The Hidden Half of the Garden

            One of the most inquisitive gardeners and writers I know is Robert Kourik of Occidental, California.  When he gets ahold of a topic, he’s like a terrier with a bone.  Whether it’s ways to drought-proof a garden, the design of graywater and drip irrigation systems, or the best recipe for a lavender-infused martini, he worries away at the research in the library  and in the field until he gets to the root of the matter.  Quite literally, too, in the case in point.

            Like most of us, Robert had passed without comment the illustrations in horticultural textbooks that represented root systems as essentially subterranean mirror images of the plants’ above-ground structures.  That is, if an oak tree rose to a height of 60 feet, with limbs reaching out to a similar extent, the image of the roots showed them sinking and spreading to 60 feet.  Except that Robert has a tendency to question the accepted wisdom, and one day this led him to wonder whether the profile of a plant’s roots was really so simple.  This in turn led to a compulsive exploration of road-side ditches and excavations to see how roots really behaved, and many hours of research in the University of California Berkeley Library, searching for actual data on exhumed root systems.  What he found, and summarized in two books, Roots DeMystified (Metamorphic Press, 2007), and Understanding Roots (Metamorphic Press, 2015), is that the conventional imagining of roots bore little relationship to the truth.

            One of Robert’s greatest finds were the detailed maps that professor John Weaver of the University of Nebraska made by excavating plant root systems in the 1920’s and ‘30’s.  Later, Robert supplemented this with a series of German textbooks with similar illustrations of excavated tree, shrub, weed, and vegetable roots, as well as books and papers by other authors abroad and at home.

            What he learned from all of this included the fact that most trees do not form tap roots, and that a shade tree growing on a clay soil is likely to grow 90-95 percent of its roots in the top twelve inches of soil.  What’s more, on a very heavy clay soil, the roots are likely to stretch five or more times as far from the trunk as the tip of the branches.  Even on a better drained and aerated loam soil, that shade tree is likely to keep the vast majority of its roots within 36 inches of the surface, and extend them half again as far as the branch tips.

            These facts have obvious implications about watering and fertilizing such trees.  For example, the rule-of-thumb that fertilizer should be applied around a tree’s dripline (the area under the tips of the branches) is obviously in error.   

            The pictures Robert presents about vegetable roots can also change how you care for them.  I had believed, for instance, the advice I was given that asparagus plants are shallow-rooted and so should never be weeded with a hoe.  Yet Robert’s German source reveals that those excavators found an asparagus’ roots to penetrate down as much as four feet into the soil. 

            Commonly, the visible structure of the plant gives little indication of how the roots grow.  Thus, a humble beet may sink its roots four feet down into the soil.  A mature horseradish’s roots may reach downward more than a dozen feet, which explains why my attempt to confine horseradish by surrounding it with a bottomless garbage barrel sunk level with the soil surface failed.

            One of the most interesting chapters in Understanding Roots is the one about mycorrhizal fungi, fungi that inhabit the soil in association with plant roots.  These fungi tap into the roots to feed on the plant’s sap, but in return, they greatly extend the roots reach, for one cubic inch of soil may contain 8 miles of mycorrhizal filaments.  This extended reach not only helps the roots absorb nutrients and water, it can actually connect one plant with another, creating a sort of super-organism.   The presence of these fungi throughout the soil also creates a persuasive argument for no-till gardening as any digging causes widespread disruption of the mycorrhizal fungi’s network.

             A look at either of these books will transform your gardening.  Robert Kourik has also written a number of other books, most recently Lazy-Ass Gardening, whose motto is “Maximize your soil; Minimize your toil.”  That sounds good to me.

The Worm Has Turned

           There’s no species of wildlife more beloved of gardeners than the earthworm.  Yet, as I’ve been learning recently, this creature can also spell trouble, both for the cultivator and for the local ecosystem.

            The earthworm has been an icon of a healthy garden ever since its cause was taken up by no less a figure than Charles Darwin.   For more than forty years, when he wasn’t working on his theory of evolution, Darwin was quite likely observing and experimenting on earthworms.  The great scientist was deeply interested in geology and he was fascinated by the very slow yet powerful ways in which earthworms had transformed the British landscape.  Darwin observed how earthworms consumed organic matter, passing it and soil through their guts to deposit the end product at the surface in little piles of fine-grained, nutrient-rich “worm castings”.   Although individually each pile of castings is insignificant, in the aggregate, their impact on the environment is huge.  Using an estimate by a fellow scholar that the average acre of soil supported 53,767 worms, Darwin calculated that this population cumulatively deposited more than ten tons of castings on the soil surface every year, for a total of 320 million tons nation-wide.  “It may be doubted,” he wrote in the book about earthworms that he published in 1881 (the year before his death), “whether there are many other animals which have played so important a part in the history of the world, as have these lowly organised creatures.”

