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

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Saturday - May 04, 2013

From: San Antonio, TX
Region: Southwest
Topic: Groundcovers, Shade Tolerant
Title: Groundcover for shade under oaks in San Antonio
Answered by: Nan Hampton

QUESTION:

I have been modifying my landscape to reduce grass and the need for frequent watering. Have turned half the backyard into native plants garden with hardscape and sitting areas. Have 2 mature Monterey oaks in middle of backyard providing lots of shade, with a large saga palm between them. Can you recommend a ground cover or low-growing plant which will grow in all that shade?

ANSWER:

The biggest problem growing something in the shade of your oaks is, well, all that shade.   Most plants prefer to grow in sun or partial shade.  There are some, however, that will grow quite happily in the shade or partial shade.  Also, there may be a problem growing plants under your oak tree, not just because it will be shady but because the oak tree has an allelopathic effect on some other plants.  Allelopathy is a situation where a plant releases a chemical that has an effect on another plant.  The effect may be beneficial or harmful, but the ones we hear about the most are those that are harmful to another plant.  One of best known of the allelopathic plants is the walnut tree (Juglans spp.).  It has a very strong effect on most plants attempting to grow beneath it.  Trees with a lesser allelopathic effect include oaks, hackberries and eucalyptus trees.  Red oaks, which in your area would be Quercus buckleyi (Texas red oak), are mentioned more often than other oak trees.  You can read more about allelopathy in this article, Allelopathic plants: nature's weedkillers.  That said, not all plants are susceptible to the oak's chemicals.  Some plants can grow under oaks.  I couldn't find a list of plants native to Texas that reportedly grow under oaks, but I have seen the following growing happily under oaks:

Carex blanda (Eastern woodland sedge) is evergreen.

Carex planostachys (Cedar sedge) is evergreen.

Carex texensis (Texas sedge) is evergreen.

Chasmanthium latifolium (Inland sea oats) is perennial and turns golden brown in the winter.  You can cut them back down to the new green rosettes forming in the early spring to show their vibrant new growth.  They are happy growing in the shade of oaks.  This grass grows 2 to 4 feet and is very attractive.

Calyptocarpus vialis (Straggler daisy) will probably remain evergreen in San Antonio and do just fine under oaks.

Geum canadense (White avens) is evergreen and can be mowed.

Packera obovata (Golden groundsel) will form an evergreen ground cover.

Salvia lyrata (Lyreleaf sage) makes an evergreen ground cover.

Paxistima myrsinites (Mountain lover)  Here are photos and more information from Southwest Colorado Wildflowers.

 

From the Image Gallery


Eastern woodland sedge
Carex blanda

Cedar sedge
Carex planostachys

Texas sedge
Carex texensis

Inland sea oats
Chasmanthium latifolium

Horseherb
Calyptocarpus vialis

Horseherb
Calyptocarpus vialis

White avens
Geum canadense

White avens
Geum canadense

Golden groundsel
Packera obovata

Golden groundsel
Packera obovata

Lyreleaf sage
Salvia lyrata

Oregon boxleaf
Paxistima myrsinites

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