Planting Info

USDA Hardiness Zone Changes

By Carl Wilson, Colorado State University Cooperative Extension Horticulturist, Denver

The USDA plant hardiness zones, last updated in 1990, are about to be revised. Gone will be the “a and b” zones. Four more numbered zones will be added for the semitropical portion of the country for a total of 15 zones.

For us in Colorado, we will go from a mountain zone 3, foothills zone 4 and plains zone 5 state to a predominantly zone 4,5,6 state. Only the very highest mountain areas (example Leadville at 10,000 ft) will be zone 3. The mountains and foothills will be a mix of 4 and 5, the populated Front Range zone 5 with zone 6 infiltrating the southern areas plus “heat island” cities of Denver and Ft Collins, and the plains a mix of 5 north and 6 in the south (Ark Valley).

The reason for the revision is the markedly higher temperatures seen in the climate data collected from 1987 to 2001. A look at the national map reveals zone creep northward confirming what you have heard discussed in Denver for the last 5 years, “this is a zone 6 plant but don’t be afraid to try it.” Now we will officially be zone 6.

The other nice feature about the new map is that it has county lines on it. The USDA ARS (Ag Research Service) contracted with the AHS (American Hort Society) to crunch the data and create the map. The revision is due to be announced later this year.

  • A color draft version of the map is available on-line as a PDF file (Adobe Acrobat) on the AHS website, The map is well worth a look. Be forewarned it is a large file to download if you have a modem connection and better suited for those with DSL service. Even if you don’t download the map, the article about hardiness zones is very interesting. Note that the AHS site also soon will include a searchable by zip code feature to find hardiness zones similar to the heat zone search feature now on the site.


Spring Tree Care

How can the homeowner keep shrubs and trees healthy during the spring and throughout the year?

For an answer to this question, it’s important to know something about the needs of healthy plants. Facts to consider include a plant’s capacity to make and store carbohydrates, enough soil moisture, and soil that has adequate nutrients and is conducive to plant growth.

Capacity to make and store nutrients: This is important to all plants and it is not as simple as it may seem. Each perennial plant must be able to store adequate carbohydrates, not only to reproduce leaves for each year, but also to “hold in escrow” the energy needed to grow new leaves, if they are killed by frost or destroyed by wind or hail.

If graphed, the carbohydrate storage curve would be high in early spring, just before trees leaf out. After leaf-out, it would plunge (because the tree has used a lot of stored food energy to put on new leaves). Then, in midsummer or later, the curve would rise again, as the tree begins to build new food stores.

Trees and shrubs use stored nutrients in early spring. By the end of spring, after a tremendous growth spurt, trees have used up a lot of these nutrients. A healthy tree will begin, through the process of photosynthesis, making new supplies of nutrients (carbohydrates).

Though summer is hot, the healthy plant will continue to make and store nutrients sufficient to carry it through the winter. In fall, plants begin to lose their leaves and go dormant for winter, and the tree’s food-making capacities slow down.

By knowing this cycle, it becomes apparent that the plant must be healthy enough to manufacture, store and use adequate nutrients throughout the year. If it isn’t, you will end up with dead branches or even a dead tree. In some cases, a tree may have just enough food stored to begin leafing out, but not enough to continue growing. In that case, the tree will die. Proper plant care, year-round, should prevent this from happening.

Soil moisture: Too little or too much moisture will result in a tree dying back or dying off. As a rule of thumb, soil needs to be moist to between 12 to 18 inches of depth for most trees and shrubs. The only way to check moisture depth is to check by careful digging or by using a soil probe after watering the root area.

Don’t assume you are watering a tree when you are watering your lawn. Most of the water may go to the lawn, which has many roots competing with tree roots. Thatch in the lawn acts to repel water, and different soil types make water penetration very difficult in many cases. Soaker hoses and root waterers can be useful tools for applying water.

Be sure to apply water during extended winter dry periods. This is vital to good tree health.

Soil types: Soils can vary greatly within a short distance. Generally, Front Range soils tend to be clayey and alkaline. However pockets of sandy soils can be found in some areas. You need to ascertain what type of soil you have and take steps, gradually, to improve it. If yours is a clay soil, aeration will help provide oxygen needed for optimum plant health.

