Plant Science for Gardeners: Essentials for Growing Better Plants, Robert Pavlis, New Society Publishers, June 2022. 224 pages, 6” x 9”, photos, drawings, diagrams. $22.99.
This is a valuable, concise, accessible book for home gardeners, homesteaders, market gardeners, small-scale and large-scale crop farmers. As I noted about Robert Pavlis’s first book in this series: Soil Science: “I recommend this book to all gardeners who have hesitated to open a soil science text for fear of dry, incomprehensible overloads of numbers.” The same is true of Plant Science. Robert Pavlis is a very good science writer. He disentangles false myths from facts, and teaches us how to make science-based decisions and grow healthier, more productive plants. I reviewed Soil Science for Gardeners, in 2020. As well Plant Science, he has a newer book: Compost Science for Gardeners, and coming soon, Microbe Science for Gardeners.
By understanding the science, we will better able to base our decisions on actual conditions, even as those conditions change in the climate chaos we are now dealing with. Our one-time rule “plant garlic in the third week of October” has gone by the wayside as the soil stays warmer later. We now plant garlic around the end of the first week of November. The author has a blog, called gardenmyths.com, that has had over 14 million visitors and discusses hundreds of garden myths.
After introducing Plant Basics, we get a tour of roots, stems, leaves, flowers, fruits and seeds, and then the whole plant. Woody plants have their own chapter, as do environmental factors, selecting seeds, vegetative reproduction, and plant names. Each chapter includes sidebars exploding common gardening myths (six in the roots chapter!) and tips for assessing plant problems and finding solutions.
After absorbing this book, you will avoid wasting money on faddish garden products and techniques. You can marvel at the information that plant roots excrete chemicals to attract beneficial microbes which then ward off root pathogens.
You can fill any gaps in your knowledge of the xylem and phloem systems for transporting water and nutrients around the plant; and what actually happens during photosynthesis, when the energy of light converts carbon dioxide into sugars and oxygen. When the sun goes down, photosynthesis stops, but roots continue to absorb water and nutrients, and the plant continues to grow and form flowers and leaves.
The awe-inspiring photo of root hairs makes it obvious that they increase the surface area of the roots by an order of magnitude. In this chapter you can also learn to distinguish between fibrous roots (lettuce), taproots (carrots), tuberous roots (sweet potatoes) and adventitious roots (growing out of the stem or leaves, such as on tomatoes).
Roots need a pH between 5.5 and 7.0; temperature between 40°F (4°C) and 90°F (32°C); easy access to nutrients, provided the levels do not become toxic; sufficient air and water.
Learn the real truth about soil mycorrhizal fungi – buying them is a waste of money! Learn about nitrogen-fixing bacteria (both free-living, and symbiotic bacteria in nodules on roots of legumes and certain other plants). The bacteria do not provide “free” nitrogen, but exchange it for photosynthetic compounds made by the plants. Annual plants excrete 40% of their photosynthates and other plants typically provide around 30%. Some of these exudates attract mycorrhizal fungi.
Some plants excrete 50% of the fixed carbon from photosynthesis through their roots into the soil, where microbes feed on them and die, after producing more microbes. The dead microbes provide lots of nutrients in the “soup” around the roots. Some of these nutrients can boost plant growth; some attract nitrogen-fixing bacteria; some inhibit the development of pest nematodes. Plants attract and herd the right microbe food sources towards their roots; they also change the pH around the roots, making insoluble minerals more available. They produce fewer sugars when they no longer need as many microbes. This is amazing plant chemistry in action. It is not a sign that plants are “intelligent” in our usual meaning of the word.
The chapter on stems will help you distinguish herbaceous and woody stems; vascular bundles of phloem and xylem; nodes and internodes; trichomes (stem hairs, scales or spines); terminal and lateral buds.
Rhizomes are not roots, but fleshy, modified stems growing laterally in the soil. Terminal buds develop along the rhizomes at the nodes, and then shoot upwards. When a rhizome is broken up, each piece can become an independent plant. Stolons (runners) are also a type of modified stem, growing horizontally on the soil surface, rather than underground. The baby plants are clones of the mother plant and can be separated to replant elsewhere.
