Selecting Quality Trees from a Nursery

In looking for quality nursery stock, consider all aspects of the plant. The above and below ground quality of each plant should be evaluated before purchasing nursery stock for your site or project.

Above ground quality

Often the first view that you get of trees in a nursery is the trunk, branches, and foliage. In shopping for plants for your landscape or tree planting project, the sheer numbers of plants and the variety of visual information which your eyes are exposed to can often overwhelm ones ability to differentiate a quality plant from a poor quality plant. This is true with either a row of ball and burlap Betula nigra (River birch) or a block of 5-gallon containers of Potentilla. It takes concentration, sharp eyes, and clear thinking to evaluate the above ground quality of nursery stock. Some factors to consider include:

1) Plant Size

The first measure of quality of a tree is its size. Except at smaller sizes, trees are sold based on their caliper. Trees less than four inches in diameter are measured six inches above ground line. Tree’s four inches and larger are measured 12 inches above the ground. The term diameter at breast height (DBH) is used as a measure for forest trees. This method should not be used for standard size ornamentals.

2) Ratio of height to caliper

The ratio is also a measure of quality. Although it will vary with species, trees that are excessively tall for their caliper should not be accepted. This occurs when trees are either collected from the wild, or are grown too close together in the nursery. Both situations produce relatively weak plants that do not survive transplanting as well as properly grown trees, and may be susceptible to blow down.

3) Plant form

It is important to select trees that have well-spaced structural branches, oriented uniformly around the trunk. Branches that are growing close together when young will grow into each other with age, and they will not be able to develop their full structural strength. Some trees, such as Bradford pear, have gotten a bad reputation for clusters of branches growing at acute angles that end up splitting in snowstorms. However, if the trees were pruned more carefully in the nursery, many of the problems could be avoided. Inspect the branch structure of the trees in the nurseries that you visit, and point out defects to the nursery operator that you feel should be corrected.

Too many nursery operators trim trees like hedges to form dense canopies. This makes the trees look better temporarily, keeps them from growing into each other, and makes them easier to harvest, ship, and handle. However, it often leads to the development of many crossing and parallel branches. Again, you should encourage the operator to remove crossing and parallel branches during the production cycle. Otherwise, they will have to be removed at the planting site.

If the tree is a type that should have a strong central leader, avoid trees that have clustered growth at the top. These branches may develop narrow angles that will be weak sometime in the future, or they may grow together and form weaknesses in the tree.

For street use, always specify single-stem trees that are limbed up as much as possible in the nursery prior to purchase. It is not good to limb up too high, too fast, because it leads to weakness in the trunk. Trees must be limbed up enough to ensure that they do not interfere with pedestrian or vehicular traffic.

4) Plant Health, Growth, and Signs of Injury

In examining the general health of nursery stock, you should examine both general health characteristics and specific signs of damage, infestation, or injury. The general health of the tree can be determined by examining the foliage color and density, and the length of shoot extension for the previous two or three years. The foliage should be lush green. Thin, light green or yellow foliage is a sign that the plants were grown under poor fertility conditions at the nursery. This results in trees that have poor nutrient and energy reserves at the time of transplanting. When trees are dug, 90 to 95% of their roots are left in the nursery, and their survival and growth in the landscape depend on the rapid regeneration of roots. The more nutrient and energy reserves in the plant, the faster it can regenerate a root system large enough to allow it to become well established in its new site.

With a little experience, it is easy to identify the annual growth in length of the branches of trees. The growth should be stunted the year a tree is planted in the nursery, but each year after that the growth should increase in length. If the growth appears to be stunted the year before harvest, you should try to determine the cause. It could be from drought, poor fertility practices in the nursery, or it could mean the tree was dug sometime during the previous year and was held above ground for many months. Again, a tree held this long will have poor nutrient and energy reserves compared to a freshly dug tree.

Next, inspect the trunk to ensure the bark has not been damaged during production or handling. Torn bark reduces the movement of water and nutrients in the tree and opens it up to infection by decay organisms. Equipment, frost cracks, and nonselective postemergence herbicides can damage bark. Reward™ (diquat) or Finale™ (glufosinate) are contact herbicides that may kill the bark they contact. This results in sunken areas in the trunk where living wood and bark grew around dead bark. Bark damaged by Roundup Pro™ (glyphosate) splits open. The bark split can range from barely noticeable to several inches wide reaching from ground level up into the canopy of the tree. The size of the split is dependent on the tree species, tree age, time of application, and the rate of herbicide applied to the tree.

At the time of delivery, carefully inspect all trees for any signs of serious insect and disease problems. Cankers can be diagnosed by the fact that they are centered on pruning wounds. Entering through pruning wounds, canker disease organisms also cause sunken areas or splits in the bark which may be confused with cultural injuries mentioned earlier (ex. postemergence herbicide injury).

Look for cankers, scale, and borer damage on the trunk. Look for scale, caterpillars, severe aphid infestations, or diseases in the foliage. Plants that are found to have an insect or disease problem should be isolated and quarantined so the problem is not spread to new or existing plantings. If the problem can be corrected, treat the plants accordingly. If not, they should be returned to the nursery.

