July Heat

What’s blooming in the area: Roses, rose of Sharon, Russian sage, buddleia, trumpet creeper, silver lace vine, morning glories, datura, daylily, purple phlox, bigleaf globemallow, bindweed, white sweet clover, goats head, pink and white evening primroses, velvetweed, toothed spurge, first pigweed Queen Anne’s lace, tumble mustard, heliopsis, Tahokia daisy, cultivated and native sunflowers, golden hairy aster, goldenrod, hawkweed, horseweed, goats beard, wild lettuce, sand burs and grama grass seed heads.

What’s blooming in my garden, looking north: Blackberry lily, golden spur columbine, coral beardtongue, hartwegii, squash, perky Sue, fern-leaf yarrow, chocolate flower, blanket flower, coreopsis, Mexican hat, black-eyed Susan, chrysanthemums.

Looking east: Large-leaf soapwort, bouncing Bess, sweet alyssum, pink salvia, veronica, winecup, hollyhock, sidalcea, California and Shirley poppies, pink bachelor buttons.

Looking south: Tamarix, bundle flower, perennial sweet pea, tomatilla, cosmos, zinnia.

Looking west: Flax, catmint, white spurge, caryopteris, purple ice plant, ladybells, sea lavender, Monch aster, purple coneflower.

Bedding plants: Sweet alyssum, snapdragons, petunia, Dahlberg daisy, tomatoes.

Inside: Aptenia, zonal geranium.

Animal sightings: Birds in cherry tree, gecko, hummingbird moths on large-leaf soapwort, ants, bees, crickets, grasshoppers becoming serious pest.

Weather: Storms continued to blow through with more clouds than water; days remained hot.

Weekly update: Burn out, brown out, heat stress. Whatever you call it, it happens every year as regularly as the earth revolves around the sun.

This year began the same. Ladybells came up as densely as a groundcover, then plants disappeared. Catmint started blooming, then stems flopped and leaves at the base turned brown. Perennial sweet peas started to bloom, then the leaves turned brown and died out.

Then something different happened. Despite day after day of 90 degree heat, some ladybells did not evaporate, but have been blooming since July 2. Six Hills Giant revived with new stems from the base that have filled in the bare spot with a mound of leaves and flowers. A couple weeks ago, my sweet peas put out new leaves, and two weeks ago their first flower.

Why?

Plants have simple imperatives. Chloroplast segments in leaf cells use energy from the sun to manufacture food from water and carbon dioxide, the one coming up from the roots, the other through pores near the chloroplasts. Summer heat disrupts the equilibrium when more water is lost through leaf stomata than roots can replace.

However, it’s not simply the midday sun. Plants like my squash adapt and wilt, only to revive in the night. Others close their pores in the day. Tamarix concentrate its photosynthesis in the hours after dawn. The controlling condition is that night temperatures must drop. If they remain high, plants can’t use their night shifts to replace water they sweated. So far this year, even if early evenings have been progressively warmer, early morning hours have stayed cool.

Water crises don’t just arise from a lack of rain. During the severe drought a few years ago, I continued to water beds at night. Even so, the native sunflowers got about a foot high, then stopped growing and eventually disappeared without blooming. This year, I have one 6' high that has its first flowers, and others still waist high and growing.

Back then winds that normally absorb water when they pass over distant reservoirs, full rivers and summer mountain snowfields arrived so barren they cannibalized water from plant tissues. Even though we haven’t had much rain since June, storms continually blow through and equalize water pressures between leaf sensors and the atmosphere.

The intensity of the sun at 6,000' and 36 07 N latitude also matters. Ladybells come from northeastern Asia, catmint from the Caucasus, and sweet peas from southern Europe. The first fails in July in Maryland, the second on the Georgia piedmont after a few seasons, the last during summers in the Kentucky foothills. Problems always appear around the solstice and begin to disappear a month or so later when sun angles change.

Microclimatic variations are so subtle that sweet peas can bloom all summer four miles away and nearer the river. Two patches are under old trees, the other in the sun in soil thrown along the side of a ditch. Closer to where I live, shaded sweet peas clinging to a chain link fence awoke from their summer hiatus Friday morning.

So what’s been special about this July?

Government weathermen only record temperature, humidity and precipitation, then for cities a thousand feet higher or lower than Española. Botanists can’t predict how individual species will respond to combinations of air and water without more research. Plants themselves are the ones who report afternoons this summer have been beastly but, if they’ve been able to protect themselves, it’s been a very good year.

Notes:
Anderson, Jay E. “Factors Controlling Transpiration and Photosynthesis in Tamarax Chinensis Lour,” Ecology 63:48–56:1982.

Armitage, Allan M. “Nepeta,” in Herbaceous Perennial Plants, 1989.

Scausey. “Adenophora Latifolia - Ladybells,” Scausey's Journal, available on-line.

University of Kentucky. “Lathyrus - Sweet Pea,” Kentucky Garden Flowers, available on-line.

Photograph: Perennial sweet peas with dead leaves in background, 28 July 2007.