The internet abounds with information on gardening and is ripe with garden tips—but not all of these hold up under scientific scrutiny. Thanks to research from the University of California Agriculture and Natural Resources (UCANR), we can separate fact from fiction and help your garden thrive with evidence-based practices.
Here are some of the most persistent gardening myths—and what science really tells us.
Myth 1: Adding used coffee grounds to your soil is good for your plants.
What Science Tells Us: Coffee grounds contain a much smaller level of nitrogen than conventional fertilizers and must be composted to release the nitrogen. Also, research conducted by the University of California has shown that coffee grounds suppress growth in a variety of flowers and vegetables. Instead of using them directly in the garden, UCANR recommends adding used grounds to your compost pile, where they contribute organic matter. For tips on using coffee grounds in your garden, see: Use of Coffee Grounds in the Garden
Myth 2: Grass clippings are great for compost, so the more, the better.
What Science Tells Us: Grass clippings are rich in nitrogen, but too much can create a soggy, smelly compost pile. A compost pile needs to have the right mix of carbon and nitrogen. The easiest way to achieve this is to aim for equal amounts of green and brown materials. Grass clippings qualify as nitrogen rich green material but adding large quantities of grass clippings creates a moisture and air barrier in the compost pile. This promotes the growth of mold, creates bad odors, and slows down decomposition. UCANR advises balancing green (nitrogen-rich) and brown (carbon-rich) materials in a 1:1 volume ratio to maintain a healthy compost system. For tips on starting and maintaining a compost pile, see: Basics of Composting
Myth 3: Magnesium sulfate (Epsom salt) prevents blossom end rot in tomatoes.
What Science Tells Us: Blossom end rot is caused by a lack of calcium uptake, not magnesium deficiency. Adding Epsom salt can actually worsen the problem by increasing magnesium levels, which can interfere with calcium absorption. In addition, adding eggshells as a source of calcium is not recommended, see below. For tips on preventing blossom end rot, see: Managing Blossom End Rot in Tomatoes and Peppers
Myth 4: Eggshells Are a Great Source of Calcium for Plants
What Science Tells Us: Eggshells contain calcium, but they decompose very slowly (years)—even when pulverized. UCANR suggests focusing on overall soil health and using soil testing to determine if the soil lacks key nutrients such as calcium. If the soil is found to be deficient in calcium using recommended amendments such as gypsum is suggested. For tips on testing and adding calcium to your soil, see: Basics of Soil Amendments
Myth 5: Adding rocks in the bottom of pots improves drainage.
What Science Tells Us: This actually causes water to collect above the rock layer, making the soil soggier. This results in roots staying wet and potentially getting diseased and rotting. It is recommended to use well-draining potting mix throughout the container instead and a screen over the drainage hole(s). For tips on planting in containers, see: Basics of Container Gardening
Myth 6: Clay soil is poor for gardening.
What Science Tells Us: While clay can be tough to work with, it’s rich in nutrients and retains moisture well. With proper management—like adding compost and other organic matter, the soil will improve with time and it can be a productive growing medium. In addition, because clay holds moisture well, it can help reduce water bills. For tips on growing plants in clay, see: The Truth About Clay Soil
Myth 7: Wilting means that a plant needs more water.
What Science Tells Us: Wilting may mean that a plant needs water but there are a number of other causes including overwatering and diseases. In fact, many of the signs of overwatering look the same as when plants are underwatered. Look for tips here on how to water properly: Plant Care: watering
Myth 8: Pruning and other wounds in trees require a wound dressing.
What Science Tells Us: Trees are resilient and often can heal their own wounds. It is generally advised not to cover tree wounds with sealants or dressings, as this can impede the tree's natural healing process and potentially increase decay. Trees compartmentalize decay by forming protective barriers and growing new wood around the wound. For more tips on tree wound care see: Landscaping Trees: Their Wounds and Treatments
Myth 9: Large vegetable gardens are required for a large harvest.
What Science Tells Us: To fit more vegetables in a bed, utilize space-saving techniques like interplanting, planting fast-growing crops like radishes and lettuce between larger, slower-growing ones such as tomatoes or cabbage. Maximize vertical space by trellising vining plants like cucumbers, melons, and indeterminate tomatoes. Additionally, you can do succession planting by following an early producing crop with a fast-maturing one. For tips on planting vegetable gardens in small spaces see: Vegetable Gardening in Small Spaces
Myth 10: Dish soap is a safe, natural pesticide.
What Science Tells Us: Dish soap should not be used as a pesticide; instead using commercially formulated insecticidal soaps is highly recommended. Unlike insecticidal soap, dish soap contains synthetic chemicals designed to strip oil and grease. The principal cleaning agents in dish soap can strip the natural waxy coating off a plant's leaves, leading to leaf burn, drying, and damage. In general, homemade remedies for pest control are not recommended. Commercial products are tested for efficacy on target pests listed on their labels and undergo extensive safety testing as well. For tips on using approved insecticidal soaps see: The Dangers of Homemade Pest Control Remedies
Final Thoughts
The science behind gardening is well advanced. Science-based gardening information is available from the University of California and other respected institutions. It’s important to question old habits and embrace practices backed by research. UCANR provides a wealth of science-based resources to help gardeners make informed decisions and grow healthier, more sustainable gardens. To learn more about the University of California Agriculture and Natural Resources see: University of California Agriculture and Natural Resources
References:
Use of Coffee Grounds in the Garden
Managing Blossom End Rot in Tomatoes and Peppers
Landscaping Trees: Their Wounds and Treatments
Vegetable Gardening in Small Spaces
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