Rockrose will shade out stubborn horsetail on bank

Q: We have a bank 15 feet deep by 40 feet wide covered with vinca. It looks terrible because horsetail grows up through it. I stopped watering it this summer, and much of it turned brown. What do you suggest I plant in place of the vinca, and what should I do to control the horsetail? Also, I would like some ideas about what to plant in the flat area at the top of the bank so it that will be attractive year-round and different from the bank below. The garden area is about 5 feet wide by 15 feet long. Because of the view, I want to limit plants to 4 feet.

A: Horsetail is very difficult to eradicate since its roots go as deep as 6 feet. A knowledgeable source tells me there are no herbicides for horsetail removal registered for homeowner use, so you would need a licensed professional to spray the bank, and repeated applications over several seasons would be necessary. During the process, your bank would be bare of plant growth and subject to erosion.

I suggest that instead of herbicides, install plants on the bank that will grow taller than the horsetail and shade it out. White rockrose (Cistus x hybridus), with its dense evergreen structure, would be a good choice. In my experience, white rockrose has a neater habit, and the foliage is denser than orchid rockrose (Cistus x purpureus) or crimson-spot rockrose (Cistus ladanifer).

Plant the rockrose right into the vinca, trimming enough away to get to the soil. Leave the rest of the vinca to provide erosion control until the rockrose grows. Space the plants no less than 3 feet apart if you want complete cover in a year or two. The plants' ultimate size is 3 to 4 feet tall and to 8 feet wide. The plant is hardy to 15 degrees Fahrenheit, so it may not survive arctic blasts and will be fine most winters. The white flowers with yellow centers are attractive in summer.

Since rockrose thrives in hot, dry sites, when you stop watering the bank after the rockrose gets established, the dryer soil will discourage horsetail, which prefers moist sites. The same goes for the top of the bank, too. Plant low-water-use plants, so you are not creating moist conditions for the horsetail. You will need to be diligent about removing new shoots by hand until it exhausts itself. Choose a mix of evergreen and deciduous shrubs along with perennials to create a changing composition through the season.

I think the classic South of France combination of rosemary and lavender is unbeatable for the contrast of the green of the rosemary and the gray of the lavender, each chiming in with flowers in variations of violet blue. Choosing among all of the cultivars of these two is daunting. If you, gentle readers, let me know which ones have been successful for you, I will pass on the information.

Sunset rockrose (Cistus 'Sunset') would be a great choice here with small gray-green leaves with magenta flowers appearing on and off though the summer that would look quite different than the white rockrose on the bank below. 'Sunset' is lower-growing, to 2 feet, than many other rockroses, so will stay in bounds. Hebe is another good low-growing evergreen with many species and cultivars available. Hebe 'Autumn Glory' is 2 feet high with dark green leaves and late-summer, deep lavender flowers. Bring in a larger leaf texture with Phlomis russelliana, a perennial with yellow flowers that form spikes that persist into winter.

This is a perfect place to tuck in a few bulbs. They need no summer water, and if you choose smaller ones, their leaves will fade away unobtrusively when they are done blooming. Species tulips and narcissus will naturalize. 'Lady Jane' tulip (Tulipa clusiana 'Lady Jane') has red petals with white margins. Hoop petticoat daffodil (Narcissus bulbocodium) has fat little yellow flowers only 6 inches high.

Phil Wood has a degree in landscape architecture and designs and builds gardens.