            Ever since, gardeners have regarded earthworms as a principal source of topsoil, as well as, with their tunneling, aerators of the soil.  I remember my mother – my first gardening instructor – teaching me that these creatures should be treated with something like reverence.

            Indeed, in Darwin’s England, where earthworms are native, their effect is all to the good.  That isn’t necessarily so, however, in the northern United States, according to Josef Gorres, a professor of plant and soil science at the University of Vermont.  Earthworms were largely eradicated from our area by the ice sheets of the last ice age, so that there are only a couple of relatively rare species which are truly native here.  The rest – the earthworms and night crawlers that populate our gardens – were introduced accidentally with ship ballast dumped on our shores and in plants brought by colonists.  As these creatures spread, they profoundly affected the soil ecology, making it less hospitable to many wildflowers and other native plants, especially in forested areas.   

            What’s of far more concern to Dr. Gorres, though, is some more recent arrivals.  These are the “jumper” or “snake” worms that arrived from Asia and which are still colonizing the northern landscape.  These were first observed in the eastern United States in the 1920’s in the Washington D.C. and Baltimore area; one theory is that they arrived with the flowering cherry trees sent from Japan to adorn our nation’s capital.  In any event, they have been moving north since then, distributed in nursery stock and plants traded among gardeners.  Once introduced to an area, they may also be spread by municipal leaf composting programs, unless the leaves are hot composted.

            These worms differ physically in a number of respects from the earlier European arrivals, but are most easily distinguished by the violent way they thrash and jump when disturbed.  The snake worms also differ in the more aggressive way they process organic litter.  They can reduce a couple of inches of organic mulch or natural forest duff to something like a loose layer of coarse coffee grounds in a single summer.  This transformation of the soil’s top layer can interfere with the growth of shallow-rooted plants.  In a woodland, this has a number of impacts, including reducing deer browse, forcing the deer to focus on tree seedlings and so interfering with the forest’s ability to regenerate.  By thinning the vegetation on the forest floor, snake worm activity also exposes the nests of ground-nesting birds, making them more vulnerable to predation.

            Once an area is infected with snake worms, the best that can be achieved is to reduce their numbers.   This can be accomplished in the garden by hand-picking them and drowning them in a bucket of water.  Reducing the numbers of worms will, over time, also reduce the number of their egg cases in the soil, decreasing their rate of reproduction and making the job of controlling them easier.

            I never thought, when my mother extolled the virtues of earthworms, that someday I might be contemplating their control.


Climate Chaos

            “Global warming” is the popular name for the often dramatic and destructive weather trends that we have been witnessing in our gardens in recent years.  My wife the climate researcher, however, prefers to call it “climate chaos.”  That’s because what we have seen in the Northeast has not just been a gradual increase in temperatures, but rather an overthrow of the familiar norms. 

            I was reminded of this recently when I had occasion to speak to Dr. David Wolfe of Cornell University.  Dr. Wolfe is a professor of plant and soil ecology who has pursued a special interest in maintaining sustainable gardens in a time of climate change.  He is internationally recognized for his expertise in this subject; in 2017, he was invited to speak about it to Britain’s Royal Horticultural Society.

            When I spoke to Dr. Wolfe, he pointed out that the effects of climate change will vary widely depending on the area of the United States in which you garden.  In the West, for example, the most pronounced effect on gardens is predicted to be a combination of greater summer heat, with accompanying heat stress and drought.  Similar changes will afflict the Northeastern gardeners.  Climate models predict that total annual precipitation will probably remain about the same, but because plants will need more water to cope with the greater summer heat, they will effectively experience more drought.  This will probably be exacerbated by a trend, already beginning, toward fewer and more violent rain storms.   In other words, periods of too little water will probably alternate with periods of too much.