Soil nutrients: In general, trees do not need as much fertilizer as do lawns. However, in our generally high pH soils, nutrients, such as nitrogen, iron, zinc and manganese, can be added. Note the color of leaves and needles. If they look sickly or light colored, that is a clue that additional nutrients may be in order. If you are concerned about soil health, you might consider having your soil tested.

By understanding these and other plant needs, you will know how to provide healthy plant care, not only each spring, but throughout the year.

Photograph courtesy of Judy Sedbrook.

Watering Newly Planted Trees

By Dr. James Klett, Dept. of Horticulture and Landscape Architecture, Colorado State University

If trees are to survive, the gardener needs to understand the basics of watering. Incorrect watering can cause more harm than good.

Overwatering, very common with newly transplanted trees, forces oxygen out of the soil around the roots and can lead to death of a tree. A good indication of too much water is yellowing of the foliage that develops first on the inside leaves and progresses to outer leaves.

This leads to the question – when and to what extent should I water? It is important to determine moisture levels of the soil in the root zone. Most roots in Colorado’s claylike soils are 6 to 10 inches below the soil surface. It is easy to determine soil moisture by carefully digging down 6 to 10 inches. If soils feels powdery or it crumbles, it is time to water. If it clings together when squeezed, it contains adequate moisture.

To water a newly transplanted tree, you might want to use a soil needle or root feeder attachment to a garden hose. This allows some water movement directly to the root zone. Insert the soil needle in a zigzag pattern around the root ball of the transplanted tree. This ensures that all new feeder roots outside the original root ball receive adequate water.

Water newly transplanted trees thoroughly to late August, than gradually cut back on water to allow for “hardening off” before cold weather sets in. After the leaves have dropped off, continue watering if natural moisture is inadequate.

To help reduce moisture loss, mulch around newly transplanted trees. These watering techniques will help maintain a high survival percentage to your newly transplanted trees.

When you transplant a tree, how much thought do you give to how it was grown? Does it make a difference whether it is a balled and burlapped tree, if it was grown in a plastic container or in a grow bag?

Perhaps not, but research underway at Colorado State University could change that. The Department of Horticulture is measuring and comparing the growth differences of B & B trees or those grown in a plastic container or grow bag. They want to know how the trees will respond under varying irrigation treatments. When they have completed their research, Colorado consumers will know more about what happens after trees are transplanted.

Although the study is not yet complete, there are some early observations:

  • During the first year, the balled and burlapped transplants showed the least new growth of the three production methods.
  • The production method is inconclusive as far as determining which trees showed the most new growth. Plastic container ash grew considerably more than the fabric container ash. Fabric container oak grew more than the plastic container oak. There is no measurable difference between the new growth of the plastic container and the fabric container pines.
  • Fabric container trees required more frequent irrigation than did the balled and burlapped trees.
  • Under high temperatures and drought conditions, fabric container trees showed stress earlier than did the B & B or plastic container trees. This was especially true with the ash, but not so obvious with the oaks or pines.
  • Because of the vigorous ash root system, the fabric bags were more difficult to remove from the ash than from the oaks or pines. Thus, more fine roots were removed at transplant time; this may account for the high stress observed with fabric container ash over the oaks and pines.
  • No fabric container trees were lost at transplant, while some B & B and plastic container trees were lost.
  • The fabric container trees were much easier to handle, as the root balls were much smaller and lighter than the B & B or plastic container tree rot balls.
  • The condition of the root ball is critical to successful transplanting any tree. Trees with damaged or dry root balls experience additional stress after transplant. A number of trees were lost in this experiment because of rootball problems and their data has been discarded from this study.

Think Before you Plant!

Robert Cox, Colorado State University Cooperative Extension agent, horticulture

If you’re thinking about planting trees or shrubs this fall, be aware that in the metro area, they need to be in the ground by October 15. At higher elevations, planting must be done sooner.

Though spring is the best time to plant, fall works well if you’re willing to properly prepare soil, and to water the new transplants through dry winter periods.

Before purchasing trees or shrubs, ask these ten questions about yourself and the plants you want.

  1. What is the purpose for the plant? Shade, specimen tree/shrub, privacy screen, windbreak, noise barrier, bird habitat, flower display, fruit display or fall leaf color may be criteria for selection.

  2. Can this tree or shrub assist with energy conservation? Evergreen trees or shrubs can be used along the north and west sides of the property to break winter winds and to keep your home warmer. If planted on the east, west or south side of your home, evergreens will shade the home even in winter when you want the sun to enter windows to assist with heating. Deciduous trees lose their leaves, allowing heat gain in winter and cooling shade in summer. East, west and south sides of the home are better sites to plant deciduous trees/shrubs.

