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USDA Plant Hardiness Zones–What do the Zones Mean, and What Information do they Leave Out?

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The USDA plant zones are based solely on mean average low temperature.  What is left out is very important.  The average low temperature is important to know.  This information makes sure you are buying a plant that can take the cold, but there is much more to a plant’s hardiness.  

2023 USDA Plant Hardiness Zone Map for Wyoming; Source: USDA Website

Starting with water, here in the west we are almost always in a drought of some level. The average yearly moisture for Laramie County, Wyoming is 8 to 15 inches. Some places in Natrona County, Wyoming, get as little as 3 inches and up to 12 inches. These numbers include the moisture in snow.  Back east, the average annual moisture starts at 21 inches and goes up to 45 inches, with an average of 38 inches.  This is a world of difference for a plant that tolerates wet soils, but dies in dry soils, but has a USDA Zone for the area. 

Soil pH is another important component to plant growth.  Some plants need an acidic soil, where other plants tolerate a more alkaline soil.  In the west, we deal with an alkaline soil or pH of 7 or higher. This is not information on the USDA plant zones. 

The best example where soil is critical for plant survival are Blueberries. For some reason the big box stores like to sell them to us here in the west. While blueberries are very cold hardy and zone appropriate for the west, they are not tolerant of alkaline high pH soils–they are acid low pH soil lovers.  Trying to change the soil pH is extremely difficult if not impossible.  A fertilizer for acid loving plants is not the answer, it only causes problems in the long run. Blueberries will not tolerate dry windy locations especially during the winter.   Like all fruit bearing plants and trees they need a lot more water than the average shrub or lawn.

If you must grow blueberries, plant them directly into a bag/container of peat moss, never let it dry out. For winter, bring the plant and bag into a shelter out of the wind and never let it dry out.  Use a low nitrogen fertilizer, but one with micronutrients. 

It is much easier to grow a plant or tree that is already adapted to our growing conditions.  It is nearly impossible to change the soil for a plant that doesn’t want to grow here.  It’s better to buy a plant that is more cold tolerant than the zone listed for your area. 

Written by Catherine Wissner, retired University of Wyoming Laramie County Extension Horticulturist.

Additional References:

  • “Use this tool to guide garden and landscape choices – but understand its limitations,” Caleb Carter and Hudson Hill, posted April 7, 2021, https://uwagnews.com/2021/04/07/use-this-tool-to-guide-garden-and-landscape-choices-but-understand-its-limitations/ ↩︎
  • USDA Plant Hardiness Zone Maps — How to Use Them: https://planthardiness.ars.usda.gov/pages/how-to-use-the-maps
  • USDA Plant Hardiness Zone Map for download: https://planthardiness.ars.usda.gov/pages/map-downloads

Two Hardy, Tough Trees to Plant

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A tree that was on the brink of disappearing is now making an amazing comeback.  These are the American Elms, Ulmus americana, they are the quintessential shade tree that keeps streets and homes cool in the summer.  Their tall, graceful limbs and open habit were nearly lost. 

St. Croix (TM) American Elm, Ulmus americana ‘St Croix,’ Photo Credit: Baileys Nursery, 2024

In the late 1990’s researchers found numerous species of American Elms that survived the great die off from the 60’s-80’s.  A die off caused by a fungus (Ophiostoma ulmi and Ophiostoma novo-ulmi) that is carried by a tiny European Elm Bark Beetle, (Scolytus multistriatus).  A beetle that lays eggs just under the bark and inoculates the tree with the fungus, which eventually kills the tree.  

However, several American Elms scattered across the northern U.S., not only survived but thrived with good resistance to the fungus.  A lot more University level research went into propagating and testing these survivor Elms for the nursery trade. 

Why plant American Elms?  To start, they tolerate a wide variety of soils and growing conditions, can deal with drought, wind, minus 40 degrees and take average water. They can get big, up to 60 to 75 feet depending on location and conditions, so plan their placement carefully.  They are long lived and fast growing up to 3 feet per year. Most have an open spreading vase shape canopy.  Dark green leaves in the summer and golden colored leaves in the fall.   More plant nurseries are carrying them. Some varieties to choose from: St Croix, Prairie Expedition, Cathedral, Accolade and Princeton, to name a few of the new American Elms.    

