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Planting for tree survival

Trees need all the help they can get in the harsh, hard-surface conditions of many schoolyards.

Schoolyards are very different places to front yards but all too often schools follow the guidelines for planting trees in front yards where trees are usually planted at the same surface or grade level as the surrounding soil or lawn. Planting in this manner in school grounds exposes trees to greater soil compaction and erosion around the tree and damage to the bark by children at play. Also, grounds maintenance vehicles such as mowers can easily cause physical damage to trees that are not adequately protected.

Over the past four years the Canadian Biodiversity Institute has developed a method of planting trees on school grounds in groupings of mounds or berms with pathways and open areas with seating for quiet activities set between them. The grade separation alone of human and tree spaces has resulted in a very high rate of tree survival because the mounds make the trees more visible and provide them with a much-improved growing space. Creating quiet, tree-shaded spaces away from active areas also lowers the incidence of accidental tree injury from boisterous play and sports activities.

When people first think of greening their schoolyard, planting trees is often a priority as a way of building a landscape structure for other planting projects and for creating pockets of shade in and around play and social spaces. Before developing a tree-planting plan, schools should consider how the site conditions and the human activities that take place in the schoolyard are likely to affect the health and survival of trees.

Growing conditions on school grounds can be problematic because the soil is often poor, heavily compacted, dry and low in nutrients. Salt used on pathways and driveways may leach into the surrounding soil. There is generally little protection from harsh Winter winds and the drying effects of Summer heat.

Taking the time to learn about the needs of trees such as water, air, nutrients, protection and care helps to ensure that the right tree species are selected for the right sites. Creating a sense of ownership by involving students in every step of the process and educating them on the benefits of trees helps prevent tree damage when they are at play in the yard. It is very important to remember the underground parts of the tree while planning because healthy root growth will be determined by the health of the soil, the size and location of the planting space and the availability of water.


Choosing and locating trees

To help you choose appropriate tree species for your schoolyard, identify trees in the vicinity of your school that are thriving in locations comparable to the spaces where you intend to plant. This will help you choose species that are likely to grow well on your site. Try to find mature specimens so that you can plan for the approximate height and spread of the plants when they are fully grown. This will help you avoid locating trees and other plants that reach large proportions too close to buildings, overhead wires and cables, and underground services such as sewer and water pipes, gas and hydro.

Check out the health of trees growing along streets and next to driveways around your school to help you choose tree species that withstand the rigours of hard-surface plantings. You can assess the health of local trees by looking at the size and colour of the leaves and checking for leaf spots, disease, die-back and other signs of stress. Find examples of trees that have been planted close to asphalt or where paving has been added next to the tree some time after it was planted.A tree with non-porous paving on one side and lawn on the other will often show signs of stress on the side next to the paving. It is not unusual in some species to find large, bright green leaves showing little or no sign of insect damage on the side of a tree nearest the lawn, and smaller, yellowish green, insect-eaten leaves on the half next to the paving. This shows that trees are healthier when their roots have enough space to spread and that they can become stunted and diseased when the growing conditions are restricted by paving. When a tree's root system can develop well all around the tree, the tree is also more stable. Its stability is weakened when root growth is inhibited on one side by paving or buildings.

Some relatively urban-tolerant and disease-resistant trees have become more disease prone because they have been over-planted as street trees. Mono pests often develop in monocultures because there are fewer checks and balances than when there is a greater diversity of plant species. In neighbourhoods where a particular tree species has been over-planted, try to avoid planting it to help prevent the spread of disease.

Choose "clean" trees for planting next to play structures. Deciduous trees with small leaves such as locusts will dry quickly and blow away in the Fall. Larches, the only deciduous conifer, are also acceptable for planting near play spaces; however, they provide less shade than locusts. During the Autumn, large leaves can reduce play safety by forming a wet, slippery mat on and around play equipment. Some trees, such as Manitoba maples and large-leaf lindens, can harbour insects which produce sticky substances that can adhere to metal and plastic surfaces of play structures.

Avoid planting trees under overhead cables and wires. Trees are often disfigured when branches are cut back to make way for overhead wires and improper trimming can adversely affect tree health.

To help solve drainage problems, use trees that thrive in moist conditions. Areas of the school grounds where excessive puddling occurs can be graded so that rainwater is directed towards moisture-loving trees. This reduces the size of the wet or puddled area and concentrates the rainwater where it can be used by trees.


Matching species to soil

Make sure that the plant species you choose are matched to the soil in each location. Some plants are quite particular about soil type whereas others will tolerate a variety of soils. Good gardening books provide details on individual plants' soil requirements including soil density and drainage, and whether they prefer acid, alkaline or neutral conditions. Students can perform a simple litmus test to check the acidity of the soil. It is advisable to test the soil in each planting location because soils can vary considerably within the same area.

Although the soil on school grounds is typically fairly poor, it is possible to both improve its condition and create pockets of varying soil types within one planting project. Material can be mixed in with the soil when planting to increase soil acidity for acid-loving plants such as conifers while, for example, crushed limestone can be added to the soil in an adjacent spot for plants that like good drainage as well as having their roots in contact with limestone.

Schools with heavier clay soils need to prepare the site carefully prior to planting. Clay soils have low porosity. When a planting hole is dug in clay soil with a spade and without thoroughly breaking up the clay around the hole, the sides of the hole resemble those of a plastic flower pot or pail sunk into the soil. Rainwater and water from melting snow run into the planting hole and saturate the loosened soil but are not easily absorbed by the surrounding clay. Waterlogged conditions deprive the roots of air and they suffocate or "drown".

