Understanding microclimates

Your garden railway has different "neighborhoods" of growing conditions
This seven-year-old dwarf azalea (Rhododendron ?Hilda Niblett') is growing happily in a microclimate with partial shade and acidic soil. Note that though the plant and its leaves are much smaller than the species, the flowers are the usual size.
Don Parker
A miniature yarrow (Achillea tomentosa ?King Edward') flourishes in full sun. The same plant, when located on the shady side of a small spirea bush, languished and eventually died.
Don Parker
A paxistima shrub (Paxistima canbyi) grows luxuriantly in partial shade with moist soil.
Don Parker
Here, a paxistima shrub planted in full sunlight and drier soil is shorter, has sparser foliage, and has lost a number of branches. It is struggling as compared to the paxistima in the above photo, even though they are the same age.
Don Parker
Just as your trains may run through a variety of diverse areas in your garden railroad (such as towns and rural landscapes), so your garden has areas of distinctly different "neighborhoods" or growing conditions. When you see a plant that thrives beautifully in one area, but struggles along miserably in another site, it's a result of something called a "microclimate."

Above-ground microclimates

Microclimates can be grouped into two categories: above the soil level and below the soil surface. When considering above-ground conditions, you might include sunlight or shade; good air circulation (or moist, humid surroundings); protection from harsh, winter winds (or the opposite); and added warmth from nearby objects. Sunlight and shade are easy considerations when we choose plants for a specific location. But there may be other factors in the area to think about. A south-facing slope, for example, will make winter sun more direct and summer sun even more intense. Reflected light from a building (especially if light colored) will increase the sunlight's effect (more heat, faster-growing plants, and quicker drying of soils). Some groundcovers, such as Corsican mint (Mentha requienii) and New Zealand brass buttons (Cotula), might look tidy in partial shade/half-day sun but become more coarse and leggy when grown in full shade.

Good air circulation is important for plants that are prone to fungal diseases on their foliage, such as miniature roses (Rosa)and bee balm (Monarda). Most plants with fuzzy, silver-colored leaves, which are adapted to dry climates, need breathing room, too. Good air circulation is achieved by locating plants in more exposed areas and avoiding overcrowding. However, during prolonged rainy spells, the chance of overall fungal problems is increased from lack of adequate drying time.

Harsh winter winds can desiccate foliage on evergreen trees or shrubs that are otherwise rated for colder Hardiness Zones. This can even occur with the ubiquitous dwarf Alberta spruce (Picea glauca 'Conica'). It's rated cold hardy to Zone 2, but can be prone to winter burn when the ground is frozen and no water is available to the tree. Cold, drying winds will increase evaporative loss from the still-green needles, eventually causing them to turn brown and fall off on the windward side. The trees typically recover and begin growing new foliage in the spring, but they may look a bit lopsided for half of the growing season. To avoid this, plant the trees in an area protected from north or west winter winds, or a make a burlap windbreak on the side of heaviest winds. If you've had an especially dry fall, make sure your miniature evergreen trees are watered well before the ground freezes. It is also a good idea to water periodically throughout the winter during warmer spells when the soil begins to thaw, especially if there is little or no snow cover.

Other above-ground microclimates, easily overlooked, are those that gain extra warmth from surrounding objects. Larger rocks or stone walls hold heat gained from exposure to the sun and release it slowly to the surrounding area at night or during short cold snaps. Plants that prefer warmer climates may be grown in parts of the country rated colder than their native Zones if planted in the lee of a large rock or stone wall.

Below-ground microclimates

Microclimates underground are harder to understand primarily because they're harder to see. Characteristics that influence these microclimates include the soil type, structure, and pH. Pick the right plants and they will flourish in these contrasting neighborhoods. Plant the wrong ones and they will inform you of your poor choice by listless performance or sudden death. You can alter the condition of the soil by adding sand and/or fine gravel to increase drainage, or organic matter such as leaf compost to increase moisture retention. You can also increase drainage by raising the planting area above the surrounding level and/or putting in subsoil drains.

Plant labels stuck in nursery pots often have symbols or information alerting you to the preferences of the plant (sun/shade, dry/moist, and Hardiness Zone). But labels don't always have complete soil information. Plants may prefer either an acid or alkaline soil. There are simple tests to determine the pH (acidity or alkalinity) of your soil. Test kits to measure soil pH are available in most garden centers and results are easily obtained at home. Before siting any plant that is pH sensitive, be sure to test the soil and make it more acid with sphagnum peat moss and cottonseed meal, or more alkaline with ground limestone. Examples of plants with a special liking for acid soil are azaleas/rhododendrons (Rhododendron), heaths and heathers (Erica and Calluna), and spruce (Picea). Alkaline-loving plants would include roses (Rosa), garden pinks (Dianthus), and speedwells (Veronica).

Large-scale microclimate

A remarkable instance of a microclimate affecting an entire garden railway is at the Franklin Park Conservatory in Columbus, Ohio. This well-done, large-scale railroad and extensively planted garden is situated in an outdoor courtyard completely surrounded by two story, concrete-and-glass walls. It is located in Zone 6 but, because of its protecting and heat-trapping surroundings, has many Zone 7 plants flourishing in their snug environment. Most of the those plants wouldn't survive their first winter if moved only a few yards away outside their courtyard setting.

Gardening choices

Gardening can be as much science or intuition as you want it to be. As you gain specific knowledge about a particular plant or gardening technique, apply it in as simple or involved a way as you feel inclined and watch for the results. If you enjoy keeping track of things, write down what you observe and use your observations in succeeding years or in other areas of the garden. Notice the unique conditions of different parts of your garden and which plants do well there. Consider these neighborhood environments when you buy your next plants and watch to see how accurate your assessment of microclimates has been. This is the more organized, or scientific, manner of gardening.

If you prefer gardening by intuition and serendipity,-then the plants themselves will become the judges of which site they prefer. Whatever lives stays, and whatever dies. . .well, dies. This method takes longer to discover (by trial and error). It is simpler and takes less mental effort, but may involve more physical work replacing dead plants with tried-and-true stalwarts.

Overall, understanding microclimates allows you to have a greater variety of plants in an area that may not otherwise be considered suitable. Long term, it means a healthier and more varied garden for you.

Microclimate considerations

Choose the right plant for the conditions it will grow in.

Choose the right site for a given plant.

Re-site plants that are not doing well to more appropriate microclimates.

Thin out plants that are crowded and prone to leaf damage from excess wetness.

Amend the soil to create more favorable conditions for a particular plant.

Improve drainage to avoid prolonged soil wetness where it is not wanted.

Address watering needs of plants, providing extra water by drip irrigation where needed.

Provide winter shelter or wind breaks for winter-damaged plants.

Consider moving or shading plants stressed by summer sun.

Check soil pH before planting acid- or alkaline-loving plants and amend the soil as needed.


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