Prairie Expedition Elm, Ulmus americana ‘Lewis & Clark’, photo credit: Baileys Nursery, 2024

A smaller tree to consider is the Prairie Gold (tm/r) Aspen, Populus tremuloides, ‘NE ARB”. This prairie native is disease resistant, fast growing and tolerates a variety of soil types.  It was found near Columbus, Nebraska, by Allen Wilke. This Aspen will get 30 feet tall but only 15 feet wide, is fast growing, but long lived. It will survive minus 40 winters and takes average water.  The leaves are dark green in the summer and brilliant yellow gold in the fall.  

Prairie Gold (R) Aspen, Populus tremuloides ‘NE Arb,’ Photo credit: Baileys Nursery 2024

Most Aspens sold at nurseries are wild harvested from the mountains.  Where the soil is cool and moist with a short growing season. It is very hard to duplicate those growing conditions on the prairie. Wild harvested Aspens are short lived and prone to numerous diseases. Make sure to get a nursery grown Prairie Gold (tm/r) Aspen suited to lower elevations. 

You can find these trees, and many others suited for Laramie County, by browsing our LCMG Bare Root Tree & Shrub Fundraiser. The pre-sale is open until April 20, 2024.

Written by Catherine Wissner, retired University of Wyoming Laramie County Extension Horticulturist and Laramie County Master Gardener.

Native Plant Profile – Ratibida columnifera

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Thanks to our LCMG Native Plants Committee for this month’s native plant profile on the Mexican Hat, Ratibida columnifera. The LCMG Native Plants Committee is a team of volunteers interested in learning more about native plants while sharing that information with others; co-chairs: Ken K. and Laura M.

Photo credit: Laura Mettler, Laramie County Master Gardener

For more information, check out the following resources:

Clover Lawns in Wyoming

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Author: Clavel Guzman, Laramie County Master Gardener Trainee

Clover lawns are slowly growing in popularity due to their low maintenance habit, as well as their ability to be lush and green compared to normal grass lawns. However, is it a good option for Wyoming residents to have clover lawns? Will clover lawns be successful for Wyoming residents? Here are some pros, cons, and other facts to help you decide the best option for your lawn.

Pros

  • Mowing lawn less.
  • Most clover hardy to Zone 3.
  • Requires less water and less fertilization.
  • Can make grass healthier.
  • Don’t need herbicides or pesticides.
  • Helps with erosion control.
  • Attracts helpful insects.
  • Drought tolerant.
  • Pet friendly (resistant to dog urine).
  • Great support with fruit and vegetable gardens.
  • Eco-friendly compared to traditional grass lawns.
  • Improves soil due to the nitrogen.
  • Crowds out weeds.

Cons

  • Reseeding sooner as a perennial *Only-clover lawns need to be reseeded every 2 to 3 years; if mixed with grass lawns, it does not need to be reseeded.
  • Stains clothes easier than grass.
  • Roots creep over edging and can enter into unwanted areas.
  • If you overseed, the lawn can look patchy, lumpy or uneven.
  • During dry spells, will need more water than usual.
  • If clover gets infected with fungus. it is toxic to horses.
  • Weed killers will kill clovers.

Types of Clover

Stock Photo, iPhoto

White clover (trifolium reopens):

This type of clover can be in a full sun or partial sun location, and it will spread quickly and rapidly. It does well with poor soil and outcompetes weeds. Can pair with Red Clover, Kentucky bluegrass, Bermudagrass, red fescue or hard fescue. This clover produces white flowers that are great for pollinators, such as honeybees and butterflies. Can vary in height between 4 to 8 inches.

Stock Photo, iPhoto

Red Clover (trifolium pratense):

Red clover likes to be planted in either full sun or partial shade. This type of clover produces flowers that bloom a reddish-purple and have natural medicinal properties. The flower has a honey-like scent and is edible. This clover can grow taller ranging 6 to 24 inches in height. This pairs well with White Clover, Ryegrass, or tall fescue.

Stock Photo, iPhoto

Microclover (trifolium repents var. “Pirouette” and “Pipolina”):

This clover is the most popular with clover lawns because there are smaller leaves and fewer flowers. While this clover can be planted in partial sunlight, it will grow the best in full sunlight. This clover only grows 4 to 6 inches in height. It is also less prone to clumping than White clover which does help integrate with other grasses. May be paired with Kentucky bluegrass or tall fescue.

