Holiday choices

With Thanksgiving passed, we’ve entered the period for buying Christmas trees. Already an outdoor stand near us has stacks of trees ready to sell, and I’m seeing station wagons cruising by with trees strapped to their roofs.

Ready to go

    In my present state of constant background worry about climate change, my reflex is to think that cutting down trees is a bad thing. But what’s the total environmental impact of Christmas trees? Predictably, tree farmers say that real trees are more environmentally friendly, while makers of artificial trees argue that man-made trees can be just as sustainable.

    Proponents of natural trees point out that used Christmas trees can be recycled to make mulch. Like many cities, ours picks up trees at curbside after the holiday for this purpose. Tree farms provide wildlife habitat, if pesticide spraying doesn’t poison animals trying to move in. Whole trees can be laid down to prevent erosion, and if all else fails, used trees can be fed to goats!

 

Christmas trees stabilize a Connecticut beach

    The trees themselves are carbon neutral over their lifetime. When harvested, they give up the same amount of carbon that they took in. There’s carbon cost for powering machines used in farming—did you know that Christmas trees have to be sheared annually to create dense, uniform foliage? 

Christmas tree farm

Shipping harvested trees to the point of sale also uses fossil fuel, so it’s good to buy local. In all, production of a live tree that makes its way to your home generates an estimated 6.8 pounds of greenhouse gases per year of the tree’s life.

    By comparison, cars and trucks account for an average of 24 pounds of greenhouse gases per gallon of gasoline.

Christmas trees are paltry polluters compared to cars

    A 2008 study from a group of sustainable development consultants in Quebec found that artificial trees have lower environmental impact only if they’re used for at least 20 years. 

Buying an artificial tree could be the start of a long-term relationship

An alarming wrinkle: many of the artificial trees currently in use are made of recycled plastic, usually polyvinyl chloride, that has been stabilized with metal. What metal did Chinese makers add? You guessed it: lead. The plastic degrades as it ages, and at nine years, it starts to give off lead. Use of lead in plastic Christmas trees has recently been outlawed in China.

    I’m not in the market for a Christmas tree, but if I were, I’d be tempted to buy a living tree in a pot and grow it in my yard once it had played its part in the holiday celebration. 

A petite balsam fir (Abies balsamea)

Clemson Cooperative Extension in South Carolina describes the pitfalls of this approach. Trees that spend more than 10 days in the hot, dry indoor environment aren’t likely to survive being planted outdoors. Planting sooner is better, but that’s assuming that the ground isn’t frozen after Christmas. As an alternative, Clemson suggests planting a tree in your yard and decorating it there.

    As always with personal environmental decisions, assigning weights to these factors is complicated. My conclusion: enjoy your Christmas tree, park the car, and try to walk more.

Mulch falling from the trees

My new perennial bed near the back of the house is covered with needles from a big white pine (Pinus strobus) nearby. 

Pine needles on the new bed in October

When the area was lawn, I didn’t notice how many needles fell from the tree, perhaps because our focus was on raking them off the grass.

    Now I realize that those pine needles are providing natural mulch without my intervention. Is that a good thing? A couple of persistent urban legends warn against pine needle mulch.

    In the South, you can buy compressed bales of pine needles, commonly called pine straw, in various shapes and sizes at the garden center. 

Bales of pine straw at Space Coast Landscape Supply in Florida

Proponents point out that this mulch is pretty, it’s slow to decompose because of the needles’ waxy coating, it doesn’t blow around or pack down to create a water barrier, and it’s less dense than bark mulch. A 40-pound bale of pine straw covers as much ground as 30 cubic feet of other much heavier mulches. By contrast, big bags of bark mulch that I purchased recently contain 3 cubic feet each. I can barely lift them, and a bag covers a dishearteningly small area.

Bark mulch, heavy as all get out

    Tossing pine needles around has got to be easier than schlepping those bags of bark mulch. Having the tree and the wind spread pine needles is even less work. (My recent mulch favorite, arborist wood chips, also has the advantage of being less dense than bark).

Arborist wood chips make great mulch too

    It’s often suggested that pine needle mulch will acidify soil. The argument is that since the needles have a lower pH than average soil, adding them to soil must make it more acidic. Many gardeners hesitate to mulch with oak leaves for the same reason. But science doesn’t substantiate this.

    Soil has tremendous buffering capacity. As Robert Pavlis points out in this blog post, rain becomes acidic as it falls, picking up carbon dioxide from the air and converting it to carbonic acid. This is true even without the pollution that causes acid rain. If rain hasn’t changed soil pH over millennia, you’re not going to do it with a few pine needles or oak leaves.

Pine needles can't overpower soil's buffering capacity

    Second, pine needles contain a group of organic compounds called terpenes that people worry may suppress growth in the garden. Terpenes act as insecticides, preventing insects from eating the needles. They may also help suppress growth of other plants under the tree. But once the needles fall from the tree and hit the ground, the volatile terpenes quickly float into air or water—that’s why the needles stop giving off that pleasing pine scent. Plants can and do grow under our white pine.

Shade lovers growing at the base of the white pine

    Recent research suggests terpenes help trigger rainfall by combining with free radicals and oxygen in the atmosphere, forming aerosols of polarized molecules that collect water and grow into clouds. Trees aren’t as passive as we used to think!

Conifers don't just wait for rain

    I’ll soon be able to assess whether pine needles are improving soil, suppressing weeds, and helping to retain moisture in my new bed. I’m optimistic.

Picky eaters

Something I learned from entomologist Doug Tallamy’s book Bringing Nature Home is that most insects specialize. Ninety percent of herbivorous insects depend for food and shelter on a few plants they coevolved with. This insight is directing me toward different plant choices.