  3. Will allergies be a problem? In general, most plants with showy flowers are insect-pollinated and do not need to produce huge amounts of pollen. Others that depend on wind to spread pollen often must produce large amounts of pollen to ensure that it is distributed. These trees are most likely to cause problems to those with allergies. Examples include pines, certain junipers, oaks, honeylocust, cottonwood.

  4. Are overhead lines, underground utilities or sewer pipes present at the planting site? Call the utility locator service to mark the location of gas, electric, telephone and cable lines before you dig planting holes. Otherwise, you may be liable for any damage.

  5. How fast will the plant reach full size? Often, fast-growing trees are sought to provide quick shade. Fast-growing trees, however, are weak-wooded and are damaged more easily by winds or the snowstorms that can visit Colorado while leaves remain on the trees in late September.

  6. Will this plant be located far enough away from the house, driveway, front entry and neighbor’s property? Take the long-term view and find out how large the plant will get. Imagine it 10, 20 or more years from now and decide if it will still be a positive part of the landscape.

  7. How will this new plant fit in with buildings and other plants already on the property? Trees which become very large might dwarf a one-story ranch style home and could dominate a small lot, shading out other plants on the property. Some could require significantly more or less water than already existing landscape plants.

  8. Will this tree/shrub grow well on my property? Most Front Range Colorado soils need amending to create a good rootzone environment. Heavy clays or sandy soils both benefit from the addition of organic matter. Local nurseries can be counted on to supply trees/shrubs that are dependable in Colorado. Of course, some may require more care or maintenance to perform well.

  9. What kind of special care or maintenance will the tree need? Am I willing to provide this? Some plants need extra attention — extra watering, pruning, fruit cleanup or insect/disease control. You may want to avoid those which are rather pest-prone and use alternatives. For example, some may be particularly prone to aphids or spider mites. If these are likely problems, you might select alternatives.

  10. Will the tree be in a lawn area? Trees develop better root systems and can have fewer problems when grown in mulched areas rather than lawn areas. When tree roots are forced to compete with grass roots for water, nutrients and oxygen, the young tree often is at a disadvantage. Fertilizer needs and water needs for the lawn generally are not the same as for the trees growing in the lawn. If the tree must be planted in an area that will be seeded or sodded for lawn, give the tree a one-year head start before lawn is added.

For more information about selecting, planting, and caring for trees and shrubs, contact your county Colorado State University Cooperative Extension office.

The Ten Commandments of Tree Planting

By Robert Cox, Colorado State University Cooperative Extension agent, horticulture

Trees evoke all kinds of images. Arbor Day, Earth Day and the coming of spring take care of that.

But, before we dream about their beauty and the cooling shade trees supply through the summer, let’s consider The Ten Commandments of tree planting.

Denver’s dry climate and generally poor, alkaline soils present some difficulties for trees. Healthy, long-lived urban trees get off to a good start when we pay attention to some tree-planting fundamentals.

Here are guidelines to help ensure that the tree you buy and plant this year will be part of your landscape in the years ahead.



    Trees need room to develop root systems underground and branches above ground. Don’t plant trees that will grow too large in small areas. Also avoid planting under power or telephone lines or too close to buildings.

    Site suitability will determine which, if any trees to plant. The designated site may be in the lawn, near a patio, along a street or sidewalk, in a garden, in sun or in a shaded spot. Soils may be clay, sandy, saline, compacted, wet or dry, gravelly or even full of old building rubble. Whatever the situation, you will need to determine if the site is suitable for growing a healthy tree.

    Consider planting for energy conservation. Deciduous trees will shade the west, south and east sides of the home in summer, and evergreen trees along the west and north edges of the lot will provide winter windbreaks.



    For what reasons are you planting the tree? You may want privacy, increased property values, a windbreak, shade, fall color, flowers, fruit or a bird habitat. Perhaps you want to create a sound barrier. Combine this information with knowledge about the site.

    This is a good time to visit your local Cooperative Extension agent. You can ask for fact sheets titled “How to plant trees and shrubs;” “Small deciduous trees for privacy and color;” and “Large deciduous trees for street and shade.”

