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Myth vs. reality: What's the truth behind some common gardening practices?

Reality can get skewed when there are so many sources of information – books, magazines, newspapers, nurseries, and, most of all, the Internet and social media open up lots of room for contradiction. So, how do you find the right answer to gardening questions?

Nine experts from Oregon State University Extension Service stepped up to bust some common gardening myths. Read on to get some research-based answers to 10 common misconceptions.

For additional questions, call the OSU Extension master gardeners in your area.

MYTH: You should top a tree to control its height.

REALITY: Trees are programmed to attain a certain height. Topping only temporarily delays the inevitable. The resulting sucker growth, which grows rapidly in an attempt to provide food for the compromised root system, is weakly attached. This creates an even greater hazard. Additionally, the trunk is not a limb and cannot use the tree’s architectural physiology to seal the wound caused by topping. This often leads to a slow death for the tree. – Al Shay, horticulturist and site manager for OSU’s Oak Creek Center for Urban Horticulture

MYTH: Lime will remove moss from your lawn.

REALITY: Lime will not fix the problem. Moss prefers to grow in wet, shady conditions. Lawns with moss need more sunlight, i.e. trimming, pruning, and thinning trees. If you like the trees the way they are you will continue to have moss and you should think about shade-tolerant alternatives to grass. Moss also grows well in infertile soils, which includes acidic (low pH) soils, but more importantly, it also includes nitrogen-deficient soils. Lawns, like a lot of cultivated plants, prefer nitrogen-rich soils. Regular fertilizer applications (four applications per year, two in the fall and two in the spring) with products containing nitrogen, combined with improved sunlight will result in a green, dense lawn that can out-compete moss. – Alec Kowalewski, OSU Extension turf specialist

MYTH: Ponderosa pine needles make the soil more acidic (low pH).

REALITY: The notion that pine needles change the soil pH so that nothing will grow or that it will damage plants has been out there for years. The truth is pine needles do not make the soil more acidic. It is true that pine needles have a pH of 3.2 to 3.8 (neutral is 7.0) when they drop from a tree. If you were to take the freshly fallen needles (before the needles decompose) and turn them into the soil right away, you may see a slight drop in the soil pH, but the change would not be damaging to the plants.

For those of you that leave the needles there on the ground, they will begin to break down naturally and the microbes (decomposers) in the soil will neutralize them. So, you can leave them there (if you’re not in a wildfire-prone area). They are a good mulching material that will keep the moisture in, suppress weeds, and eventually add nutrients back to the soil. You can also add them to a compost pile; they will slowly break down over time. If you run them through a shredder they will break down faster. A general rule of thumb is not to add more than 10 percent of pine needles to your compost pile.

If you are having difficulty growing other plants under your pine trees it is likely due to the fact that evergreen roots are numerous and shallow and compete for water and nutrients. The shady conditions under a tree can also make growing other plants a challenge. – Amy Jo Detweiler, OSU Extension horticulturist

MYTH: Just add more compost to the soil.

REALITY: Adding organic matter to the soil in the form of compost helps to improve soil structure and promote long-term plant health, but adding too much compost at once or over time can lead to problems. If the soil organic matter is much higher than ideal (5 to 8 percent), the soil can have too much available phosphorus, which can stunt plant growth and potentially leach into the water table. Also, some composts can be high in salts, which can also impact plant growth. – Weston Miller, OSU Extension horticulturist

MYTH: Bee houses help promote and conserve bee diversity.

REALITY: Although some bee species nest in the cavities provided by bee houses, most bee species nest in the ground. Research out of Canada shows that most cavities in bee houses are colonized by native wasps (that help control pests), and not native bees. So, bee houses still do good, but not necessarily the good that you might think. – Gail Langellotto, OSU Extension entomology specialist and state coordinator of the Master Gardener program

MYTH: Tree roots go only as far as the drip line.

REALITY: Many trees extend many times beyond the branch crown diameter. For instance, magnolia extends 3.7 times the diameter; red maple 3 times; poplar 3; locust 2.9, and ash 1.7. – Steve Renquist, OSU Extension horticulturist

MYTH: Epson salts are a must for great tomatoes. Use them in every garden.

REALITY: If you have done a soil test and your soil lacks magnesium and your plants are not growing well, give it a try. In most soils and gardens, they can do more damage than good. Steve Renquist, OSU Extension horticulturist

MYTH: When you plant a new tree or shrub, dig the hole and add an amendment to the soil before you backfill the hole.

REALITY: Although the amendment of soil with organic matter is often a good idea, it should be done on an area-wide basis, not just in a planting hole for an individual plant. Adding an organic amendment to the soil only in the planting hole will tend to reduce the growth of the plant. This happens because roots may stay within the amended soil and not grow into the native soil, creating a root-bound plant within the amended soil. If the organic amendment is not completely decomposed it may require nitrogen for further decomposition, which will compete with plant roots for minerals, thereby resulting in reduced growth. And in the case of large shrubs or trees, decomposition of the amendment will cause the plant to settle and the root collar will sink below the soil. – Neil Bell, OSU Extension horticulturist

MYTH: Brown recluse and hobo spiders are common in Oregon.

REALITY: It is commonly thought that hobo and brown recluse spiders cause necrotic bites in this state, when in fact the brown recluse is not found in Oregon and the hobo spider does not cause necrotic bites. The hobo spider was in fact removed by the Centers for Disease Control and Prevention’s list of venomous spiders in 2015. The black widow is the most harmful spider to humans in the state. – Heather Stoven, OSU Extension entomology specialist

MYTH: Watering on hot sunny days will burn the plants because the water droplets magnify the sun’s rays.

REALITY: It rains during the summer all around the world and plants are just fine. More of an issue is that the irrigation water will evaporate and not be as effective. Note though those sensitive houseplants like African violet can show leaf damage from very cold water applied to the leaves. – Brooke Edmunds, OSU Extension horticulturist

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Kym Pokorny, Communications Specialist for Oregon State University

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