    I used to think that leaf-eating insects ate any leaves they could get. Tallamy explains that, on the contrary, over the millennia insects have survived by tailoring their behavior and physiology to be able to sense and locate a few plant species. 

Not just any leaf will do for food

They’ve synchronized their life cycles with the plants’. They’ve developed ways to get around the plants’ defenses. As they’ve invested in these food sources, they’ve become less able to live off others. That’s why most need plants from their local region and can’t eat plants from other parts of the world.

North American native insects can't eat plants that originated in Asia

    This year I’ve expanded my collection of milkweeds for monarch butterflies, the poster children of insect specialization. Monarch caterpillars have evolved to be able to eat milkweed leaves, which contain defensive chemicals called cardenolides that make the caterpillars and adult butterflies poisonous for other animals. 

A monarch on showy milkweed (Asclepias speciosa)

When eggs laid on milkweeds hatch, the caterpillars have the food they need to grow and metamorphose into another generation of beautiful adult monarchs.

    Monarchs are now under great stress. They need to fly from their winter home in Mexico to summer habitat in the US. 

Wintering in Mexico-photo Steve Bridger

Here they find less and less milkweed because of herbicides such as Roundup (glyphosate) that are widely used in industrial agriculture. Organic farmers make a point of leaving weed strips around their fields for the sake of pollinators, including butterflies. Until that becomes universal practice, home gardeners are encouraged to grow some milkweed for the monarchs.

    I started with swamp milkweed (Asclepias incarnata), which turned out to be quite easy to grow. Swamp milkweed’s dusty pink flowers are clearly an insect magnet. Not only the monarchs benefit.

Swamp milkweed attracts lots of pollinators

    This year I tried butterfly weed (Asclepias tuberosa) again. I’d planted this species twice before, only to see it dwindle without flowering. With a sunnier spot and plenty of rain this spring and summer, it didn’t flower, but it seemed to take hold. I’m optimistic about seeing some of the orange flowers next summer.

Butterfly weed, not as easy as I'd hoped.

    I also started seeds of common milkweed (Asclepias syriacus) and planted a couple of seedlings. This could turn out to be a mistake, because the plant is reputed to be quite a spreader. I like the seed pods and the seeds with their parachutes of white fleece, though, and I’m hoping to see some monarch caterpillars on the leaves.

Common milkweed seed pods opening in fall

    As an experiment, I laid a half-open common milkweed pod that I found in the neighborhood on the ground in an open area near some new shrubs. Maybe the wind will pick up some seeds and sow them as it would in the wild.

    I spotted just one monarch butterfly in the yard this summer. I hope she or he will send friends.

Too many trees

Back in 1997 during a major garden expansion, I had fun shopping for trees with garden designer Betsy Brown. She drove me around Massachusetts to pick out saplings of favorite tree species I’d dreamed of including in my ideal garden—in retrospect, too many of them. Betsy artfully placed new trees and shrubs in open ground we’d annexed, standing back to direct the landscape crew to move them a few feet this way and that until they were spaced just right.

    The new trees on the berm at the back of the lot looked a bit forlorn with lots of mulch between them. Back then it was hard to imagine that they’d ever fill the empty space.

Newly planted trees on the berm, 1997

Well, twenty years later, they’ve done that and more.  First the young trees grew taller and fuller. Then they reached out and touched branches with their neighbors. 

Touching shoulders in 2006

This year I looked at some photos and realized the trees on the berm were crowded. We’d planted them too close together.

This week--too crowded

    When my friend Marlyn invited me on a tree walk through Newton Cemetery, I got to see a more farsighted planting approach. Groves and single trees were amply spaced to create painterly vistas. I observed what several of my trees would look like if they’d grown up with unlimited room to spread out and no nearby competition for sunlight and moisture.

Japanese umbrella pine at Newton Cemetery

And trying to grow in my crowded yard

     






     I’ve taken down some trees over the years to reduce crowding. Betsy’s original design involved a circle of lawn lined by four graceful white-flowered redbuds (Cercis canadensis f. alba). I’ve since regretfully removed two of them. When they were young, their lithe branches lined with tiny white blooms in May looked like dancers in a green woodland. 

One of the redbuds in its early days

Unfortunately, the trunks couldn’t grow straight because of expanding shade from taller trees.  

One of two remaining redbuds, reaching for sunlight

As the ill-fated pair leaned farther and farther out over the lawn and shaded out perennials I wanted to grow nearby, I made up my mind that they’d have to go. Their stumps still make me feel like an assassin, as do the empty spaces left by hemlocks we took down this year.

What terrible person cut down this tree?

    At this point there’s no easy way to make more room for the trees. I can let the inter-weaving of branches continue or make more hard choices and edit out more trees. As I plant in the empty space left by the hemlocks, I notice that as they expanded, they distorted the growth of surrounding trees. A Hinoki false cypress (Chamaecyparis obtusa) has no living branches on one side where a hemlock muscled in on it. 

Branches died on the side where the hemlock shaded out this tree

    If I did remove trees from the berm, the ones remaining wouldn’t have symmetrical shapes. Once they’ve shed their needles because of too much shade, branches of these conifers won’t send out new foliage.

    What to do? I’ll probably delay action until one of the trees dwindles or becomes a danger and has to come down. It hurts to cut down trees.

Attention friends: I'll be speaking at the Massachusetts Horticultural Society's Ecological Gardening Symposium next Wednesday, November 8, at Elm Bank in Wellesley, MA. Here's a link to the program information. I'd love to see you there.