Make certain soil provides good drainage for new sedum

Q: I want to try sedums as ground cover on a hot, dry, sunny spot I have covered with an inch or two of small-size bark mulch. Can sedums root and spread through mulch? Or should I remove the mulch around them to encourage them to spread more quickly?

A: It's more important that you prepare hospitable soil with good drainage to get your new sedum off to a good start. If you choose sedum that tolerate heat and drought, and water them well through the first summer and autumn, they'll root and spread through the bark just fine. Sedum also does well with a gravel mulch.

When selecting your new sedum, remember it's a misconception that they all love heat and drought. For example, the pretty little yellow Sedum makinoi 'Ogon' needs protection from hot afternoon sun. The most vigorously spreading sedum in my garden, planted in full sun, is the coppery golden S. rupestre 'Angelina.'

Q: I have a big bog in one corner of my garden that is always damp no matter how many drains we put in. Would this area work for a rain garden? I'm tired of fighting with it because most things I've planted there die off.

A: In Northwest winters like the last one, we all have rain gardens. So it's best to accept that fact and work with the natural topography and water levels in our gardens. This is the idea behind rain gardens, which are environmentally sound because they help to slow erosion and filter rainfall and storm water runoff.

There are many aspects and purposes to rain gardening, from installing rain chains to re-landscaping your garden with eco-swales. More importantly for most of us, it means an acknowledgment of, and the resolution to work with, weather patterns and our gardens' natural conditions.

Whether you decide to plant your bog with natives like skunk cabbage and red osier dogwood (Cornus stolonifera), or go for drama with the giant-leaved Gunnera manicata or showy flowers such as Iris pseudacorus 'Variegata,' be sure to choose plantings that thrive in damp conditions.

Information

Seattle Public Utilities: www.seattle.gov/util/naturalsystems.

Rain Garden Network: www.raingardennetwork.com.

To learn more, check out Seattle Public Utilities' and Rain Garden Network's Web sites, and the new book, "Rain Gardens: Managing Water Sustainably in the Garden and Designed Landscape," by Nigel Dunnett and Andy Clayden (Timber Press, $34.95).

Q: I've admired allium in neighborhood gardens this spring, but don't know when to plant it or what it needs.

A: Alliums are ornamental onions, and there are a great many kinds from tiny white willowy ones to Star of Persia (Allium christophii) with basketball-size flower globes of lilac-colored stars shooting out like sparklers.

Allium grow from bulbs planted in autumn, and like all bulbs enjoy sun and require good drainage. When planted in free-draining soil, allium are truly perennial and will multiply over the years.

Be aware that allium have a somewhat unusual growth pattern. Rosettes of leaves arise from the ground in early spring, followed by tall, strong stalks that develop spherelike flower heads in late May and early June. By the time the flowers come on, the foliage begins to wither unattractively.

My favorite allium is 'Globemaster,' which is tall with gloriously purple tennis-ball-size flowers. I'm newly enamored with the grape-colored 'Purple Sensation,' which blooms a little earlier than 'Globemaster.' Both have great impact for the little garden space they take up, and make showy, long-lasting cut flowers.

Valerie Easton also writes about Plant Life in Sunday's Pacific Northwest Magazine. Write to her at P.O. Box 70, Seattle, WA 98111 or e-mail planttalk@seattletimes.com with your questions. Sorry, no personal replies.