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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.

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. 

Extreme Weather and Trees

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Extreme Weather and Trees

Written By

Catherine Wissner

Laramie County Master Gardener and Retired University of Wyoming Laramie County Extension Horticulturist

Saturday, January 13, 2024, the temperature registered minus 20 degrees…add wind and it’s dangerous to be outside. So how does -20 degrees plus wind impact plants? A number of factors play into plant hardiness and surviving Wyoming winters.

The most important is planting USDA Zone 4 and Zone 3 Hardy Trees and Shrubs which are better equipped to survive Wyoming Wind and Arctic Cold Fronts. Wind, regardless of temperature, can dry out and kill tender twigs and less hardy branches. Zone 5 plants will only survive with good protection from those winds — maybe. An article1 published in April 2021 in the University of Wyoming AgNews by Caleb Carter and Hudson Hill, provides additional information as to why the USDA Plant Hardiness Zone Map (which recently listed Laramie County as Zone 5a-5b) has its limitations when choosing plants for landscaping and gardening.

Tree roots are active year-round, always looking for a drink of water. In early October, it’s important to give trees and shrubs a good deep drink of water (no fertilizer). You’ll want your trees to go into winter well hydrated. If possible, give your trees and shrubs a drink when the winter weather warms above 45 degrees F. Watering goes a long way for tree survival by keeping tender twigs more hydrated and better able to withstand extreme weather and drying winds.

For additional information on the effects of severe freezes on trees, please check out this article2 by Laurie Stepanek, published February 16, 2023, in the UW AgNews.

  1. “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/ ↩︎
  2. “Understanding the Effects of Untimely Severe Freezes on Trees,” Laurie Stepanek, posted February 16, 2023, https://uwagnews.com/2023/02/16/severe-freezes/ ↩︎