Answer: The first order of business is to get rid of the weedy vegetation on the embankment. I've had good success with an organic herbicide called AllDown. Alldown Green Chemistry Herbicide is a non-selective weed and grass killer made from organic acetic acid, citric acid, garlic, yucca plant extracts and water. An alternative is to use a flamer in the area (to kill the tops of the weeds) and then digging out the roots by rototilling and raking up the debris.
To keep the weeds from reappearing before your groundcover becomes established, you might want to lay down a weedbarrier or a thick (3-4" deep) organic mulch. If you use weedbarrier material, cover the entire area and then cut slashes or holes in it to plant. If you use organic mulch material, plant first and then cover the bare soil with your organic material.
Periwinkle (vinca) makes an excellent groundcover. Space your plants 12-18" apart, in a checkerboard pattern, for fast fill-in. You might also consider Fragaria (ornamental strawberry), Ajuga (carpet bugle), or Rosa Rugosa (groundcover roses) as groundcover for your embankment.
You can plant at any time of year, but spring and fall are considered the best months to plant because of typically cooler temperatures and abundant rainfall.
Good luck with your project!
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