Vines may curl clockwise or counter-clockwise, despite myths that abound. 90% of vines curl anti-clockwise as viewed from above. Cucumbers can curl either way. Pole beans and runner beans both curl counterclockwise. If you are giving your beans a helping hand, and you are right-handed, you may need to work against your instinct to twine them clockwise.
Leaves have upper and lower epidermis layers, sandwiching the mesophyll and the veins. The epidermis of some plants has extensions such as hairs, that secrete sticky, bad-tasting or smelly substances that can provide a defense against insects and other animals. The mesophyll is where photosynthesis happens. The surfaces of a leaf (especially the lower side) have fairly large openings called stomata. These control the water and air passing out, in response either to the internal state of the cells, or the dryness of the soil.
If you are spraying soft-bodied insect pests on your plants, the reason to use insecticidal soap made from potassium salts, rather than detergents like dish “soap” or bar soap (made from sodium salts) is to protect the waxy cuticle on the leaves. Sodium salt soaps and detergents can strip off all the wax coating, leaving the plant more vulnerable to insects.
Hardening off transplants for a couple of weeks before setting them out in the garden gradually acclimates the indoor-grown plants to brighter, windier, hotter or colder outdoor conditions. The leaf cuticle can grow thicker, the stomata smaller or fewer, the plant can grow smaller, tougher leaves. Read this chapter to learn why purple- and red-leaved plants grow slower, and why brassicas can show purple leaf coloration in cold and dry conditions (phosphorus isn’t moving fast enough).
The chapter on flowers provides an introduction to botany for those who have not met the information before, and a refresher to those who have. It includes information about night-length sensitivity (often called day-length sensitivity, although it is the length of the dark period that acts as the trigger to flowering), along with vernalization (a cold period preceding the night-length of the right duration.) In specific situations and climates, flowering may be triggered by strong far-red light, heavy rainfall, or sunlight intensity.
Outdoor lighting in your garden can interfere with blooming triggers. Red light is the most critical. If you have outdoor lights, it’s best to turn them off when you go indoors at night, to enable plants to get enough darkness.
Plants will not have more blooms if you give them extra phosphorus! “All parts of the plant need all the nutrients. An excess of one nutrient, like phosphate, does not make the plant grow better, nor does it cause a plant to bloom more. Don’t waste your money on bloom booster type products”
This chapter is mostly about plants grown for their flowers, rather than as vegetables. As a vegetable grower with a bit of a background in botany, I skimmed this chapter. Many vegetable growers also grow fruit trees or cut flowers, and this book covers all the basics.
The next chapter is about fruit. Fruit protects the developing seed, provides suitable humidity for the seeds, and sometimes provides nutrients for the seeds too. If the taste of the fruit attracts animals to eat the fruit, the seeds might then get dispersed further afield than if the fruit simply dropped near the tree.
Should you sucker tomatoes? See the YouTube video: https://youtu.be/1p6TC4-hj5E. Suckering is a type of pruning, where side-shoots are pinched out while small. Some gardeners never sucker tomatoes, and deal with the ensuing mass of greenery and fruit. Some prune hard, especially for greenhouse tomatoes, leaving only one main stem. Others take a middle road, and let two or three main stems grow. The author has compiled a table, comparing three degrees of suckering (none, plants sprawled; none, plants in cages; single stem, all suckers removed). What are your tomato-growing goals? Hard suckering reduces the number of potential fruits on each plant, allowing you to plant closer together, increasing your total yield. Suckering also produces an earlier crop, as does any kind of staking, compared to sprawling your tomatoes on the ground. Suckering achieves earliest fruit, tidiness, large fruit, least chance of pests and diseases. These benefits come with the costs of much extra time on maintenance, medium numbers of fruit and lower yield.
