Extension News

Ask the ISU Extension Garden Experts: Protecting Trees, Crown Gall and Houseplants

Note to media editors: Got gardening questions? Call the Hortline at (515) 294-3108, Monday-Friday from 10 a.m. to noon and 1 to 4:30 p.m., or e-mail us at hortline@iastate.edu. For more gardening information, visit us at Yard and Garden Online, http://www.yardandgarden.extension.iastate.edu


What effects will construction activities have on nearby trees?

Construction of buildings, patios, garages, driveways, sidewalks, and roads often compromises the growing environment of nearby trees. The majority of a tree’s roots are located in the top six to 18 inches of soil and often extend well beyond the edge of the tree canopy (dripline). Changing the grade by removing or adding soil around existing trees can cause extensive root damage. Removing soil can disturb and injure many of the tree’s roots. Adding soil can reduce the movement of oxygen to tree roots, causing them to die.

Trenching often severs major portions of the tree’s root system. Soil compaction by heavy equipment and foot traffic reduces the supply of oxygen to the root system. Mechanical damage to the trunk of the tree caused by construction equipment can strip off bark and damage vascular tissue, reducing nutrient and water movement in the tree. Open wounds created by these injuries can serve as entryways for insects and decay-causing fungi. Severe construction damage can cause affected trees to decline and die.

The best way to minimize damage to a tree during construction is to do nothing around, in or on top of a tree's root system. Construct a sturdy fence at least at the outer dripline of the tree and allow zero activity within this area. Prohibited activities inside the dripline of the tree include lowering the grade, adding soil, trenching, parking or operating machinery in the area, and storing supplies, soil or excavation materials.

There is a large, hard growth on the cane of one of my roses. What is it?

The large, hard growth is probably crown gall. Crown gall is caused by the bacterium Agrobacterium tumefaciens. The bacterial disease can infect roses, grapes, apples, raspberries, willows, euonymus and many other woody plants. It also affects some herbaceous plants. The bacterium enters through wounds, especially on root and crown tissue. Rough, woody galls develop and interfere with the flow of nutrients and water. Galls may enlarge to an inch or more in diameter. To prevent crown gall, avoid injury to the roots and crown of plants when planting and when doing routine maintenance chores. Eradicating crown gall from infected plants is difficult. In most cases, infected plants should be dug up and destroyed. Also, discard the soil in the area as the crown gall bacterium can survive in the soil for two or more years. When replanting, select plants that are resistant to bacterial crown gall.

Do houseplants actually improve indoor air quality?

Houseplants are attractive additions to homes and offices. They also can improve indoor air quality by removing formaldehyde, benzene, xylene, ammonia and other harmful chemicals. Plants that have been shown to improve indoor air quality include Chinese evergreen (Aglaonema spp.), umbrella tree or schefflera (Schefflera spp.), spider plant (Chlorophytum comosum), dumbcane (Dieffenbachia spp.), corn plant (Dracaena fragrans), pothos (Epipremnum aureum), weeping fig (Ficus benjamina), rubber tree (Ficus elastica), English ivy (Hedera helix), philodendron (Philodendron spp.), snake plant (Sansevieria trifasciata), peace lily (Spathiphyllum spp.), arrowhead vine (Syngonium podophyllum) and palms (various species).


Contacts :

Richard Jauron, Horticulture, (515) 294-1871, rjauron@iastate.edu 

Del Marks, Extension Communications and External Relations, (515) 294-9807, delmarks@iastate.edu