    You’ll want to consider that fast-growing trees often are weak and subject to storm damage. Think about the mature size and shape of trees and learn whether their roots might invade sewer lines, lift and crack sidewalks or make bumpy lawns. Learn which trees are likely to harbor insects or diseases.



    Before digging, contact your utility company to mark the location of any underground lines. You could be liable for damage done to such lines.

    To prepare the site, mark a circle or square at least 3 times the diameter of the tree’s rootball. Excavate the area with a pick and spade. In clay soil, dig to a depth 2-4 inches shallower than the height of the rootball. In sandy soil, dig to a depth equal to the rootball. Leave the bottom of the hole firm and undisturbed.

    To the excavated soil, add 25 percent, by volume, of a coarse organic amendment, such as sphagnum peat, compost or aged manure. Mix it well with the excavated soil; this becomes your backfill.



    Try to plant trees when the weather is cool, cloudy and humid, but not windy. If you can’t plant right away, keep the tree in a cool, shady, protected spot and keep the roots moist. It helps to soak bare root trees and shrubs in a bucket of water overnight before planting.

    Remove any plastic or metal containers from the rootball. Place the tree upright in the center of the planting hole. If the tree is in a fiber pot, tear off the sides. If the roots of a containerized tree are potbound, “tease out” some of the roots and shallowly slit the rootball’s sides with your finger or a knife.

    For balled and burlapped trees, cut any rope tied around the trunk and pull the burlap away. Cut any reinforcement wire, removing as much as possible, but be sure the rootball stays intact.

    Shovel backfill into the hole; continue until roots are covered and most of the backfill is used. Don’t tamp the soil with your feet.



    Don’t put fertilizer into the planting hole; it may cause root injury. Next spring, fertilize young trees lightly.

    Root stimulator solutions have negligible value. You can use them, but they aren’t necessary for transplant success.



    Water the soil at relatively low pressure, using the hose or a “bubbler.” Let the water, not your foot, settle the soil. If the soil settles below grade, add more backfill. When done, the planting area should be well-soaked and moist backfill should barely cover the top of the rootball. Watering frequency depends on the soil, not the calendar. Dig with a trowel on the edge of the planting area. Soil that feels moist and holds together when squeezed doesn’t need water. Overwatering drives air from the soil, causing root suffocation. Frequent, light watering promotes shallow root development. Mulching will reduce watering frequency.

    Send your trees into winter with a good supply of moisture by watering them thoroughly in fall. Water during extended warm, dry periods of winter to prevent drought damage to roots. This is especially important for trees planted the previous year.



    A newly planted tree needs only minimal pruning. Prune out only dead, diseased or injured branches. Research shows that transplanted trees establish quicker when as much foliage as possible remains. If you do prune, don’t use pruning compounds on pruning cuts.



    Trees can be staked too tightly or for too long. Don’t stake small trees or those not in the wind’s path.

    Large evergreen trees, planted in a windy site, will need staking. To stake,do not use garden hose and wire. Instead run wire through grommeted staking straps or use wide strips of carpeting. This way, the straps, not the wire, passes around the trunk. A year of staking usually is sufficient.

    Rigid staking of a tree is counterproductive; research shows trees don’t develop normally if they’re not allowed any sway.



    A forest tree provides its own mulch with several inches of leaves on the ground. We can imitate this by mulching the planting area with 3 to 4 inches of wood chips, chunk bark, straw, pine needles or shredded leaves. Don’t use plastic beneath the mulch; water or air can’t penetrate it. Fabric-type weed-barriers are preferable.

    One thing you won’t see in the forest is manicured lawns around a tree. Research shows that newly planted trees are at a disadvantage when they must compete with grass for water, air and nutrients. Keep grass from the planting area for at least one year. If you mulch around trees, instead of planting grass, you also prevent possible trunk damage by lawn mowers or string trimmers.



    Use crepe paper or other wraps on your deciduous tree trunks about Thanksgiving time; remove the wraps around Easter. Do this for the first 2 to 3 seasons. This protects young trees from winter sunscald. If you’ve purchased a tree with the trunk wrapped, remove the material now; otherwise the wrap could harbor insects and diseases over the summer.

Selecting Trees & Scrubs

By John Pohly, Colorado State University Cooperative Extension agent, horticulture

Coloradans can plant trees and shrubs throughout the growing season.

Until fall, however, plan to purchase these plants locally. The mail-order houses that many gardeners patronize during spring won’t ship stock during the hot summer months. They’ll resume again once the weather cools in the fall.