            One way to cope with a cycle of this sort, Dr. Wolfe suggested, is to boost the organic content of the soil.  By adding copious amounts of compost, for example, the gardener can enhance both the soil’s ability to absorb and retain water, and also make it more porous, so that excess water drains away more promptly.  In addition, to conserve the existing organic content of the soil, the gardener should avoid unnecessary digging and tillage.  That’s because turning the soil over mixes air into it, which in turn accelerates the breakdown of the soil’s organic portion, and its escape into the atmosphere as carbon dioxide, the principal greenhouse gas that is driving global warming.

            Rising temperatures mean that winters will be warmer on average.  This could be an opportunity, Dr. Wolfe noted, for gardeners to experiment with less winter hardy plants, plants which may also be adapted to warmer summers.  This change in weather patterns has also had the counter-intuitive effect of increasing frost damage.  Dr. Wolfe explained that because New York’s apple and grape crops had flowered several weeks earlier than their historic norm in some recent years, they became more vulnerable to late frosts.   The same effect translates to spring-blooming garden plants, which are likely to bloom earlier, coming out of dormancy at a time when they are likely to be damaged by late frosts.

            Warmer winters are also allowing southern pests and weeds to migrate north.  Insects such as the hemlock woolly adelgid which were formerly barred from our region by winter’s cold are making inroads here, killing the hemlocks in our forests.  Even more familiar, long established insect pests are affected by global warming.  Longer summers mean more time for reproduction, with some pests adding more generations and more population growth per year.

            Dr. Wolfe recommended enhancing the diversity of the garden, not only by trying new plants but also by varying planting dates to take advantage of and guard against earlier springs and late frosts.  One further recommendation is to reduce the amount of greenhouse gases your garden generates.  Besides reducing tillage, conscientious gardeners will also reduce their use of synthetic fertilizers, which consume large amounts of fossil fuels in their manufacture.  Organic fertilizers are preferable in this respect, since they largely consist of agricultural byproducts.  Even these should be applied judiciously, however, because nitrates, a common ingredient of most fertilizers, can escape into the atmosphere as nitrous oxide, a greenhouse gas more than 200 times as potent as carbon dioxide.

Garden Grafitti, July 12th, 2019

             I love the spiritual side of gardening: the feeling of communing with nature, and giving back to the earth.  But if I dwell on that too long, I start to take myself too seriously.  That’s when I shift gears and focus on some silly fun.  Like garden graffiti.

            This is a trick I invented through necessity more than thirty years ago.  I was newly married and commuting between my work as a writer in New York, and College Station, Texas, where my wife was working as an environmental scientist at Texas A&M University.  My wife was quite proud of the garden we had created around our Texan home.  She bragged about it to colleagues and volunteered to host a barbecue in the garden for a gathering of colleagues.  She hadn’t told me about this, however, and about a month before the barbecue, I decided I was tired of the plantings in the back yard and ripped them all out.  My wife was stricken when she got home that day.  When she told me why, I promised to rescue the situation, though I didn’t know how I would accomplish that.

            The only plant I could think of that would mature sufficiently quickly was curly garden cress (Barbarea vernapraecox – also known as “upland cress”), the peppery green with which generations of English cooks have made sandwiches for tea.  Garden cress will grow from sown seed to lush maturity in about three weeks.  So three weeks and a couple of days before the party, I raked the back yard beds out flat and smooth, and began sketching patterns on the soil by dribbling garden lime between my fingers.  If I didn’t like a pattern when I had finished it, I erased it by raking the soil again.  Eventually, though, I completed a wild complex of curlicues, lightning bolts and crisscrosses that satisfied me.  Then everywhere I saw white, I sowed cress seed, working it into the surface of the soil gently with a claw-shaped hand cultivator. 

            I watered daily and very soon, a green inscription began to emerge from the ground.  Curly cress is a low-growing, thick and parsley-like green.  When planted thickly, it makes a nice miniature hedge.  By the night of the party, my freehand micro parterre was complete.  When I turned on the floodlights in the back yard, the view was unforgettable.  One English scientist told me he had never seen anything like it, which I decided to take as a compliment.  My wife was pleased as the garden was the talk of the evening, especially after the wine and beer had circulated for a bit.

            Years later I revived this technique when our son had started kindergarten.  He was learning his letters, and, without warning him, I planted his initials in cress in a bed at the end of the driveway where we waited every morning for the school bus.  He noticed his initials emerging in green and asked me about it.  I suggested that the garden was trying to communicate with him.  He didn’t fall for that, but he did watch that bed with a proprietary pride.   When the cress matured, I harvested it with shears and added it to salads.

