Building a Safer Vegetable Bed; Coaxing Palms, Gardenias and Magnolias to Grow

Q: Is pressure-treated lumber safe for building a vegetable bed, or should I use composite wood or stone blocks? -- David

A: My preference is to use natural materials for building gardens that contain edible plants, and "natural" does not include chemically treated wood. Consider using West Virginia or Pennsylvania fieldstone, which is flat, stacks well, fits curves and is ornamental. Use pieces three to five inches thick, and stack them three to four high. Concrete is available as preformed retaining walls at garden and home improvement centers. The structure should complement your other garden spaces.

This fall, the leaves on my 15-plus Japanese maples were beautiful. To my knowledge, they are all the same species, and the soil doesn't vary much. What accounts for their amazing variation in colors? -- Frank

When temperatures drop and days turn shorter, the chlorophyll that makes leaves green dissipates. Chlorophyll production dwindles at varying rates, even on the same plant. For example, foliage on the side of a tree directly exposed to sun might turn red, while the shady side might be orange or yellow. Reds, purples and bright oranges emerge from another group of chemicals, called anthocyanins, that some trees produce to help extract all the nutrients from dying leaves.

Do you know of native shrubs that can survive under closely growing mature black walnuts without being stunted by juglone? -- Robert

Juglone is a chemical produced by black walnuts that discourages most other plants from germinating and growing. Juglone-resistant plants include crossvine ( Bignonia capreolata), black raspberry ( Rubus occidentalis), tree hydrangea ( H. arborescens), pinxterbloom azalea, and two Exbury azalea hybrids, "Gibraltar" and "Balzac," as well as many ferns, asparagus, wild ginger, broomsedge ( Andropogon virginicus) and broad-leafed switchgrass ( Panicum clandestinum).

Of those, all but the Exbury azalea and asparagus are native to the United States.

The American Horticultural Society's Web site ( http://www.ahs.org) has some facts about plants that will survive under such trees.

We just bought a home with a six- to eight-foot magnolia tree that isn't very full. I would like to "top" the tree. Is this a good idea? -- Lynn

I never recommend topping a tree. It usually causes excess sucker growth; weakens the wood; and subjects the tree to breakage from wind, ice and snow. Your magnolia must be growing in too much shade. Encourage more light so it will grow full and keep its natural form by thinning branches from trees overhead or remove plants that might be creating too much shade. If it continues to lose ornamental value, remove it and replace with a tree more appropriate for the site.

I've been advised to lightly fertilize trees, shrubs and small perennials in late October to encourage root growth in fall and early winter. Your column recommended 10-6-4. Isn't the last number the chemical that promotes root growth? Why is it smaller than the nitrogen and phosphorus? -- Nancy

Plants' needs for nutrients are very complex phenomena. Each of the macronutrients nitrogen, phosphorus and potassium plays a role in all of a plant's growing processes. The numbers on the label are the percentage of each of three major nutrients that your plant might need. These numbers are a little like vitamins for people. Needs vary from one season to another. Perform a soil test to analyze all nutrients and necessary microorganisms, and find out which amendments are needed. Thorough plant and soil assessments are available from Soil Foodweb ( http://www.soilfoodweb.com, 541-752-5066).