In the meanwhile, you can have confidence in reputable nurseries and garden centers. Most select plants that are hardy in Colorado’s climate. In addition, full-time garden centers and nurseries usually will buy stock from a supplier with a climate similar to ours, so the plant doesn’t need to go through a long and sometimes difficult acclimatization.

Most local nurseries carry stock that grows in soil or a potting mix. This means the plant can be transplanted at any time of year except the hottest days of summer. And, you can look at the plant and select one that is healthy and insect-free.

Here are some pointers to help you select that special tree or shrub:

  • Look for a plant with the desired branch structure. If the branches are primarily on one side of the trunk, it will be difficult to prune to gain the symmetry most of us want. If the branches are clustered at the top of the tree, in most cases, the plant won’t produce more branches below.
  • Some nurseries will wrap the trunks to keep damage from occurring to the tender bark on the young trees. Other nurseries wrap the trunk to hide damage. Before you buy, unwrap the trunk to look for damage. If you fail to do this before you leave the nursery, it’s your word against the seller’s if you discover damage when you get the plant home.
  • When buying container-grown or balled-and-burlapped stock, grab the trunk and give it a gentle push back and forth. If the root system is well-established in the soil, the container or soil ball will move with the trunk. If the root system is damaged, the trunk will move and the soil mass will stay stationary.
  • Check for insects that might be feeding on leaves or diseased areas of the plant. Aphids often feed on the under side of leaves. Look for holes in the leaves that could be caused by chewing insects.
  • Diseases show up on branches or on the trunk as discolored areas. These usually are darker than the surrounding bark and often will be slightly sunk in. Don’t buy a diseased plant. Chances are great you won’t be able to cure the problem, and you’ll be paying the same for a diseased plant as for a healthy one.
  • If purchasing the plant is part of a day-long shopping trip, make the nursery your last stop. The plant can’t tolerate much time in a hot trunk or car. If you carry the plant in the back of a truck, cover foliage that might be whipped around in the wind.

Trees can be a big investment.

Do your homework and pick out an appropriate tree for the planting site. Think about how fast the tree grows, how big it will get, how much water it needs, whether it bears flowers and fruits and how susceptible it is to certain pests or diseases. This is a relationship! Ask yourself how well your tree’s needs match yours.

Prepare the soil before planting by adding organic materials, such as compost or composted manure. Spread the organic matter about an inch thick, then dig it into the soil to a 12″ depth.

The best time to plant trees is in early spring, though in the fall is fine, too. In any case, avoid planting just as trees are breaking bud and putting on soft new growth. Container stock can be planted all season but it also is most risky at budbreak. If you plant in the fall, do so no later than mid-October.

When planting balled and burlapped trees, dig a dish-shaped hole 3 to 5 times wider than the root ball. Plant shallow in clay soils so oxygen can reach the roots. In sandy soils, the hole should be no deeper than the root ball. Place the plant on firm soil at the center of the hole. Remove all materials wrapped around the top and sides of the root ball. Use a wire cutter to remove at least the top two-thirds of wire baskets. Backfill the hole and water slowly.

For container stock, dig the hole slightly larger than the tree’s pot. If, after removing the plant from its container, you see a tangled mass of encircling roots, use a sharp knife to make 4 or 5 cuts, a half-inch deep, up and down the ball. If most of the roots are at the bottom of the ball, use a sharp spade to split the root ball all the way through, halfway up from the bottom. Spread the two halves over a mound of soil in the planting hole. Backfill and water immediately. Container stock that is not pot-bound with roots requires no special treatment.

Other tips for tree-planting success include:

  • Use loose mulch 3 or 4 inches deep on top of the soil in the planting area, but keep the mulch 12 inches away from the trunk.
  • Don’t fertilize until the second growing season.
  • Avoid light, frequent watering. Water less often but deeper. Don’t forget to water in winter.
  • Until the tree is well established, prune only as necessary. Leave the bottom branches; they are the closest food energy source for your tree.
  • If guying is necessary, use cloth straps 2 to 3 inches wide, tied to stakes. Avoid wire and hose guying; it can injure the trunk and girdle growth. Remove guys after 2 or 3 years.
  • Protect thin-barked trees from sunscald in the winter by using a crepe-type tree wrap. Use only for the first two years and only between the months of November and April.