            These days I practice garden graffiti as a decorative way to divide up beds, planting other greens such as arugula or beets in the pockets created in this fashion.  This, and the memories it evokes, always makes me smile, and reminds me that gardening should be playful as well as serious.

Be a Better Gardener, June 30th, 2019

No bad bugs


            I’ve been thinking about my neighbor, Brian Stewart, a lot recently

            I came of age as a gardener at a time when any insect in the garden was regarded with suspicion.  We labeled them indiscriminately as “bugs” as if they were just glitches in our otherwise perfect landscapes, something to be eradicated as thoroughly and quickly as possible.  There were a few exceptions.  There were the “good” bugs such as ladybugs and praying mantises, predators that assisted us in our crusade to slaughter the plant-eating insects, the “bad” bugs. 

            Things changed dramatically with the publication of Douglas Tallamy’s book, Bringing Nature Home in 2007.  Tallamy, an ecologist at the University of Delaware, presented irrefutable evidence that plant-eating insects play a crucial role in the ecology of our landscapes.  If we want our gardens to be hospitable to birds, in particular, we have to tolerate their food source, herbivorous insects.  Indeed, with biodiversity on the decline throughout North America, cultivating native plants that in turn support rich and diverse insect populations is a duty of the enlightened gardener.

            I think, however, that at this point we need to take a step even further.  Which brings me back to Brian.

            Brian is a scientist, a professor at Wesleyan University.  In his spare time, he is, among other things, a keen gardener.  He’s also an admirer of wildlife.  He enjoys the birds that flit through his ¼-acre yard, and the various mammals that wander in from an adjacent nature preserve.  But a decade ago, he began exploring close-up.  After participating in a local “Bioblitz,” a crash program of assessing the local biodiversity, he decided to make a collection of local insects.  Instead of killing the insects and pinning them to boards, however, he chose to take photographic portraits of what he found.  He started in his own yard, thinking that he could broaden his search when he had exhausted its supply.  Ten years on, he has never stirred beyond the boundaries of his own property.  He has taken some 10,000 photographs of roughly 500 different insects, and he is still finding species unfamiliar to him right outside his front door.

.           Unlike most insect collectors, Brian doesn’t harm any of his finds.  The most he ever does is to slip them into a container and chill them in the refrigerator for an hour or so, to numb the insects so that they will hold still for their portraits.  After photographing them, he returns them all to the wild.

            What Brian has discovered with his macro lens is a bizarre and beautiful world, one that is all around us but which we typically overlook.   His portraits exhibit brilliant colors and metallic sheens, with strange, often extravagant structures.  My favorites are his shots of the female giant ichneumon wasp.  An anorexic black and yellow creature, this stingless wasp trails from its rear end a long ovipositor, a sort of tubular drill which it inserts into tree trunks or stumps to inject an egg into insect larva burrowing in the wood.  If Dr. Seuss had turned to science fiction, this, I believe, is what he would have created.

            Identifying what he has photographed is a challenge.  For this Brian has turned to communities of naturalists on the internet, notable at BugGuide (  With this help and his own field guides, Brian has identified the species of some 320 of his finds.  Much of the pleasure he finds in the photographs are more aesthetic than academic, however.  He will focus in on a part of an insect to admire the patterning: the vivid striping on the side of a swallowtail caterpillar feeding on his fennel, or the network of veins in a grasshopper’s wing.  If sufficiently close up, such a picture loses all sense of function, becoming an abstract piece of art.

            I haven’t equipped myself with a macro lens yet, but merely knowing of Brian’s project has changed my attitude toward my garden.  Stepping outdoors now feels like going on safari.  I find a greater richness to my landscape as I contemplate all the diversity it supports.  What I used to view as pests, I more often think of now as assets.



Launch of Growing Greener

I’m pleased to announce the launch of Growing Greener, my radio show dedicated to making your landscape healthier, more beautiful, more sustainable — and more fun.

You’ll hear:

  • interviews with people exploring this relationship between gardening and a greater awareness of nature,

  • practical gardening tips,

  • and readings from outstanding new garden writers.

You can hear us live every Wednesday evening from 6:00 - 6:30 at 88.1, or streaming online at You can also download past and present versions of the program from the archive.