Below Ground Quality

Often the quality of the nursery stock stops with a cursory look over the trunk, branches, and general health of the foliage. This is an error and can lead to replacing landscape plants and increasing your landscape and management costs. The quality of the root system of a tree is the most important factor in determining how well the tree will survive transplanting, and how fast it will become fully adapted to its new site.

As mentioned above in the section on height to caliper ratio, trees that are collected from the wild should not be accepted. The only roots close to the base of collected trees are large support roots. The smaller feeder roots are farther away and few are included in the soil ball when dug. Nursery grown plants are root-pruned several times during propagation and transplanting operations prior to being dug for landscape use. This results in the development of many more fine roots close to the base of the tree, which are dug with the soil ball for movement to the landscape. If the root system is of poor quality, restricted, undersized, or damaged the chances of the plant surviving and growing is limited.

1) Burlap Choice Impacts Root Quality

There are many questions asked about the different types of burlap used to wrap soil balls. Natural, untreated burlap is the best. If buying locally it can be specified. However, it only lasts about six weeks when heeled-in in a mulch bed. Many growers use treated, or rot-resistant, burlap. This burlap is treated with a copper compound that retards the decomposition of the burlap that is caused by fungi in the soil. It lasts three to six months in mulch beds or the soil. Both untreated and treated burlaps bind well with soil after planting and allow the free movement of water in and out of the soil ball. Both are acceptable.

Plastic burlap used to be green and shiny, and anyone with even a limited knowledge of plants knew it should be removed at planting. However, sometime around the mid-1980’s brown woven plastic burlap with the same texture as natural burlap was introduced into the nursery trade. This introduction was a disaster for the industry. Many experienced landscapers did not recognize this new woven burlap as plastic and planted it with the root ball. Severe losses of trees and shrubs followed. When ordering trees, state specifically that you will not accept trees with root balls wrapped with plastic burlap.

2) Growing System Impacts Quality and Size

Trees are available in a variety of forms for planting in urban areas. They may be bareroot, balled in burlap (B&B), or grown in gro-bags or containers. Bareroot plants are not commonly planted in cities because they are highly susceptible to vandalism prior to establishment of a dense root system. However, bareroot plants are a very economical form for marketing a large number of trees for planting in lightly traveled areas. Bareroot trees must be kept cool (35 to 40 °F) and moist prior to planting. These trees should be planted immediately after their arrival to ensure their survival. If the chance of long delays between arrival of the trees and planting exists, bareroot trees should not be ordered.

Balled in Burlap

Balled in burlap is currently the standard marketing form for trees in the industry. There are several standards of quality by which the soil balls of B&B trees are evaluated. The size of soil balls should meet the minimum standards established by the American Association of Nurserymen (AAN) in the publication, the American Standards for Nursery Stock (ASNS, 1996). The diameter and depth of the soil balls should both be measured and compared to the standards. It should be recognized that the standards published are minimums, and there is nothing wrong with receiving soil balls that are somewhat larger than the specified sizes.

In addition to size, several other characteristics should be examined. The burlap and wire baskets used to wrap the soil balls should be very tight. Wire baskets should be crimped as tight as possible. The looser the wrapping, the greater the risk the ball will be damaged during handling. The trunks of the trees should be in the center of the soil balls. Sometimes trees dug with a tree spade are badly off-center in the ball. This reduces the tree’s chance of survival, and may make it unsteady in the planting site. Ensure that the burlap and twine used to wrap the balls are not plastic or nylon. If they are, they must be removed at the time of planting.

The twine around the trunk of some of the trees should be removed to determine where the crown of the plant is in relation to the top of the soil ball. The thickening of the trunk and the development of buttress roots can identify the crown. They should be right at the soil line. If they are buried more than two or three inches, the tree will have an undersized root system. The widest part of a tree spade dug soil ball is the top. If there are no roots in the top six inches of the ball, the root system may be reduced by 30 to 40% below the standards. These trees should not be accepted.

Pot-in-Pot

Another system of growing trees that has become very popular in recent years is the pot-in-pot (PIP) system. Trees are grown in containers sunk in the ground. As in other containers, the medium for growth is usually all organic matter generally a mixture of peat moss and composted bark. The trees grow well in the containers in the nursery, but when transplanted to the landscape they do need to be watered more frequently than trees moved with a soil ball (B&B or gro-bags). They are also easier to rip out of the ground, so they should not be planted in areas in which vandalism is major concern.

Successful landscape management begins with choosing the right plant for the right place followed by purchasing and planting quality plants. Taking a little time to examine the plants before purchase can go a long way toward increasing the success and life span of the plants in the landscape. The above are some tips to help you in choosing plants for your clients.

Written by Dr. James Sellmer, Penn State Ornamental Horticulture Extension Specialist.

Resources for Selecting Quality Trees

Selecting Quality Tree in the Nursery – an excellent publication written by Dr. Ed Gilman at the University of Florida

Guidelines Specifications for Nursery Quality Trees

Selecting Quality Trees Slides – Dr. Ed Gilman’s PowerPoint Slides

Buying High Quality Trees – a short ISA publication for consumers

American Standard for Nursery Stock – ANSI Z60-2004 – an industry standard and must read when ordering large qualities of trees from a nursery

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