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Bad Gardening Advice

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Ask gardeners where they learned about plants, and most will tell you about a parent, grandparent, or favorite aunt or uncle who passed along a love of digging in the dirt. Certainly these green-thumbed mentors did a lot right. It was, after all, their colorful perennial borders and bountiful veggie patches that inspired the next generation to pick up a trowel. Amid all their sound advice, though, they probably passed down some less-than-scientific lore, too. Many homegrown garden tricks simply don't live up to their hype when researchers put them to the test. But old ways are slow to change. After all, most gardeners learn from one another rather than by brushing up on the latest university study. So we've done the research for you. The next time a well-intentioned neighbor offers up one of these time-tested "tips," you'll be able to weed out fact from fiction.

1. Buried Banana Peels Give Roses a Potassium Boost

Illustration by Hadley Hooper

Bananas and their peels do contain high levels of potassium, an essential nutrient that roses—and all garden plants—need for everything from stimulating growth to producing flowers. But burying whole peels can backfire. As soil microorganisms work to break down the peels, they extract significant amounts of nitrogen from the soil, which results in less nitrogen for greening up plants. The best place for banana peels is in a compost pile, where they can break down alongside other nutrient-rich table scraps. To give plants the balanced nutrition they need, top-dress with compost instead.

2. Drought-Tolerant Plants Don't Need Watering

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Drought-tolerant plants may need less water than other plants, but that doesn't mean you'll never have to pull out your garden hose. If the garden or container soil around your plant is dry, water it. Young plants are especially susceptible to drought because their roots are getting established. Be vigilant about keeping soil slightly moist, but not soggy, throughout a plant's first year, regardless of its reputation for resiliency.

3. Organic Pesticides are Safer Than Synthetic Ones

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Snake venom, arsenic, poison ivy—they're all natural, but that doesn't make them safe. By the same token, there are many natural toxins used in organic garden products that are potentially harmful. If misused, natural poisons, such as pyrethrin (an insecticide extracted from chrysanthemum flowers), are hazardous to people, pets, and the beneficial inhabitants of our gardens, such as frogs and bees. If you must use a pesticide, base your selection on how dangerous the active ingredients are, and how effective. Safer choices include products that contain Bacillus thuringiensis, aka Bt, and insecticidal soap.

4. Sprinkling Coffee Grounds Around Acid-Loving Shrubs Lowers The Soil's pH

Illustration by Hadley Hooper

Coffee grounds are acidic, and mixing them into the soil can affect pH—slowly. But here's the catch: Fresh coffee grounds can inhibit plant growth because they tie up nitrogen in the soil as they decompose (just like banana peels), especially if large quantities are added. To lower your soil's pH without causing a nitrogen deficiency, purchase a sulfur-based soil acidifier (available at garden centers) and amend soil following the package instructions. Many popular shrubs, including azaleas, heathers, rhododendrons, and blueberries, will appreciate soil with more acidity.

5. Adding Fertilizer to The Hole Helps Transplants Establish Faster

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No fertilizer, or other soil amendments, on hand? No worries. Adding them to a planting hole isn't necessary and, in some cases, can actually discourage a vigorous root system. Nutrient-rich planting holes can give roots less incentive to branch out to absorb nutrients and moisture from the surrounding area; and fertilizers, including the phosphorus-rich fertilizers frequently marketed for new transplants, contain salts, which can burn tender new roots if they're not incorporated into the surrounding soil. If you're concerned about soil fertility, you're better off giving plants a nutrient boost by spreading a 1- to 2-inch layer of compost, then 1 to 2 inches of mulch over the planting site. Just be sure to leave a few inches of breathing room around each plant's stem, especially when mulching trees. Mounding soil or mulch around a tree trunk can cause girdling roots that encircle the trunk and slowly strangle the plant.

6. Adding Sand to Clay Soil Improves Drainage

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Waterlogging is common in clay soil because clay particles are extremely fine and easily compacted. Sand is just the opposite. It drains quickly because its particles are coarse and less tightly spaced. So adding sand to clay soil must speed up drainage, right? Not quite. The tiny clay particles simply fill in the gaps between the sand grains, resulting in a substance similar to concrete. If you want to improve clay, the secret ingredient is compost, not sand. For immediate results, till a 2-inch layer of compost over the entire planting area. Or, if the site is already planted, you can reap similar benefits by top-dressing with compost. It will just take a year or two for the compost to infiltrate the clay.

7. Newly Planted Trees Require Staking

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Unless your tree is top-heavy or in an especially windy site, it does not need staking. A little movement is actually good for young trees. Just as our muscles grow larger with exercise, tree trunks grow thicker and stronger when they're allowed to move. Staked trees tend to grow taller, but their trunks are skinny and weak, so if you decide to stake, be sure to stake as loosely and as briefly as possible. Rarely is staking necessary for longer than six months. Use something soft, such as a length of garden hose, against the tree bark to keep from cutting into it.

8. Painting Pruning Cuts Protects Trees From Disease and Insects

Photo by Karen Bussolini

Professional arborists gave up the practice of painting tree wounds and pruning cuts years ago. The fact is, there's little evidence that pruning tar, or any other compound, prevents disease or insects from entering tree wounds. Research even suggests that this practice slows trees' natural healing process of sealing cuts with a tough layer of "woundwood." The best ways to avoid damage are to make clean cuts with sharp tools and prune during late winter, when diseases and insects are dormant.

9. Gravel in The Bottom of Containers Improves Drainage

Illustration by Hadley Hooper

This is one myth that never seems to die. But rather than preventing root rot, adding gravel makes it more likely to occur. Water is pulled down through the container by gravity and builds up near the drainage hole. A layer of gravel at the pot's base serves as the drainage hole and collects water in the same way. So instead of preventing roots from sitting in water at the container's base, the gravel simply moves the pool of water higher up the pot, where it can do more damage.The best way to ensure adequate drainage is to use a potting soil made with coarse materials, such as pine bark. You can also stir in several extra handfuls of perlite, which helps keep potting soil light and airy.

10. Baking Soda Cures Black Spot

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Although it works fairly well for powdery mildew, baking soda is not effective on black spot. But here's a home remedy that does work: Mix 1 part milk with 2 parts water; spray affected foliage once every week or two, before black spot becomes a serious problem. This solution can also help control powdery mildew. But don't use it on edibles, since milk sours.