En Español

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

Is a wasp colony in hollow in my oak tree harming it?
November 26, 2013 - I have wasps colonizing a hollow portion of a live oak here in my backyard in Austin ten feet from my backdoor. They are yellow and black banded, maybe an inch long. Two questions: 1. Are they hurt...
view the full question and answer

Stump sprouting of Oak trees in the wildfire area in Bastrop, TX.
May 13, 2012 - We live in Bastrop, Texas, in the wildfire area. We lost all of our trees. The oak trees have "suckers" growing from the base of the burned tree that has been cut down. They look like little bushe...
view the full question and answer

Trees for home in Kansas
April 01, 2012 - I am needing help planting trees in my yard. It's a new construction home and I would like a tree that won't get into the septic system easily. Also, I have to plant 3 other trees as well due to the...
view the full question and answer

Need trees to screen view of parking garage in Houston, TX.
December 29, 2011 - We live in Houston, TX with a beautiful lot except a 4 story parking garage has been built behind us. How can we screen this and the lights out of site. It looks terrible from the second story espec...
view the full question and answer

Why do my Possumhaw Holly berries fall of in the summer in Euless, TX?
June 28, 2011 - My possumhaw holly has LOTS of green berries in the spring but they fall off in summer, so that I have only a handful of red berries in the winter. What is going wrong?
view the full question and answer

Support the Wildflower Center by Donating Online or Becoming a Member today.
E-NEWSLETTER | BECOME A MEMBER | DONATE NOW | MEDIA | JOBS | SITEMAP | STAFF INTRANET
© 2016 Lady Bird Johnson Wildflower Center