Extension News

Ask the ISU Experts

Note to media editors:

Got gardening questions? Contact the Hortline at (515) 294-3108 (M-F; 10-12 & 1-4:30) or send an e-mail to hortline@iastate.edu. For more gardening information visit us at Yard and Garden Online at www.yardandgarden.extension.iastate.edu


Sap is oozing out of an old pruning cut on my elm.  Is this a serious problem? 

The sap oozing from the elm wound is probably due to bacterial wetwood or slime flux. Bacterial wetwood or slime flux is a common disease of elm, cottonwood and mulberry. It also occurs on maple, birch, ash, linden, redbud and other deciduous trees. Symptoms include the bleeding or oozing of sap from tree wounds. The sap is oftentimes colonized by fungi, yeasts and other bacteria, resulting in a frothy, slimy, foul-smelling liquid. 


The bacteria associated with wetwood are common in nature. They enter the tree through wounds in the trunk, branches or roots. High pressure builds inside affected trees from bacterial activity. Eventually the gasses and fluids work their way out through cracks or wounds on the tree. Wetwood is more common in years when trees are suffering from drought stress. 


Wetwood or slime flux may weaken affected trees somewhat, but usually doesn’t kill them. Preventing wounds and avoiding stress are the best ways to deter wetwood problems. Once a tree is oozing sap, there is no way to eliminate the disease. Proper pruning, watering when needed and avoiding soil compaction over tree roots are ways to minimize wetwood or slime flux problems. 


Will an application of gypsum improve a clay soil? 

Advertisements for gypsum sometimes claim that gypsum will help loosen heavy, clay soils and improve soil drainage.  However, the addition of gypsum to Iowa soils is of little benefit.  Gypsum is chiefly used to amend sodic soils. Sodic soils are found mainly in arid regions of the western United States. 


Core aerification is the best way to improve growing conditions for lawns established on clay soils. The core aerifier should remove soil cores that are approximately three-fourths of an inch in diameter and 3 inches long. There should be 20 to 40 holes per square foot.  April and September are the best times to aerify lawns in Iowa. 


Vegetable and flower gardens can be improved by applying and incorporating organic matter, such as compost, well-rotted manure or peat, into the soil. Work the organic matter into the top 8 to 12 inches of soil. 


When should I harvest raspberries? 

Raspberries are ripe when the fruit are fully colored. Also, ripe berries slip easily from their receptacles, which remain on the plant. If possible, harvest in the early morning hours before heat builds up in the fruit. Raspberries are very perishable. When harvesting, handle the berries carefully. Refrigerate or freeze the fruit immediately after harvest. The maximum storage life of raspberries is two to three days at a temperature of 31 to 32 F and a relative humidity of 90 to 95 percent.



Contacts :

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

Jean McGuire, Extension Communications and Marketing, (515) 294-7033, jmcguire@iastate.edu