially for borderline-hardy
plants, and is less preferable. Gradually harden off (acclimate) nursery
or greenhouse grown containerized plants, especially if they were purchased
at lower elevations, or choose plants that have been grown outside at
local nurseries. About two weeks before the anticipated planting time,
reduce the amount of water the plants receive and expose the plants to
increasingly longer periods of outdoor conditions. Start by placing the
plants in a protected location, and gradually increase the exposure to
sun and wind. Be prepared to provide temporary cover (frost caps, floating
row covers, or even bed sheets) if the temperatures threaten to dip below
freezing at night.
Watering
Determine your source of water. If it is a well with a household-only
use permit (where no outside watering is permitted; this applies to most
wells drilled after 1972 on less than 35 acres, see fact sheet 6.700,
Private Wells for Home Use), you can minimize or eliminate
your water inputs by ‘planting with the precipitation.’ Plant
drought-tolerant plants when the rainy season begins in your area which
is often in early to mid-July. Plants should be watered on the day they
are planted and then mulched to retain moisture. Trial gardens indicate
a very high survival rate when planted this way (visit the High and Dry
website at www.extension.
colostate.edu/gilpin/highanddry.html). Wildflowers and native grasses
sown in the fall also have little to no watering needs, see fact sheet
7.233, Wildflowers
for Colorado.
If you have city water, or a well that permits outside watering, you
may want to let the water warm up before watering. Mountain water is often
very cold and can decrease the soil temperature, slowing growth.
Mulches
Use organic mulches such as weed-free straw, bark chips, or shredded
bark to protect plants from severe drying where snow cover does not persist
during the winter, and to increase soil moisture and decrease weeds in
the summer. Organic mulches can delay spring growth; remove them in early
spring to allow the soil to warm up. You may need to use netting or branches
to keep the wind from blowing the mulch off. Pea gravel or rock mulches
are more wind-resistant, provide frost protection, increase the soil temperature
for enhanced growth, particularly if it is dark-colored, and may allow
self-seeding of plants.
Wildlife in the Garden
Excluding animals such as deer, rabbits, voles, and ground squirrels
from the garden is the most effective long-term solution. Each animal
requires a somewhat different technique (contact your local Colorado State
University Extension office for information, or visit www.extension.colostate.edu/wildlife/).
If this is not possible, plant resistant plant species. While nothing
is foolproof, in general, most wildlife will avoid plants that are very
aromatic; have prickles or spines, tough, leathery leaves, milky sap;
or are toxic. See fact sheet no. 6.520,
Preventing Deer Damage for some plant suggestions.
Fire Mitigation
Many mountain homes are surrounded by wildfire-susceptible forests, so
fire resistance should be considered when landscaping. In general, flammable
plant materials such as conifers should be removed within 15 feet of the
home, and nothing should be planted within 3 to 5 feet of the house, particularly
if it is sided with a flammable material.
For more information, see the fact sheets 6.302,
Creating Wildfire-Defensible Zones, 6.303,
Fire Resistant Landscaping, 6.305,
Firewise Plant Materials, and 6.306,
Grass Seed Mixes to Reduce Wildfire Hazard.
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