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Garden Q&A: Is the American giant millipede a pest and what’s a microclimate?

Q: This worm-sized, non-worm appeared in the garden near my stumpery. Plant pest? Beneficial? I’m stumped!

A: Stumperies are lovely garden features, like a vignette of the forest floor, that I wish were more common for added variety. They use dead root flares and various pieces of tree stumps or logs to emulate the forest floor environment, where ferns and other perennials or shrubs can be tucked-in to set the mood. Picture a rock garden with tree pieces instead of boulders.

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An American giant millipede, which can reach up to nearly 4 inches in length.

This fun find is one of our American giant millipedes, widespread and native in Maryland. The legs can give you a hint, other than it not being a worm — two pairs of legs under each body segment means millipede, one pair means centipede. (Good luck having a speedier centipede hold still long enough to figure that out, though. With their legs positioned closer to their sides, they’ll be easier to see from above than a millipede’s legs. Plus, centipedes might bite if held. Millipedes would rather coil up and wait for you to go away.)

Millipedes like this giant plod around in rotting logs and leaf litter consuming organic debris. Sometimes they might snack on ground-level vegetables, but the rind would probably need a cut in it first so they can access it. Like earthworms, they help aerate the soil, which gives roots better access to oxygen and helps drain excess water. As members of the composting team they also incorporate organic matter on the soil surface — nature’s little rototillers.

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When disturbed, they are capable of secreting defensive chemicals from glands on the side of their body, and this liquid might irritate skin if you handle them without gloves. Otherwise, they’re harmless. Here’s your weird biology fact of the day, courtesy of BugGuide (a database hosted by Iowa State University): “[the] female lays a single egg in a cup formed from regurgitated food.” Aww.

Q: What is meant by “microclimate?” I’ve occasionally encountered this term used in gardening but don’t understand it. Is it related to climate change?

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A: Although it can be influenced by climate change, microclimate is a stand-alone concept. The “micro” in this case refers to scale, referring to the climatic conditions of a relatively small area within the larger landscape. As gardeners we focus on temperature, but this also includes aspects like airflow and humidity. Microclimates can be natural or human-made.

For example, a home’s yard can have several microclimates where growing conditions can differ a bit from their surroundings. Structures that absorb, retain, reflect and radiate the sun’s warmth like house walls, solid fences, pavement and stone retaining walls create warmer, drier microclimates for plants. A roof overhang over a foundation or the dense canopy of a mature shade tree creates a drier microclimate. The outflow area of a roof downspout or a gully collecting drainage from a street creates a soggier one. Our winter winds often blow out of the northwest, so that area of the yard or a balcony facing that direction will feel more of the brunt of desiccating and chilly air than a south-facing balcony or part of the yard. Plants growing on hilltops are more protected from frost (somewhat counterintuitively) because the cold air flows downhill and settles in lower-lying areas.

Microclimates can be beneficial if you want to grow species that are well-suited to those conditions, or it can be a detriment if it causes them stress. It’s always simpler and easier to garden successfully when working with the microclimates you have rather than trying to change them to suit particular plants.

Warmer microclimates can help overwinter species that are borderline hardy or more at risk of suffering winter damage, like fig, rosemary, camellia and Yaupon holly. Colder microclimates can help plants that need a minimum chilled dormancy period to bloom well (like lilacs and many fruit trees). They can also discourage plants from breaking dormancy too early after a winter warm spell, where subsequent freezes injure foliage and may kill flower buds.

Pest and disease potential is also impacted by microclimate. Spider mites, for instance, thrive in warm conditions with limited airflow and drier foliage. Plants grown in narrow planting spaces between houses with high fences, or in soils next to paved roads or driveways, may be more vulnerable to mite population booms. Scale insects are known to outbreak on street trees because that microclimate of heat from the road, drought-prone soil, limited root space and airborne pollution and dust stresses trees and hinders the activities of natural enemies preying on scale.

You’ve probably heard of urban heat islands caused by the collective warming effect of all those hard surfaces in cities compared to the leafy suburbs. This can be considered a large-scale microclimate of sorts, which is why cities tend to have longer growing seasons than outlying areas — the spring frosts end sooner and the autumn frosts arrive later.

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University of Maryland Extension’s Home and Garden Information Center offers free gardening and pest information at extension.umd.edu/hgic. Click “Ask Extension” to send questions and photos.


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