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Originally published February 15, 2012 at 3:01 PM | Page modified February 16, 2012 at 6:42 AM

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Using USDA zone-hardiness map to chart your plant purchases

Ciscoe Morris, Seattle Times garden writer, says Seattle did get a new number on the USDA zone-hardiness chart, but our weather does not always correspond to the number. Tips on pruning climbing roses and using coffee grounds in your garden.

Special to The Seattle Times

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Q: Do the changes to the USDA zone-hardiness map affect what plants we should buy?

A: The USDA created the Plant Hardiness Zone Map to help gardeners select plants that will survive in their particular climate zone. The zones are based on the average annual lowest winter temperature in a location, and each numeric zone spans 10 degrees (F) and is further divided into 5-degree "a" and "b" sections.

In theory, if you buy a plant listed as hardy in your climate zone it should be able to survive winter cold fairly reliably where you live. The problem is that trends and average cold temperatures can't predict what will happen in a given winter.

The Seattle zone was just changed from 8 to 8b. Zone 8 plants are hardy down to 10 degrees F. Zone 8b plants are hardy down to 15 to 20 degrees. Considering that the last two winters massacred practically every zone 8 plant in my garden, including ones that I covered, it's obvious that anything listed for 8b wouldn't have stood a chance.

Like all tools, you have to take the USDA Zone Map with a grain of salt. It doesn't mean you shouldn't try to grow plants that are listed for zone 8b. Pushing the zone limit is half the fun of gardening. Just be aware that sooner or later, another plant-murdering winter will come along and you'll be visiting the nursery to replace all of those 8b plants you were dreaming you could keep alive in your garden!

Q: I have a climbing rose that is about 20 years old and is very leggy with foliage only at the top about 12 feet up. Can I cut it back and restart growth?

A: Cutting back some, but not all, of the canes is a good way to renovate a climbing rose. Prune it in late February or early March. Begin by removing all dead, diseased, dying and weak shoots; plus saw off any dead stumps at the base. Then cut about 1/3 of the oldest canes to the ground. That should stimulate several vigorous new shoots to grow up from the base.

Roses tend to form flowering shoots only at the highest point on the cane. To encourage blossoming lower on the plant, tie in the new shoots as horizontally as possible. If the rose is growing against a fence or a wall, pound in horseshoe nails and use them to tie in the new canes horizontally.

If it's growing up a pillar or arbor, twist the pliable new shoots gently around the uprights, keeping them as horizontal as feasible. Stimulate additional side branching by snipping off the tips of the canes.

Keep your climbing rose well-fed and deadhead spent roses regularly. Your old rose will reward you by growing plenty of new shoots, and flower like it's a spring chicken!

Q: Is it true that coffee grounds make good fertilizer for rhododendrons and camellias? What's the best way to use them?

A: Used coffee grounds contain about 1.5 percent nitrogen and trace minerals including magnesium and calcium. It's a good organic fertilizer, but don't use it on your rhododendrons, azaleas, blueberries and camellias. That's because once brewed, the grounds lose their acidity and have a pH of about 6.9 which is great for tomatoes, roses and most perennials, but too high for acid-loving plants.

It's best to dig it into the soil around the root zone of your plants. They'll benefit from the nutrients, and the grounds will attract hardworking earthworms to aerate the soil as they devour their share of the grounds. Avoid applying coffee grounds on the surface around your plants as a mulch. Most of the nutrients would be wasted, and the grounds will develop a gross-looking mold.

By the way, if you've heard that used coffee grounds kill slugs and snails, it's not true.

Only unbrewed coffee grounds have a detrimental effect on mollusks, but they should never be used in the garden because they are deadly to dogs and earthworms.

Ciscoe Morris: ciscoe@ciscoe.com; "Gardening with Ciscoe" airs at 10 a.m. Saturdays on KING-TV

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