Planting

After deciding which type is best for your lawn it’s time to start planting the seeds. The best time to plant any of these clover types is early spring around mid-March to mid-April or when temperatures will stay above 40 degrees F.

To start preparing soil, lightly rake the lawn. Seeding depth should be 1/8- to 1⁄4-inch deep. For the amount of seeds, it will be about 6 to 8 oz of seed per 1,000 square feet.

After sowing your seeds, water the soil lightly and mist until clovers take root. No fertilization is needed at the time of planting or the time it takes for seeds to show up. It will take about 2 weeks to germinate, water daily until you see leaves, reduce watering to about twice a week.

Summary

Are clover lawns a good option for Wyoming residents and will they be successful? Yes, clover lawns are a good option and successful for Wyoming residents. They will hold up in Wyoming’s climate. They are a great low management lawn with lush results.

Additional Resources

  • Forage Identification: White Clover, University of Wyoming Department of Plant Sciences, https://www.uwyo.edu/plantsciences/uwplant/forages/legume/white-clover.html
  • Forage Identification: Red Clover, University of Wyoming Department of Plant Sciences, https://www.uwyo.edu/plantsciences/uwplant/forages/legume/white-clover.html
  • “Should You Move Over to Clover?,” Tony Koski, PhD, Colorado State University Extension Turfgrass Specialist, available for download from https://cmg.extension.colostate.edu/wp-content/uploads/sites/59/2021/02/Should-you-move-over-to-clover_Koski_2020.pdf

No, It’s Not Too Early 

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Author: Catherine Wissner, retired University of Wyoming Laramie County Extension Horticulturist and Laramie County Master Gardener

This in-between season of seed catalogs, but not being able to go out to garden, is a great opportunity to plan your vegetable and/or flower gardens.  The best vegetable varieties and choices will be from catalogs and starting from seed.  

First, put pencil to paper, or use a computer program, put down what vegetables you want to grow. What will the family eat, and do you want to preserve any? Fine tune your list by putting in days to maturity or harvest. That number can be found on the packet of seeds.  Draw out a map of where you want these vegetables to grow.

Seed packets have a lot of important growing information on them. Find the days to fruit harvest or fruit maturity. If the packet shows 80 days to harvest add another 7 to 21 days for seed germination, or what is listed on the packet. Add another 14 to 21 days for the plant to grow to transplant in the garden size.  If planting the last week of May, start your seeds mid- March.  

Photo credit: Suzanne Sauls, 2024

A couple rules for starting seeds inside. Once planted never let the seed starting soil dry out, this requires daily inspections.  Only mist water over the soil, never pour water on. Seeds need to start in a consistently warm environment of 65 to 75 degrees for proper germination.  Only, use good quality seed starting soil, never soil from your garden.  The packet will tell you how deep to plant the seed.   

Tomatoes are the easiest to start, germinating in 5 days, peppers can take 2 to 3 weeks to germinate.  No need to start root vegetables, like carrots or beets, from seed, they have a high transplant mortality rate. 

Your plan must include an irrigation system. Vegetables are not drought tolerant, skimping on water will lead to a low harvest.  Garden vegetable need water every 24 to 48 hours during the growing season.  Never let the garden dry out. The best watering method, a soaker hose or drip tape system that is on a timer.  Timers are inexpensive, battery operated, easy to set up and work consistently, even when you are away.   The worst way to water, is with an overhead sprinkler system. 

PLANT PROFILES

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Blanket Flower, Gaillardia aristata

Plant Profile provided by the LCMG Native Plant Committee

(Co-chairs: Ken K. and Laura M.)

Native perennial, short-lived (about 2-3 years)

Height: 24-36 inches; spread: 12-24 inches

Flowers in early summer until killing frost

Hardiness Zone 3

Full sun

Soil – poor; well-drained; pH not an issue – very tolerant

Water: water well initially for good root establishment; then it is very drought tolerant

Propagation from seeds; or dividing the roots & planting the roots in a new site

Deadheading (removing old flowers) prevents seed formation and additional flowering

Medicinal uses: a tea prepared from the powdered root is used for gastroenteritis; chewed root powder is applied to skin disorders