Though plants, soils and weather conditions are always evolving, it seems as if some gardening practices become embedded like fossils. I once encountered, for example, a person whose family had heard that whacking the trunk of a tree with a newspaper every night would increase its diameter, and so he whacked his trees regularly, too. With newspaper, the practice may be harmless, but substitute a board or a stick and it wouldn't be hard to seriously damage vital tissue just under the bark.
Tree whacking is easy to pooh-pooh, but what about tree staking? Recent studies at the University of California at Davis have shown that most people stake their young trees needlessly and for too long. The cords and wires (even cushioned with a piece of rubber hose) can damage the cambial layer and retard growth of the trunk. It's an entrenched practice that injures plants rather than helping them get stronger. Although time-honored gardening techniques and folk wisdom can be valuable resources, gardeners who don't constantly check their assumptions run the risk of working hard to no good end, or worse, actually harming their plants.
Here's a checklist of six widely held gardening assumptions for you to measure against your own practices. The trick to a lifetime of effective gardening is to stay flexible and be willing to change the things you do, based on your own observations and the guidance of modern research. Remember, if you don't keep growing, your plants may be suffering too.
A good gardener is a good observer. Have you ever seen a forest (or a garden) littered with the carnage of leaves fried by the sun shining through rain droplets? Thunderstorms arise every summer with amazing speed and the sun often comes out afterward. Plants are well adapted to deal with this. You'll never find burn damage on leaves after watering plants with a sprinkler of any kind. Any holes "burned" in leaves are due instead to fungal or bacterial diseases and have nothing to do with sunlight shining on droplets.
A good reason not to water at midday, however, is that the moisture you supply will be lost much more quickly to heat or wind then. Water promptly whenever you see that plants need it, but the ideal time is in the morning when the water has a chance to drip from the foliage and soak through mulches.
According to the Lawn Institute, thatch is produced more by the misuse of strong fertilizers and pesticides than by clippings alone. Thatch is a layer of dried grass clippings that builds up on the soil surface. As long as this layer is less than 1/2 inch, it's harmless and in fact has some of the same benefits as a mulch. A healthy soil biota can easily decompose the normal amount of clippings to keep the thatch layer from building up. But earthworms, which perform much of this task, are especially vulnerable to popular lawn chemicals.
For a healthy lawn, mow frequently. Short clippings are tender and rot quickly. Set the mower blade high -- thicker lawns shade out weeds. Adjust the pH to 6.5 to 7.5, the range favored by most grass species. Fertilize only if the lawn really needs it; if you are leaving the clippings, they will return fertility to the lawn.
In the early 1970s, Carl Whitcomb, a researcher at Oklahoma State University, disproved this oft-repeated advice. In controlled studies using percentages of different amendments (up to 40%), roots of ornamental trees and shrubs were consistently larger in unamended soils. The amendments seemed to encourage roots to stay in the vicinity of their planting holes and not grow out into the unamended soil, leading to stunted root systems. Whitcomb concluded that it's best to let the roots begin to grow in the nativ soil right away and to use organic matter on the surface as a mulch, rather than mixing it with the soil. If a tree isn't suited to native soil, you are better off growing it in a container than trying to change the soil with amendments.
Though U.S. Forest Service worker Alex Shigo debunked this myth some 20 years ago, the news is just now getting around. A truly flush cut does serious damage to a tree. You should leave the branch "collar," a swelling or shoulder around the base of each limb. Shigo discovered that this collar has the ability to produce chemical changes in the area behind a break or cut that preserve tissue and confine decay to a small area, preventing it from moving into the core of the wood. Look at trees in your walks around town or in the woods, and you'll begin to notice these collars, which differ in size and shape with each species.
When a bare-root tree begins to grow, it's able to limit new shoot and leaf growth to the capacity of the root system on its own. Pruning takes away some of the healthiest buds and robs the tree of stored energy. More ground-breaking work by Carl Whitcomb in 1979 demonstrated that removing apical buds provided little benefit to ornamental trees. He showed that trees pruned by more than 15% exhibited "reduced visual quality" as they matured.
If, however, a new tree has a broken limb, prune it back to the nearest strong bud or side shoot. After the tree has grown for a season and recovered from the stress of transplanting, begin pruning for a balanced shape with well-positioned main limbs. Use the least number of cuts possible.
A little common sense disproves this chestnut as well. If the soil in the bottom of a pot stays wet, a plant's roots get waterlogged and rot. But it's the holes in the bottoms of containers that allow the water to flow through. In pots with no drainage holes, a layer of gravel will do no good. In fact, any gravel in the bottom reduces the volume available for potting soil, as well as making everything heavier than it needs to be. All that's needed is a small fragment of terra-cotta over the pot's drainage holes to keep the soil from slipping out. If the pot rests in a shallow saucer, don't habitually keep the saucer full of water or roots in the bottom layer of soil will be damaged.