Railroad Ties, Vegetables Are Not a Good Mix

QUESTION: We are using railroad ties for a border around vegetable beds on a moderately sloping hill. From their appearance, it is evident that they are heavily treated. A friend from Germany was concerned about the treatment leaching into the soil and affecting the plants. Is my friend’s concern valid?

--J.R., Palos Verdes

ANSWER: Although people have been using railroad ties for years to hold back hills and make raised beds, I would not use them near vegetables. Any material that prevents wood from rotting cannot be good for the vegetables or you.

Railroad ties have been treated with creosote, which is toxic, said Garn Wallace at Wallace Laboratories, a soil and plant testing service in El Segundo. He pointed out that creosote is a wood tar, so it is an organic compound that will decompose and dissipate in time, but then the wood begins to rot.

Wood treated with chromated copper arsenic, commonly called “pressure-treated,” also leaches toxic materials, including arsenic, only these are minerals that last a long time in the soil. Organic Gardening magazine researched pressure-treated wood and the conclusion was that no type of chemically treated wood should be used near edibles.

Better bets for vegetable beds are natural redwood or cedar boards, or concrete blocks, though new concrete will leach calcium hydroxide into the soil for about six months, which inhibits plant growth.

Best of all on a hillside would be old, broken concrete slabs, laid in layers to make short retaining walls. These hold back the soil but let excess water pass through and are nearly inert chemically. For regular raised vegetable beds, redwood is the most commonly used material, though it will begin to rot after several years.

Black Soot on Citrus Caused by Insects

Q: What causes orange and lemon trees to have black soot on leaves and fruit?

--I.K., Los Angeles

A: Scale insects are the cause of “sooty mold.” They feed on the plant, secrete sugars and the black mold grows on the sugars. Scale are normally controlled on citrus by several predators, including certain wasps (Metaphycus) and a lady beetle (Chilcorus cacti), but if these are killed by spraying with pesticides, an accumulation of dust on the leaves or ants, the scale get the upper hand.

First, wash down the tree to remove dust that inhibits predators (a common problem in urban L.A.), then control any ants in the tree by applying a wide band of a sticky barrier, such as Tanglefoot, sold at nurseries. Make sure no leaves or branches touch the ground or fences or walls, making ladders for marauding ants.

To kill existing scale, spray with a horticultural oil, such as SunSpray Ultra-fine, which suffocates these pests. It works best on young scale, which are most active in late summer and early fall, so spray now and again in late summer. Be sure to follow label directions, especially those dealing with the daytime temperature. Even highly refined paraffinic oils like this shouldn’t be used when it is very hot and humid. After treatment, do not spray the trees with any other pesticides and the natural predators should again get the upper hand.

Herbicide Can Save the Fescue From Invader

Q: I have recently noticed that some Bermuda grass is showing up in my tall fescue lawn. Is there a product that I can obtain to selectively kill the Bermuda and not harm the fescue?

--R.H., San Gabriel

A: Surprisingly, yes. Normally, herbicides kill broad-leafed weeds in grass lawns or kill grasses in broad-leafed plantings (or kill everything). But there is an herbicide that will kill warm-season grasses, like Bermuda, in cool-season lawns, like the tall fescues (Medallion and Marathon are two well-known trade names for tall fescues).

This herbicide is called Turflon, and though you may be able to buy it at an agricultural chemical dealer, it is probably easier to have a lawn-care company buy and apply it for you. If you do apply it yourself, carefully follow label directions. Monterey Chemical Co., which packages Turflon, is working to get it relabeled so it can be sold at nurseries and garden centers.

Bermuda grass is not listed on the label, but kikuyu, another pesky warm-season grass, is. But John Rector, the agronomist at Pacific Sod, tells me it works well on Bermuda grass in fescue lawns. He also said it kills oxalis in cool-season grass lawns like fescue, which was a question sent in by another reader.

Yet another reader asked what herbicide to use to get oxalis out of a dichondra lawn, but these two plants are too similar so there is no selective herbicide to take one out of the other. Hand-weeding is the only way.

Solving the Mystery of Little Holes in the Lawn

Q: On my frontyard, I find little holes dug up--not big enough to be from squirrels, though some are pretty big. I find them during the colder and wetter times of year, and often after I have watered. How can I identify this annoyance?

--B.R., Glendale

A: If you lived back East I’d say they were caused by mole crickets, but out here they are probably caused by nightcrawlers, those big earthworms that deposit their castings on the soil surface. When the soil is wet, they come to the surface to escape the water and can be quite numerous. They might be an annoyance, but they aren’t hurting the garden. Venture out at night with a flashlight to make sure they are the culprits.

Questions should be sent to “Garden Q&A;” in care of the Real Estate section, Los Angeles Times, Times Mirror Square, Los Angeles, CA 90053. Please include your address and telephone number. Questions cannot be answered individually.