The next chapter puts all the plant parts together and looks at the whole plant. While there are features common to all parts of all plants, there are also differences. The author divides plants into four types: annuals, biennials, trees and shrubs. Another difference explained in this chapter is that between determinate and indeterminate growth patterns. Some plants stop growing at some point. Determinate plants grow to a genetically pre-programmed size, then stop getting bigger. Most deciduous trees are determinate – each species has a maximum height.
Indeterminate plants are not genetically limited in size. They may be environmentally limited: tomato plants die with the frost. Some indeterminate plants (some evergreen trees) do continue growing, but the rate of growth slows to perhaps ¼” (6 mm) per year, an amount you might not notice. Determinate vegetable types (of peas, beans, cucumbers and tomatoes) are often called “bush types.” Despite labels, there are not determinate and indeterminate types of potatoes. This is a myth that I fell for. All potatoes are determinate. Some are faster-maturing than others.
Some plants go “dormant,” meaning there is no visible activity above ground. They are not truly dormant, as the roots are still active. Fertilize plants when they are actively growing, not when they appear to be dormant. You can’t usually “wake them up”, although you can overcome summer dormancy of grasses to some extent by watering a lot. After the summer solstice, the increasing night length signals deciduous shrubs to start the complex process of shedding leaves and going dormant.
The cycles of water in a plant are fascinating, and well-described. Water travels from the roots up the xylem channels to the leaves. As the leaves transpire (give off water), a suction force pulls water molecules upwards. The surface tension of the water, caused by the shape of the molecules, causes them to stick to each other as chains of magnets do. Once water leaves the xylem, it moves from a place of high water concentration to a place of lower water content, in a process called osmosis. Water flows towards cells that have less water.
A third water movement process is called guttation. You’ve probably seen it as beads of water hanging on the edges of leaves, in times of high humidity, such as early morning. The roots have absorbed too much water and sent it upwards to leaves that cannot hold any more, and cannot transpire water while the humidity is so high.
Nutrients also move through the xylem from the roots to the rest of the plant. Once at their destination, small, mobile nutrients such as ammonium, potassium, phosphate, and magnesium ions, can move through the phloem cells to other parts of the plant. Larger molecules, like calcium, iron, manganese, zinc, boron, sulfur, and copper ions, are immobile, showing as color changes in older leaves. Mobile nutrients move out of the leaves in the fall, causing the color changes we see. Plant biology is complicated and there is no simple way to determine which nutrients are deficient in your soil, by looking at the leaves. This is another myth I had bought into, but found surprisingly hard to make use of! One fact is that deficiencies of mobile nutrients show up in the older leaves first, then move to the younger leaves. Deficiencies of immobile nutrients show up in the new growth.
Foliar feeding is widely misunderstood. Nutrients absorbed from foliar sprays enter not via the stomata, but through the transcuticular pores. Heard of those? These are small holes admitting small molecules only, in small quantities. Only 15-20% of nutrients applied to leaves actually get absorbed. Roots are much better at absorbing the large quantities of nutrients plants require. Save money and get better results, by fertilizing the roots!
This book provides explanations of how plants respond to damaged leaves and roots; grow taller; turn to follow the sun (until the seeds are set); and respond to gravity, growing into the typical shape for that plant. When light levels drop (as fall arrives, or after transplanting in shade), plants focus on the more essential root and leaf development, and stop making flowers. Nothing compensates for a shortage of light.
Perennial woody plants are the focus of the next chapter. Many perennial herbs like sage, thyme and lavender are sometimes simply called perennials, masking the secret of good care for them: prune them as shrubs. Learn about the structure of woody stems, the existence of lenticels (openings in the bark) and the differences between softwood and hardwood cuttings. Most trees do not need staking after planting, instructs Robert Pavlis, as he explodes a few more myths. Here are directions for removal of tree branches for best recovery. It doesn’t involve any paint.
Apical dominance is explained, along with a photo of a fruit tree trained flat on a wall, an effect created by managing apical dominance – bending down leaders where you want to create a new branch. Positioning the apical bud lower than the other buds lets the next one back grow out.
