Planting trees in your yard is one of the best long-term investments you can make as an urban gardener.

Shade from a tree can lower your air conditioning costs by 25 to 50 percent. Trees can increase the property value of your home by as much as 20 percent. They help clean the air by absorbing dust, ozone, carbon dioxide and other pollutants.

Trees are also the backbone of your garden landscape. They create definition for a disorganized space. They give you micro-environments or zones that you can build on by adding plantings of perennials, shrubs, ground covers and vines.

Trees can be divided up into two basic categories. There are really big trees including oaks, ash, maple and beech, that take 20 to 30 years of growing before they begin to show their true size potential. But these trees can grow over 100 feet tall and are best planted in parks or suburban and rural landscapes.

City trees need to be a little smaller because the typical yard of an urban home is smaller than their suburban cousins. They also need to be able to withstand automobile exhaust pollution, compacted soil, hot and dry conditions and a cramped space for their roots to grow in.

Trees are a little more expensive than shrubs and perennials. Expect to pay anywhere from $50 for a small tree up to $500 for a larger tree. If you need help planting, expect to pay even more.

Here's a list of small-sized trees that have pretty flowers in spring and colorful leaves in fall and that can withstand the rigors of growing in an urban area.

Redbud (Cercis canadensis) This native tree is one of my all-time favorite. It grows up to 20 feet tall and is covered with reddish purple buds along its branches in early spring. IN summer it is covered with green heart-shaped leaves. IN fall its leaf color is yellow. It grows in any well-drained soil in full sun.

Korean dogwood (Cornus kousa) Korean dogwood is a welcome alternative to our native dogwood (Cornus florida) which is increasingly attacked by anthracnose blight during summer. It grows up to 20 feet tall and produces dark green oval leaves in summer that are graced with pretty while flowers in June. The leaves turn crimson purple in fall. Grow in full sun to part shade.

Shadblow (Amelanchier) This native tree grows up to 20 feet tall and is dotted with small white flowers in very early spring, followed by green leaves that turn orange in fall followed by berries much beloved by song birds. Be sure to get an upright shadblow rather than a multi-stemmed shrub like variety. Grow in full sun to part shade.

Japanese maple (Acer palmatum) These are the graceful trees you see in Japanese paintings. There are several selections that grow from 10 to 20 feet tall. The leaves of these trees are the real treat. Some selections are green, some are red and some yellow and variegated. Grow in full sun to part shade.

Carolina silverbell (Halesia carolina) This little known tree will grow up to 30 feet tall but it is well worth the extra room. In spring it produces snow-white flowers that dangle like bells in clusters from the twigs. Grow in full sun to partial shade.

Crab Apple (Malus) Flowering crab almost did not make it to this list because so many of them are disfigured with blight and other diseases during the summer. Luckily there are new disease resistant varieties coming on the market all the time such as 'Coralburst,' 'Lancelot' and 'Snowdrift.' They grow up to 20 feet tall and are covered with sensational displays of flowers in mid to late spring.

Franklinia (Franklinia alatamaha) This tree was once thought to be extinct but it has begun to make a come back in recent years. It grows up to 15 feet tall and has an upright arching shape to it. It produces fragrant white flowers in summer and the leaves turn orange/red in fall.

Stewartia (Stewartia pseudocamellia) This little-known tree only grows up to 10 feet tall. It is covered with white flowers in May and June with yellow to 0range-red leaves in fall. In winter it has interesting looking peeling bark. A four season tree.

Planting Trees.

You can plant trees or shrubs anytime between now and the middle of October. Now is a good time to buy trees because nurseries are often offering discounts at this time of year to sell off the trees they have in stock. They will have plenty of time to settle back into the soil before the ground freezes in January.

The tree you buy will be either bare-root, which means the roots are not surrounded by soil, or balled and burlapped, which means the tree's root system is still surrounded by soil and then wrapped in burlap.

Bare root trees are usually smaller and since their roots are exposed, they must be planted immediately so they don't dry out. Balled and burlapped trees can be kept unplanted longer as long as you keep the root system watered. In any case, plant your trees as soon as you can.

Measure the height and diameter of the root ball or root spread. Dig the hole 2 to 3 " shallower that the root depth. The hole should be 1 ½ to 2 times the diameter of the root size. Set the tree in the hole so it is even or 2 to 3 inches higher in the hole than it was in the nursery. Planting the tree any deeper than soil level can kill them over time.

The jury is still out on whether it is more or less beneficial to remove the burlap from the tree. If you do remove the burlap, be sure to do it carefully to not damage the roots or trunk.

Refill the hole and water thoroughly. Create a little moat around the outer edge of the hole so the water will flow back towards the tree. Cover the ground around the hole with 2 to 4 " of compost or shredded bark mulch. This keep the weeds and bay and the roots cool.

It is not necessary to wrap trees with paper or to stake the tree. But it is important that you water the tree once a week for an hour with a slow trickle from the hose. Except in the winter when the ground is frozen, you should water like this for at least the first year of the trees growth.

Your new tree should grow and prosper blessing you with lovely flowers in spring, cooling shade in summer and colorful leaves in fall.