Rent Shop Volunteer Join

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.

Help us grow by giving to the Plant Database Fund or by becoming a member

Did you know you can access the Native Plant Information Network with your web-enabled smartphone?

Share

Ask Mr. Smarty Plants

Ask Mr. Smarty Plants is a free service provided by the staff and volunteers at the Lady Bird Johnson Wildflower Center.

Search Smarty Plants
See a list of all Smarty Plants questions

Please forgive us, but Mr. Smarty Plants has been overwhelmed by a flood of mail and must take a break for awhile to catch up. We hope to be accepting new questions again soon. Thank you!

Need help with plant identification, visit the plant identification page.

 
rate this answer
2 ratings

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

Holding an Acer rubrum in a container for two years
October 10, 2008 - Dear Mr. Smarty Plants, I am thinking about ordering a Red Maple tree that is cultivated from Mount Vernon. I appreciate the historic nature of such a tree. The tree will be shipped to me and is ...
view the full question and answer

Determination of the sex of Mexican persimmon (Diospyros texana)
January 30, 2008 - Last spring, I planted a persimmon fruit from a Mexican Persimmon. I now have 6 small seedlings coming up. Since they all came from the same seed source - 1 black persimmon, will they all be male tree...
view the full question and answer

Fruit trees for Kempner, Texas
November 29, 2013 - I just moved to Kempner , TX and would like to plant a couple of fruit trees in my 1 1/4 ac yard. I would like to plant a species that will do well and produce edible fruit. Any assistance will be app...
view the full question and answer

Transplant shock in Chinkapin oak from Copperas Cove TX
June 18, 2012 - I have a newly planted chinkapin oak, appx 14' tall, in the Copperas Cove TX area. It has done great for the first two weeks. Now the leaves are yellowing (June) and beginning to dry up. I water it ...
view the full question and answer

Oak trees losing leaves in Longview, Texas
August 18, 2009 - One of my oak trees is losing its leaves (it is the first week in August). They are turning brown and falling at an alarming rate. The ground under this tree is covered, but my other trees seem unaffe...
view the full question and answer

Support the Wildflower Center by Donating Online or Becoming a Member today.