Native Plants
![](../_images/smarty_plants.gif)
Q. Who is Mr. Smarty Plants?
A: There are those who suspect Wildflower Center volunteers are the culpable and capable culprits. Yet, others think staff members play some, albeit small, role. You can torture us with your plant questions, but we will never reveal the Green Guru's secret identity.
Did you know you can access the Native Plant Information Network with your web-enabled smartphone?
Ask Mr. Smarty Plants is a free service provided by the staff and volunteers at the Lady Bird Johnson Wildflower Center.
![](../_images/mr_smarty_plants_logo_web_200w.jpg)
rate this answer
![](../_images/star_30.gif)
Friday - September 19, 2008
From: Foristell, MO
Region: Midwest
Topic: Trees
Title: Sappy dew killing plants under oaks in Missouri
Answered by: Barbara Medford
QUESTION:
There is a sappy dew killing my perennials.I have several large oaks in my yard. I had different kinds of shade perennials around each base of the trees. But as years have gone by, the different varieties of perennials would eventually die out; as well as any grass under the tree out to the drip line. I was told that the oak trees produce a sappy dew to kill anything under the tree that would threaten the tree's nutrients. Is this true? What can I do to get my beautiful perennials and grass back under the trees?ANSWER:
There are five oaks that are native to Missouri, and we are assuming your oaks are one of these: Quercus alba (white oak), Quercus bicolor (swamp white oak), Quercus macrocarpa (bur oak), Quercus palustris (pin oak) and Quercus rubra (northern red oak). While the acorns and young leaves of oaks are usually mildly toxic, none of these were indicated to be very allelopathic. Allelopathy involves a plant's secretion of biochemical materials into the environment to inhibit germination or growth of surrounding vegetation. Allelopathy enhances tree survival and reproduction. In other words, the plant is protecting itself, its nutrition and water sources, as well as its space, by inhibiting other plants in competition with it.
We are inclined to believe that the failure of your grasses and perennials are more to be attributed to the shade of the oak, and the fibrous root system. Roots of oaks will often extend far beyond the drip line of the tree itself, and the majority of tree roots are in the upper 6 to 12 inches of the surface of the soil. Also, leaves of deciduous trees can accumulate and smother emerging plants, or promote mold and fungus diseases. Most grasses have difficulty in surviving in that environment, as well as flowering plants; the biggest disadvantage being the amount of shade cast by the mature oak. Basically, you will have to make a choice - big oaks providing shade for the property or flowering perennials? There are shade plants that can be planted under an oak, mostly low groundcovers, but even they have trouble competing.
So, back to the sappy residue you are experiencing in your yard. This sounds a whole lot like aphids, which generate a substance called honeydew, that will drip on lawns, houses, cars and people. It also can cause a sooty mold on the leaves and be very unattractive. This Colorado State University Extension article on Aphids on Shade Trees and Ornamentals will give you some information. They usually are not harmful, and you should avoid pesticides, which are more likely to kill the predators of the aphid than the aphids, themselves.
More Trees Questions
Roots in mulch around Live Oak in Austin
March 10, 2011 - I have a 20 yr old Live Oak that has about 6-8 inches of mulch buildup around the base (I had landscaped around it). I went to break up the mulch to remove it from the base of the tree, but found it ...
view the full question and answer
Young oak damaged by falling tree from San Diego TX
June 27, 2012 - My neighbor's Palo Blanco tree was struck by lightning and fell over our fence and on to a young oak tree in our yard. We waited a few days to see if the neighbor would offer help, but he never did,...
view the full question and answer
Care of desert willows
September 10, 2007 - We have three desert willows. Two are doing well, but the third, which was planted at the same time as the others, is about 1/3 the size of the other two, the foliage is thin, and the leaves have dry...
view the full question and answer
Insects along branches of cedar elm
April 14, 2011 - I have a 10 foot Cedar Elm planted three years ago. The smaller branches have what looks like incubating red pellet-like insects in a soft, putty colored glob--one insect per glob. They are all alon...
view the full question and answer
Non-native Japanese maple for East Northport NY
August 20, 2013 - I live in NY and I am looking to plant a Japanese maple in front of my house. It would be in front of a window so I'm thinking should I get a dwarf? Or a semi dwarf? I know I want a red color but un...
view the full question and answer
Support the Wildflower Center by Donating Online or Becoming a Member today. |