Explore Plants

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
    
 

Can't find the answer in our existing FAQs, submit a question to Mr. Smarty Plants.
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

Cold tolerance of Anacacho Orchid Tree (Bauhinia lunarioides)
January 13, 2010 - I live in Austin, and I'm considering planting an Anacacho Orchid Tree. How cold tolerant is it? Would the tree have been damaged in the recent 18 degree temperatures we experienced?
view the full question and answer

Distance for Escarpment oak to house from Lewisville TX
August 22, 2010 - I am planting an Escarpment Live Oak about 15' from my house. Thats as far away as I can plant it. Will this be a safe distance? How large will it be in 20 years?
view the full question and answer

Positioning a bald cypress among cattails in Silver Spring MD
April 30, 2009 - We have a rain garden, half of which is fairly overrun with broad- and narrow-leaf cattails. We've learned to be aggressive in thinning these out 2 to 3 times during the growing season. We also have ...
view the full question and answer

Probably non-native crapemyrtle trees damaged by hurricane
January 15, 2009 - I have 5 crape myrtle trees. I live in Galveston, Tx and when Hurricane Ike came through in September the salt water I think killed them. They have not come back since then and are brown with no leave...
view the full question and answer

Trouble with Redbud in Illinois
March 13, 2010 - I have a red bud bush, we live in northern Illinois, it is shedding its bark. Is this normal or is the bush dying?
view the full question and answer

Smarty Plants's Facebook profile Support the Wildflower Center by Donating Online or Becoming a Member today.

Mr. Smarty Plants wants you to be his Facebook friend. Click the Facebook icon to add yourself to Mr. Smarty Plants list of friends.