As a New York City transplant, now with a backyard, I feel I should know more about gardening by this stage in my life — but I don’t. So it was with great pleasure that I serendipitously met Meleah Maynard, a notable master gardener who writes a Southwest Journal column I had been clipping for months, gearing up for the 2015 planting season. I learned about the excellent book she co-authored with Jeff Gillman, a Department of Horticultural Science professor at the University of Minnesota. Decoding Gardening Advice: The Science Behind the 100 Most Common Recommendations, which is an excellent look at the proven scientific logic of gardening.
In each chapter they explain the science behind “Good Advice,” “Advice That’s Debatable,” and “Advice That’s Just Wrong.” They cover Soil, Water, Pest/Disease/Weed Control, Mulch, Annuals/Perennials/Bulbs, Trees/Shrubs, Vegetables/Fruit, and Lawn Care.
Here are examples of information they share about Soil, which they describe as perhaps the most misunderstood ingredient. The mysteries of what happens beneath the surface — and whether plants are getting what they need to thrive — leads to myths that are not borne out in the science.
Plants need 1) water, 2) nutrients, 3) oxygen. Ideally these elements will exist in a sustainable, interconnected way involving minerals, organic material, microbes, plants and animals. Alternatively, healthy produce can come from man-made hydroponic tubs. [More on hydroponics to come on this website.] The job of soil is to hold and provide water, nutrients and oxygen to plant roots. Ideally soil is well-drained, moist, with appropriate amounts of micro-organisms to process nutrients — without poisoning the plants.
Organic material from years of natural cycles of plant growth, death and regrowth is good to add before planting. This organic material largely consists of carbon — which comes easily from compost or mulch, or any organic fertilizer made from dead and ground plant material. Ideally turn 2-3 inches of compost into the top 6 inches of soil in a new garden. For established gardens, add 2-3 inches of compost to the top of soil, followed by 2-3 inches of organic mulch such as straw or wood chips to control weeds — preferably every year. In time, this requires less watering and likely less fertilizer.
Compost, the authors write, “is without doubt the most important soil amendment you can add to the garden.”
Also good (get the book for details):
- earthworms and vermi-composting
- getting a soil test done — based on results, add lime powder to raise pH and add sulfur to lower pH
- don’t fertilize during hot weather, when plants are stressed, because it stimulates growth and compounds water availability issues
- Change soil in flower boxes every few years. Instead, use potting soil that includes organic materials (sphagnum peat, coconut coir, composted pine bark) and inorganic material (sand, perlite — aka porous volcanic glass) which holds more water and nutrients. Changing this somewhat expensive material annually is likely overkill. Better to mix with new compost each year. See the book for more details.
- Diluted urine (1 part to 9 parts water), yes, can both help and harm plants. Best to simply include in compost piles. Don’t let it sit around before using. And don’t use on compost for edible crops.
- Fertilizing in spring is best for annuals, a week or two after planting so the roots aren’t burned. It’s useful for perennials — though not as useful as early to mid-autumn — if done before buds break on trees and perennials begin to sprout. Fertilizing helps perennials get nutrients before winter, as long as you don’t overstimulate new growth before an early frost. Use a fertilizer with a first number that is higher than the second and third (represents percent of nitrogen, phosphorus and potassium). Vegetable gardens need more nitrogen. For less intense ornamental plants, a ratio of 1 to 2 pounds of nitrogen for 1000 square feet is recommended. You might be best served fertilizer in two batches during the year. Best is to use more compost, so you don’t need to fertilize at all.
- The book also discusses the debatable advice about peat moss for improving soil drainage, annual fertilization of perennials and shrubs, annual tilling of vegetable gardens, use of ammonia vs. nitrate, use of bacteria for bean plants, and use of mycorrhizae to promote plant health.
- Using a balanced fertilizer, with equal parts nitrogen, phosphorus and potassium. Nitrogen moves more quickly than the other elements, so a balanced fertilizer simply leaves you with too much phosphorus and potassium over time. Best: get a soil test and find out exactly what your soil needs.
- Using pine needles to make soil more acidic. Needles can work as mulch — they are great at blocking weeds because they prevent seeds from sprouting — but it doesn’t help with creating a more acidic environment, if that’s your aim.
- Using compost tea as a soil enhancement. Researchers are unable to demonstrate that air bubbled through the tea gives the soil a good shot of microbes. In fact, bad bacteria can infest the tea and actually potential poison you with E. coli-laced mixture on your vegetables. The use of simple compost is best — it not only provides beneficial organisms, but gives them a place to live.
- Adding sand to clay soil to improve drainage. Clay is very efficient at holding water, which can cause headaches for gardeners. But organic amendments, such as compost and wood chips will help make the soil easier to work with in time. Sand will simply make it like concrete. Plants like columbine, New England aster, purple conflower, daylily, blazing star, Russian sage, stonecrop, and common lilac grow well in clay soil.
- Decoding Gardening Advice: The Science Behind the 100 Most Common Recommendations
- Good examples — and frustrations — of Minneapolis sustainable design
- Giving Tree Gardens: why our urban center is vital
- Q&A: Composting
- Q&A: Raingardens
- Edible weeds
- Planting for pollinators