Summer tilts into fall, and I go looking for monarch butterflies on Peters Hill. The late bloom is on the meadow, yellow-flecked with yolky dollops of tansy, while asters and the nodding umbels of Queen Anne’s lace hover everywhere amid the green.

It’s the milkweed above all, however, that makes the meadow a monarch waystation: blooming singly here and there, Asclepias tuberosa or butterfly milkweed flares red-orange in the undergrowth, while the taller stems of A. syriaca or common milkweed, buckled and brown, lean into each other like tarnished candelabras. Bold in their black- and yellow-banded livery, the caterpillars feeding on milkweed here in late August will become the adults that fly all the way to Mexico. Last February, in the mountains of the Trans-Mexican Volcanic Belt, the great-grandparents of these caterpillars shivered loose from their winter shelter amid the branches of oyamel trees (Abies religiosa); soon, their descendants will return following the southgoing autumn. No single monarch travels the entire circle. What guides them along the way? How does such a memory—if it’s reasonable to call it “memory”—take root in the world?

Both insects and memory may be said to teem, and the meadow is a crowded place, full of late-season insect life: crickets weave a blan- ket of buzz torn by sudden silences, grasshoppers popcorn out of the shadows to grasp at stems of grass, and bursts of late-season cicada song shimmer in the trees. My own Peters Hill memories come crowding, too—of summers past, in years before the Arboretum let the meadow grow uncut, when we climbed the hill on Fourth-of-July nights to watch fireworks bloom above the city. Or more recent recollections, like that of the coyote I watched one morning as it reclined in the shade of a hawthorn (Crataegus). It was the first year of the pandemic, the road below Peters Hill dotted with covid-weary visitors turning to the landscape for respite. The coyote watched them unseen, and I watched its watching until it stood to stretch, yawn, and rake a mouthful of ripe haws from a low-hanging branch before trotting off.

Memory traces abound on Peters Hill. A path cuts sharply through the timothy and vetch, a vein of bare soil compacted by visitors marching upslope and down. Joggers and dogwalkers seek the most direct route to the top, and the hill remembers their efforts. The dirt of the path is studded with rocks; when the rains come, they roll out with the waters draining off the summit, leaving an outwash delta smeared across the grass at the base of the hill. This sediment is glacial till, piled here by the churning of the kilometer-thick ice sheet that covered Boston until about twenty thousand years ago. Drumlins like Peters Hill are the memory of its passage. Landscape architect Rosetta Elkin has described the Arboretum’s drumlins as “remnants of a larger erased regional landscape of undulating, swelling, sloping life grounded in the contours of the earth’s past.” Boston “was built from the extracted hills,” she notes, in a process of development “that did not really see the virtues of the landscape.” By incorporating these hills in the design of the Arboretum, Elkin suggests, Olmsted and Sargent undertook an act of preservation, of deep-time memory.

Across the brow of Peters Hill, butterflies orbit here and there, urgent in their looping flight. Stomping through dew-wet turf at the meadow’s mowed edge, I count some half-dozen individual monarchs on patrol. Some circulate intently, sailing long reaches over the bobbling blooms before coming about, while others coast down the wind with paper-airplane aplomb. They show no preference for north or south; at home in their ellipses, they are content to browse the meadow.

To find their way up and down the continent, monarchs rely on textures of sunlight that shift with the days and the seasons. Equipped with a neurological “sun compass,” they take their bearings from the sun’s polarized light, which orients them to the advance of spring and, now, to autumn’s south-slipping radiance. These inputs are coordinated in the “mushroom bodies”—specialized clusters of dendrites in the insect forebrain that convert sense into learned behavior. Dendrites, mushrooms—trees and fruiting bodies rise in the minuscule garden of the insect brain and flower into memory. It is thought that their way- finding also relies on magnetic fields, temperature, and a host of other sensory cues. The migration, then, is a memory made of many things: shifting angles of light, the rolling bloom, the bitter taste of milkweed. Such a memory is less like a written record than a performance, or a collection—or perhaps a meadow.

Certainly, it is more than a map of the route from Maine to Michoacán—for it tells the milkweeds’ tale, too. With the retreat of the same ice sheet recorded in the gravel and drift of Peters Hill, Asclepias spread north into the meadows of North America. Investigators also have suggested that the monarch’s migratory flight remembers the colonial deforestation of the continent, which created a bounty of new open-field landscapes for milkweeds to flourish. The great annual migration of the butterflies, then, might be understood as the reenactment of these millennial metamorphoses of climate and biome, danced by the butterflies on the wheel of the year. Or why not call it a memory, vast as a continent, written in brightness and bloom, slant of light, shift of temperature?

Such a memory is less like a written record than a performance, or a collection—or perhaps a meadow.

Elsewhere throughout the Arboretum, deep-time memories abound: hedgeapple and honey locust recall hungry mastodons; the ginkgo offers up its smelly seeds and dreams of dinosaurs. I realize, however, that calling such relations “memory” might not sound right to some. Is it meaningful to claim that the monarch remembers the retreat of the glaciers, the advance of the milkweeds, and the clearing of the trees? Perhaps it is more palatable to say that they are part of a broader memory system, which is the memory of the earth. And we too are particles of this planetary memory. For the Arboretum is more than a topo- graphical archive; it’s crucially a library of biodiversity, a living memory, which takes shape in the generation-spanning work of curation and care as much as in living plant tissue. Like a sun-compassing butterfly; like a murmuration of starlings; like a forest. I started out wondering what kind of memory persists in the mind of a monarch; high on Peters Hill, I find a saving hope in the recognition that we are memories ourselves. 

Matthew Battles is the editor of Arnoldia. He is writing a book about the natural history of memory.

From “free” to “friend”…

Established in 1911 as the Bulletin of Popular Information, Arnoldia has long been a definitive forum for conversations about temperate woody plants and their landscapes. In 2022, we rolled out a new vision for the magazine as a vigorous forum for tales of plant exploration, behind-the-scenes glimpses of botanical research, and deep dives into the history of gardens, landscapes, and science. The new Arnoldia includes poetry, visual art, and literary essays, following the human imagination wherever it entangles with trees.

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