• River birches are big thirsty trees with shallow root systems.David De Lossy/Photodisc/Getty Images
  
  River birches are vigorous, fast-growing trees that enjoy moist environments. They are big thirsty trees with shallow root systems. Birch branches have diamond-shaped leaves that cast dappled sunlight and partial shade on plants growing underneath. Use companion plants that can compete with the trees for moisture in drier times, yet accept periods of inundation. Plant companions that withstand summer heat in Southern areas and are cold-hardy to USDA zone 4 in Northern areas. Choose plants that accept a slightly acidic soil of 6.5 pH or less.
  
  

Buttonbush

• Buttonbush is an environmentally friendly plant that grows in wet areas along marshes, rivers and streams. They usually grow 2 to 3 feet tall, although 15 feet is possible under the right conditions. Buttonbushes bear ping-pong ball size white flowers from June to August, which are followed by brown balls of seedy fruits that last from September to October. Butterflies and bees visit the flowers for the nectar, and ducks and shorebirds enjoy eating the plant's seeds. Songbirds build their nests in buttonbush and wood ducks roost in them. Buttonbush plants are associated with trees such as red maple, black oak and river birch.
  
  

Celandine Poppy

• Celandine poppy (Stylophorum diphyllum) is an uncommon poppy that enjoys growing beneath trees such as river birch and maple, where it finds dappled sunshine and partial shade. The butter yellow flowers grow from 1 to 2 inches across and bloom in mid- to late spring. The poppies have a restricted range because they are threatened by invasive garlic mustard plants (Alliaria petiolata) that occupy the same areas. The flowers are pollinated by insects and the seeds are distributed by ants. The plants tolerate USDA zones 4 to 9. The poppy, a protected species in some areas, grows between 12 and 18 inches. Both celandine poppies and river birch trees are suitable for use in pollution-fighting rain gardens.
  
  

Virginia Bluebells

• Virginia bluebells are inch-long trumpet-shaped flowers that are blue or purple. They are pink when first opened and deepen in color as they age. Occasionally, bluebells can be found with white blossoms, but those are very rare. The bloom time is April and May. They grow in the moist woodland environments such as bottomlands and river woods, just like river birch. Only certain bees can pollinate the tubular flowers because it takes long proboscises, although butterflies can perch on the lip of the flower to get access to the nectar and pollen. They are found in many places in the East, South and Midwest.