In clay and compacted soil conditions, break up the ground to a depth of about two feet in an area at least 10 feet in diameter and then import new soil to form a mound about one foot high in the centre. If you lack the space for a large mound, make the planting hole at least twice as wide and twice as deep as the root ball. The soil around and under the planting hole should also be loosened up to facilitate root growth.

If a clay and shale sub-soil is encountered, it is advisable to excavate the planting hole to a depth of about three feet. The excavated soil can be used to create a berm elsewhere in the yard. Although this type of site preparation is not normal practice for trees planted in parks, along streets or in front yards, it helps trees survive in schoolyards. The mound of soil can be defined as a tree space by edging it with seating or a variety of materials such as wood and small boulders or stones. The planting hole is prepared after creating the mound. When the tree has been planted, a layer of mulch such as wood chips can be applied to protect the soil and further define the area as a "tree space". Since wood chips deplete the soil of nitrogen as they decompose, nitrogen-fixing plants such as clovers, vetches and peas can be grown around the tree.

If you have sandy soil, water will drain out of it fairly quickly. Your choice of trees will be limited to species that tolerate dry conditions such as hackberry, black locust, honey locust, white ash, green ash, grey birch, ginkgo, white ash, green ash, silver maple, amur maple, Manitoba maple and hornbeam. Alternatively, you can prepare a large planting hole and mix the original soil with good top soil to help retain moisture and nutrients in the planting space. Peat moss is often recommended as a material to help retain moisture in the soil; however, it increases soil acidity and the mining of peat contributes to the destruction of wetlands. In addition to mixing moisture-retentive soil with sandy soil, you can spread a six-inch layer of broken up clay soil over the entire planting area. The rain will slowly wash the small particles of clay down through the larger particles of sand. The particles of clay stick to the surface of the particles of sand and gradually increase the soil's ability to retain moisture.

In any type of soil, remember to create planting spaces that are large enough to promote healthy root growth. Trees planted in square holes along sidewalks do not grow as well as those planted in a row in a trench that has been capped with bricks, cobbles or gravel to permit greater penetration of water to the roots. Trees' roots have more room to develop in trenches than in small planting holes and therefore trees planted in trenches grow better. Consider digging a trench instead of making several individual holes when planting trees in a row near non-porous surfaces.


Native species

Plant native species wherever possible but remember that some non-native trees tolerate hard-surface, dry and salty conditions better than native species. Most schoolyards in no way resemble native tree species' natural habitat. The built environment traps heat and temperatures are generally higher than in the surrounding countryside. Many native species of trees are intolerant of urban conditions. Some are able to survive polluted air, compacted soil, light deprivation and dry conditions and even put up with a certain amount of salt; however, the choice of native species that will grow in such stressful surroundings is fairly limited. It is preferable to plant a hardy, non-invasive, non-native tree than a native species that will not survive or no tree at all.


Ways to protect your trees

To give your trees a good start, make mounded "tree spaces" or groves of trees rather than planting trees singly and in rows at grade level around the edges of sports and active areas. Groves of trees with seating away from active play areas provide teachers with an ideal outdoor classroom space to educate students on the benefits of trees. Students can directly experience the benefits and understand how protecting the trees from harm results in the trees protecting the soil, air, water and wildlife.

It is preferable to plant large-caliper trees because they are less susceptible to damage. Smaller trees require more protection. To help small deciduous trees survive, create a protective barrier between them and children at play and involve the whole school in a planting event. Particularly in Winter, small trees often look like sticks poking out of the snow and children often hold onto the stems and swing around them. This usually results in the gradual wearing away of the bark and the eventual death of the tree. Install snow fencing or have children weave basket-like fences around young trees helps to prevent this kind of damage.

Take care not to plant conifers in places that will obstruct "sight lines" across the schoolyard or it may become necessary to cut off the lower branches as the trees grow. When trimming conifers, never remove more than one third of the live branches from the tree.

Most conifers are sensitive to salt. Plant conifers in places where they will not be affected by road salt sprayed from passing traffic. Salt will cause the foliage to turn brown and die. In severe cases, this results in the death of the tree. Salt aerosol sprayed from roads by passing traffic can travel well over a hundred feet.

Shrubs and ground covers can be planted to form a barrier between newly-planted trees and areas of hard surfaces such as concrete and asphalt. Shrubs and other plants help to retain moisture by shading the soil, cool the air around trees, reduce the drying effects of the sun and wind, prevent soil and mulch from blowing or washing away, provide additional habitat for wildlife, and add interest and colour.

The success of your tree-planting projects depends on your planning for tree survival by taking into account the environmental conditions, site uses and soil types on your school grounds and learning about designing planting spaces that meet the needs of trees. It also depends on getting people involved in weeding and watering the tree spaces over the Summer vacation. In fact, tree care during the growing season is one of the most important ways of protecting your trees. The maintenance plan should include having students and their families thoroughly soak the soil around the trees to a depth of two to three feet weekly throughout the growing season.

For the first three or four years following transplanting, the trees need lots of water to help promote healthy root growth. Do not fertilize new trees for at least three years. They need time to recover from transplanting shock and for the roots to develop. When trees are fertilized too soon after being transplanted, tree growth above the ground is promoted at a faster rate than the still-developing root system can handle.

You can protect your trees by identifying the places where snow is stored to avoid exposing them to chemicals that may be present in the snow such as salt, anit-freeze, oil, grease, windshield fluids and tire particles. Snow ploughs can cause considerable damage to plants by pushing snow and ice into them.

When planning to plant near driveways, parking lots and roads, allow plenty of turning space for vehicles help avoid mechanical damage to trees.

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