Environmental factors are discussed next, including how plants adapt to environmental changes. US winter-hardiness climate zones are explained, along with the limitations of this classification. Dave’s Garden (https://davesgarden.com) is recommended as a reliable site for its classifications of hardiness of various plants. Read about how plants cope with the cold. Soil temperature can make a lot of difference to plant survival. Hence the usefulness of organic mulches (including snow), and of hoophouses. Sugars, fats, proteins and minerals in the cells act as antifreeze and prevent the cell liquids freezing until the air temperature is colder than a mere frost. Buy hardy plants, hardy varieties, give them enough water.
Plants actually have more difficulty acclimating to heat than to cold. Water loss, lower photosynthesis and respiration rates all take a toll. The author explains that plants, animals (including people, I assume), and microorganisms, produce proteins called heat shock proteins, that act as a protective coating around enzymes and nucleic acids. Leaf rolling minimizes exposure to the sun. Hairs on leaves provide some shade (every bit helps!).
For new gardeners, figuring out when the soil has dried enough to require watering is one of the hardest things to learn. Push your finger into the soil, and if the soil feels dry, water. Not otherwise. When you do water, water deeply. Shallow watering produces only shallow roots, leaving deeper roots to die. Shallow roots are not drought-resistant.
Climate change is more than warmer average temperatures. It includes increased frequency and severity of droughts, heavy precipitation, strong winds. Plant a diversity of crops, and watch how they do. Consider plants that grow on riverbanks, those that grow in dry places, sites with high elevation and harsh conditions.
Chapter 10 is a guide to understanding and starting seeds. Some seeds are “recalcitrant” (slow to sprout). Once fully mature, some seed can remain dormant for a very long time. Some requires light to break dormancy; some a particular ratio of one plant hormone to another; some need the seed coating to be degraded before germination can happen (walnut trees are a good example); some need fire or a hot temperature (redbud trees). Most vegetables (and 80% of all seeds) are “orthodox” sprouters, and can be stored for that year’s use in paper packets at room temperature. For use in future years, make sure they are mature and dry, then store in an air-tight container under refrigeration. Recalcitrant seeds need different conditions – read the book.
For starting seeds there are links to a series of five videos covering pots, damp paper towels, and special outdoor winter-sowing methods. In this chapter are pros and cons for each, and tips on which method is best for different situations.
Chapter 11 includes information on plant genetics, heirlooms, hybrids, and GMO seeds. Unlike me, the author considers GMO seeds a safe way to produce food. Be reassured that GMO vegetable seeds are not available to home gardeners, being sold only in large quantities and requiring the purchaser to sign a contract. White Russet potatoes, and very small amounts of summer squash and zucchini may have reached retail sales. (https://www.cfsanappsexternal.fda.gov/scripts/fdcc/index.cfm?set=NewPlantVarietyConsultations)
See also https://foodinsight.org/genetically-modified-organisms-and-our-food-supply/
Chapter 12 covers vegetative reproduction, which is energetically less demanding on the plant than producing flowers and seeds. The downside is that all plants produced vegetatively are genetically identical. When stolons or rhizomes break, the fragments can grow as separate plants. Bulblets, bulbils, cormels are forms of baby bulbs or corms that can grow into new plants. Gardeners use vegetative methods to increase the numbers of plants for the next season. Layering, stem and root cuttings, leaf cuttings, root division – you can learn these from this book. Be skeptical of homemade “rooting hormones”. Willow water does contain low levels of rooting hormones; small amounts of aspirin, cinnamon, peroxide, may help control fungal or other infections or help root hairs grow. None of aloe vera juice, vitamin C, apple cider vinegar has any scientific evidence whatsoever, so don’t waste your time on these.
Chapter 13 is on plant names, and explains the conventions used when providing Latin botanical names. Conventions for naming hybrids are explained. The distinction between cultivar and variety is made clear. Varieties are naturally occurring in the wild. Cultivars are human-bred, including both hybrids and established open pollinated types. If you develop your own strain, you should give it a unique cultivar name. Don’t reuse a name already in existences, as this is inaccurate and leads to confusion.