• Japanese tree lilacs prefer cooler climates such as the one found in Iowa.Verity Welstead/Polka Dot/Getty Images
  
  Hawkeye state residents have to consider environmental factors when they select ornamental trees to plant. Horticulturist Richard Jauron of the Iowa State University Extension advises that factors include size and hardiness of the trees, their susceptibility to insects and diseases, and particular soil requirements. Iowa is situated in U.S. Hardiness Zones 4 and 5, which means winter temperatures can go as low as -20 degrees Fahrenheit. Selected trees also need to be tolerant of the bright, summer sun.
  
  

Crab Apple Trees

• Crab apple trees are in danger of diseases such as scab apple and fire blight. Apple scab is a fungus that causes leaf drop, which destroys the tree's beauty. Fire blight is a bacterial disease that causes the flowers to brown and wilt. These threats can be avoided by choosing disease-resistant varieties. Jauron recommends Hopa, Radiant Royalty and Vanguard. Prariefire has purplish flowers that open later than other crab apples and it has proven to be disease-resistant. Ormiston Roy does well in zone 4 and produces rose-red buds that form white flowers, and yellow fruit that turns orange in the fall.
  
  

Japanese Tree Lilac

• The Japanese tree lilac is a favorite of Jeff Iles, horticulturist for Iowa State University Extension. Iles recommends the lilac for Iowa yards and gardens because it stretches the season by providing color for several weeks. The tree is native to northern Japan, and prefers cooler climates such as the one found in Iowa. The creamy, off-white blooms appear like fragrant clouds of white lace. These trees prefer full sun and well-drained, slightly acidic soil. Japanese tree lilacs have growth habits similar to other lilacs and grow no larger than 30 feet, making them popular selections for use as street trees.
  
  

Thuja Giant Evergreens

• Thuja evergreens grow as high as 60 feet, and have tall, symmetrical forms which serve as standalone eye-catchers or, planted in a row, a privacy screen. These trees are drought-tolerant, pest-resistant and deer don't like to eat them. One of its best qualities is that it is not susceptible to snow and ice damage, unlike some other evergreens. It holds up well to the summer heat and frigid winters of the portion of Iowa that lie in Hardiness Zone 5. This tree is planted in the National Arboretum and won the Pennsylvania Horticultural Society Gold Medal Plant Award in 1998. Thujas are rapid growing and have dense foliage. They grow in many different soil types and require no pruning.