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November 29th, 2011

Toxic Trees

We like to till in leaves with manure in the fall in our veggie beds. Are there any leaves that are toxic? In our county, we have cottonwood, aspen, oak, and maple.

blog-dougBlack walnut (Juglans nigra) and tree of heaven (Ailanthus altissima) are two trees to beware of as you gather leaves to use for soil improvement. All parts of black walnut trees contain juglone, a chemical that is toxic to certain other plants, including tomatoes and rhododendrons. The juglone breaks down after a year of composting; gardeners who compost the leaves of walnuts or related species (pecans, hickories, and butternuts) should make a separate pile from the rest of their yard waste. The growth-inhibiting chemicals produced by tree of heaven are found mostly in the roots but may also be present in the leaves.

This ability of some plants to influence the growth and survival of neighboring plants by producing a toxic substance is known to botanists as negative allelopathy. Plants such as black walnut use this trait as a way to reduce competition from other species. Some of the most aggressively spreading weeds—think garlic mustard—literally exterminate the competition with allelopathic chemicals.

Allelopathy can also work to the gardener’s benefit. Corn gluten meal, sometimes used as a pre-emergence weed killer in lawns, contains an allelopathic compound that stifles the germination of weed seeds.

But back to your original question: Cottonwood, aspen, oak, and maple leaves can all be tilled into garden soil with no fear of toxic repercussions.  —Doug Hall

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November 22nd, 2011

Termites and Mulch

I’m worried about using mulch near my house for fear of attracting termites. What is the best alternative to leaving a strip of bare soil next to the foundation?

blog-dougThe bad news is that termites are ever-present in all but the coldest climates. The good news: You don’t have to invite them into your home.

The strip of bare soil you’re trying to avoid is actually your best defense against these destructive, wood-eating insects. A study at the University of Maryland showed that termites colonize the moist soil beneath mulches of all types, including non-wood mulches like pea gravel and plastic film. Entomologists report that soil moisture, more than the presence of wood, determines where termites establish their colonies. Because their soft bodies are susceptible to desiccation, termites avoid dry soil.

Your strategy, then, is to keep a “dry zone” a foot or two wide around the house. Contour the ground so it slopes away from the foundation. Install roof gutters and direct the downspouts out into the yard. Keep plants—especially plants that offer a dense cover of foliage—at least a foot from the house. Design landscape beds nearest the house so they can be sustained without supplemental irrigation, and use no more than an inch of mulch in these beds.

Keep a watchful eye out for signs of termite infestation: pencil-thin mud tubes on foundations and walls, the occasional “swarming” of winged adults, and, of course, damaged wood.  —Doug Hall

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November 15th, 2011

Winter Storage for Dahlias and Cannas

How can I keep my dahlia roots for next year? Frost killed the tops last week. I spent good money on the roots and I’m not willing to let them become expensive compost.

blog-dougI’m with you. It takes just a few minutes to dig and store dahlia tubers, so why not keep them for a repeat performance next summer? Dahlias survive the winter outdoors in Zone 9 and warmer; everywhere else, you can dig up the tuberous roots and overwinter them indoors. The same technique works for cannas and other tender summer bulbs.

After frost, cut the stems to stubs. The sweet-potato-like dahlia tubers extend outward from the stem about 6 inches, so be careful not to slice into them while digging. Shake the soil off the roots and let them dry in the garage for a few days. If you have more than one variety, write their names on the tubers with a felt-tip marker.

Some gardeners divide their dahlias at this point, leaving one growth bud or “eye” per division and dusting the cut surfaces with garden sulfur as a fungicide. I prefer to store dahlia and canna clumps intact, waiting until spring to divide them, because I’ve had better success that way.

Pack the roots in paper bags or cardboard boxes with dry sphagnum peat moss, sawdust, or vermiculite surrounding them. Store them at a steady temperature between 35°F and 45°F. Back in the days of unheated, damp basements and root cellars, it was easier to store tender roots without their shriveling. Nowadays drier basements prevail, and it’s wise to check on the stored roots monthly. If they start to shrivel, lightly moisten the sawdust or peat.  —Doug Hall

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November 8th, 2011

Knotty Tomato Roots

I just pulled my tomato plants out and the roots are a mass of ugly bumps and warts. They were planted in new raised beds with a mix of soil and compost from the city. What am I dealing with, and how do I correct the problem?

blog-dougWhat you describe sounds like the galls of root-knot nematodes. Nematodes are small soil-dwelling roundworms; when they parasitize or feed on plant roots, crop yields suffer. It’s possible that the tomato transplants you purchased were infected. Destroy the infested tomato plants—don’t add them to your compost pile.

There are thousands of species of nematodes, and most are beneficial. Good nematodes are part of the soil “food web” in which nutrient components of organic matter are released in forms that plants can use. Other beneficial nematodes prey on cutworms, beetle larvae, and destructive nematodes.

Even though many vegetable crops are susceptible to nematodes, each is parasitized by different nematode species, so crop rotation helps to minimize the damage. Plant your tomatoes in a place where tomatoes or related vegetables (peppers, eggplants, potatoes) have not grown for at least three years. Another control tactic is to fortify your soil with plenty of compost, which feeds the beneficial microbes that keep root-knot nematodes in check.

Next spring, look for nematode-resistant tomato varieties, or invest in grafted tomato plants, which are grown on a vigorous rootstock that is resistant to nematodes and soil-borne diseases. If these efforts fail, consider solarizing the soil by covering it with a sheet of plastic for 6 to 8 weeks in summer. Because solarizing kills beneficial microbes as well as pests, consider it your option of last resort.  —Doug Hall

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November 2nd, 2011

Pruning Storm-Damaged Trees

A freak snowstorm last weekend broke a branch off my new Japanese maple, leaving a huge hole on one side of the tree. I just planted it this spring and now I’m heartbroken. Is it fixable?

blog-dougThe beauty of some trees is enhanced by their symmetry, with branches spaced evenly along ramrod-straight trunks. Fortunately for you, Japanese maples don’t need a perfect framework in order to look good. In fact, these small trees are often admired for their free-form, sculptural architecture. Sinuous, craggy branches add to a Japanese maple’s venerable character.

Another factor in your favor is the tree’s youth. Mature shade trees that lose a major limb in a storm will probably never regain their symmetry. Young trees, on the other hand, will likely outgrow their lopsided appearance, even if it takes several years.

That said, you need to prune the damaged spot to promote quick healing and prevent decay organisms from entering the wound. If the branch left a stub when it broke, use a sharp pruning saw or loppers to cut it back to the point where it connects to another branch or the trunk. Instead of a “flush” cut that would remove the branch collar and leave a large wound, prune just beyond the collar where the branch is of a smaller diameter. If any bark has been torn, cut it off carefully to leave no loose, hanging bits. There’s no need to use tree paint to seal the wound; nature does a better job of healing on its own.  —Doug Hall

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