Growing Concerns 250: Enriching sandy soil.perennials for dry sun

We moved at the end of last year's growing season. I've spent my entire life gardening in heavy clay and deep shade -- these are two conditions I know very well. Now I have sandy soil and lots of sun. I took one elevated flower bed last year and added several bags of very rich peat thinking this would help, but the soil is still too sandy. What can I add to the soil to enrich it for a new perennial bed if I can't even get a contained bed to work?

I also could use some suggestions for a good "specimen" plant to use in my new cottage perennial bed. It needs to like sunny, sandy soil and I'd like it to be around three feet tall and have some interesting feature for three or four seasons. Would an oakleaf hydrangea (like 'Snow Queen') work? This was great in my last garden but with the conditions so different I'm not sure how it would do. - M.B. -

How thick was the layer of peat that you added to that sandy bed? If it wasn't at least four inches deep, it didn't make a significant difference in that soil's ability to hold water and nutrients. Spread four inches of well rotted manure, compost, or peat and fork or till it into the top eight to twelve inches of soil. That's one cubic yard of organic matter for every 80 square feet of garden -- about 40, 40-pound bags of composted cow manure or 13 two-cubic-foot mini-bales of Canadian peat.

Your work's not done yet. That organic matter will break down, be used up or leach away so it has to be replenished regularly. In an annual bed or vegetable patch that means yearly top dressing and tilling. In a perennial bed the year-round plant presence prevents tilling so we employ "tiller worms" instead. In our own and clients' sandy beds we maintain two inches of organic mulch -- shredded leaves, cocoa hulls, compost, or finely ground bark. We continually top up this mulch as it decays and keep the soil moist, thus providing both food and a hospitable environment for worms that churn the organic matter into the soil 24 hours a day, 200 or more days a year.

For a more permanent change, you can mix in a three inch layer of clay. I don't recommend this unless you know where the clay is coming from, since subsoil clay from a foundation excavation should not be your first choice of material. Also, clay is best dug and moved in fall -- the pressure of digging and tilling can ruin spring-wet clay by turning it into impermeable bricks.

Hydrangeas evolved in and along the edges of woods. They aren't suited to sand and sun. They produce undersized foliage, few flowers and may even die unless they have consistently moist, cool soil and some midday shade.

Butterfly bush (Buddleia davidii) flourishes in sandy, dry soil, however. We enjoy this plant's graceful arching shape in winter and early summer, love seeing and smelling the late summer and early fall flowers, and count the dependable appearance of butterflies and hummingbirds at the violet or white flowers as another "plus." This species is five to six feet tall when grown as a die-back shrub in zones 5-6, but the 3-4' dwarf 'Nanho' varieties would fit your size limits.

Whether it dies back or not, we cut butterfly bush to the ground every spring. So those that we grow change from six foot woody sprays to two-inch stubs every April, and don't regain their stature until mid-summer. Yet the stages they occupy aren't bare, even now. Butterfly bush has stringy, wide-spreading roots and doesn't cast any real shade until late June, so there's plenty of room at its feet and below its roots for early, deep-dwelling spring bulbs that also love warm, sandy soil. We cut Buddleia down every March and then stand back to admire the appearance in that same spot of dwarf bulbous iris (Iris reticulata), then early tulips (Tulipa praestens and T. daysystemon are favorites), and finally a combination of giant allium (Allium giganteum) and hardy foxtail lily (Eremerus himalaicus and its hybrids). These sprout, bloom and fade away before the butterfly bush recaptures the spotlight.

Perennial fountain grass (Pennisetum alopecuroides) provides the same time-share attraction when combined with bulbs. It's three feet tall in full, early September bloom. It needs division every five or six years to remain vigorous, however, and since it responds best to this operation in spring you'll have to wade into emerging bulb foliage to lift the grass, slice it into quarters and replant it.

Russian sage (Perovskia atriplicifolia) and blue mist spirea (Caryopteris x clandonensis) are two more die-back shrubs that enjoy the sun, heat and dry soil. Both are fragrant in all parts and provide color in late July and early August -- dusky violet Russian sage, powder blue Caryopteris. Cut back hard every spring as their companion bulbs break the surface, they just reach three feet in height as they begin to bloom.