Question #1: My beautiful huge oak trees have killed off all the grass under them in my yard. Do you know of a groundcover that grows well in the shade, is easy to establish in Oklahoma, and will also tolerate traffic well?
Answer: I don't know of any groundcovers that are tolerant of traffic except grasses. If you have a lot of traffic under the trees then it may not just be the shade that is causing the problem. Soil compaction caused by too much traffic will also kill plants. Turf type tall fescue is the only turfgrass that we recommend for bright shade conditions. Bermudagrass will not tolerate shade. Even so, tall fescue is a cool season grass and will need ample water during the summer months. In addition to the summer heat, the compaction and shade (if heavy shade) will make it difficult to grow even tall fescue.
Here is what I would recommend. Carefully till the area under the trees to loosen the soil being careful not to damage tree roots. If you can till to a depth of six inches that would be ideal. You may also thin the trees a little by removing selected branches to allow more light in if you wish. Some groundcovers that will tolerate shade and to a degree root competition include English ivy, periwinkle, creeping Jenny (moneywort), ajuga (bugleweed), bishop's weed, euonymus (wintercreeper), lamiastrum, liriope (monkey grass), and mondo grass. If there needs to be access through this area then a path should be constructed. A layer of bark would be fine or you could use some other paving material such as crushed gravel, concrete pavers, etc. *********************************************************************************************************************************** Question #2: What is the fastest growing groundcover to prevent soil erosion from water in sandy soil and heavy shade?
Answer: You do pose a bit of a challenge. Sandy soil and heavy shade can be a difficult combination to work with. Large Periwinkle (Vinca major) or English ivy (Hedera helix) would probably be my first recommendations and then Bishop's Goutweed (Aegopodium podagraria 'Variegatum') or St. Johnswort (Hypericum spp.). In addition to picking the right plant, the following steps should also be considered.
A soil test is recommended if you have not done so in the past few years. A soil test will indicate if there are deficiencies of nutrients that should then be added before planting. This is especially important in sandy soils.
Of course additions of organic matter to the soil will help improve moisture and nutrient retention and will benefit anything you plant in there.
A light layer of mulch, like clean straw, would be very beneficial at the time of planting to reduce erosion and water loss from the soil until the plants can become established.
Proper irrigation is important in the establishment of the groundcover. Keep soil moist, but not wet, until the roots have begun to grow. Then water thoroughly and less frequently for deep water penetration and thus deep root growth.
Any plants, trees or shrubs, or limbs from trees or shrubs that can be removed to allow more light into the area will also help.
This may be more than you bargained for, but when dealing with situations like yours, species selection is only one step out of many that are necessary for success.
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