Can you plant a very low ground cover in the beds? Something that will hold the soil, but not compete in height to the plants there. We have used a very low growing creeping thyme in the past, as both a filler between stepping stones and patio stones, and in very steep planting beds. It doesn't care too much for direct sun, but it did well in the beds. Grows/spreads quickly and has deep roots to hold the soil. It won't compete with the other plants for nutrients. You local nursery will have other suggestions for low plants that you can walk on.