I agree with the Crossbow. It is very effective on woody plants. I add a teaspoon of Dawn dishwashing detergent to help it penetrate further into the plants. There will also be a residual seed bank of weed seeds. Let them germinate so you can eliminate them. Otherwise you'll be fighting them as they try to overcome the semps.
Remember that semps do like to grow in a flatter spot so save little flat crevices to plant the semps and add some good soil to these spots.
At my folks' place kin MA we converted a slope into a rock garden using the abundant native rocks.
GOOD LUCK!!
Kevin
