Sweet Pea

What’s still green: Juniper and other conifers, roses, Apache plume, honeysuckle, prickly pear, yucca, red hot poker, vinca, rock rose, sweet pea, sea pink, hollyhocks, pinks, bouncing Bess, snapdragon, golden spur columbine, Saint John’s wort, some grasses.

What’s gray, blue or gray-green: Piñon, winterfat, saltbush, buddleia, loco, snow-in-summer.

What’s red: Cholla, coral bells, beardtongues, soapworts, pink evening primrose, purple aster.

What’s blooming inside: Christmas cactus, aptenia, rochea, bougainvillea, zonal geraniums.

Animal sightings: Sounds of water fowl drifted up from the river yesterday morning as the sun came out after snow in the night.

Weather: Snow dustings last Sunday and Friday night; freeze and thaw between so the drive has been too muddy to drive in at night and too icy to walk out in the morning.

Weekly update: These are the times that try plant’s souls. Those whose leaves survived the killing frosts have since faced dumps of snow, then mornings below 15 degrees, and now daily freeze and thaw cycles where temperatures range between 20 and 40. Only perennials with extraordinary biochemistry can survive.

My sweet peas so far have stayed green near the base, even though leaves farther along the stems have turned to paper. Green peaks from mounds of dead leaves on vines near the village that pile on themselves in summer when there’s nothing for their tendrils to clasp.

The plants are an improbable alien in arid New Mexico. They don’t tolerate salt, they prefer a pH between 5.8 and 7.5. In their homeland, they range from Morocco and Iberia to the Balkan peninsula, then up the Danube. They quail at the deserts of Africa and Asia and haven’t scaled the Alps of Switzerland and Austria. On this continent they balk at the cold of North Dakota and the Canadian prairies, won’t grow in humid Florida, and aren’t a success in the Mexican deserts.

The rosy racemes need 14 hours of daylight, which begins here the end of April, to begin developing. If temperatures remain around 40 degrees, which they did in the mornings until the first of this past June, they take eight weeks to develop. In the best years, they come into bloom around the solstice, just as the drought season sets in.

Lathyrus latifolia needs at least 26" of rain a year. The ones that have naturalized here grow on the banks on irrigation ditches, where they keep both cool and wet. To keep mine from browning out, I dump water on them whenever the drip pan under the outside faucet fills. They stay alive with only sporadic flowers.

Then, after being babied through the summer, the yellow veined leaves surprise and stay green into winter.

Staying green means photosynthesis is continuing at some rate. For that to occur, the roots must be functioning, as must the stem. Even though the rhizomes store some nutrients, they need to extract others, including water, from the soil. In this part of New Mexico, the average frost line is 10-15". Established sweet pea roots typically reach down more than two feet, at least 9" below frozen ground.

Photosynthesis occurs in the leaves which must remain undamaged by cold temperatures which can destroy cells when the surrounding water turns to ice. Bengt Lindforss first noted evergreens in central Germany that survived the frost contained no starches, because they had converted the carbohydrates to sugar. He then established the ability of sugar to protect plants by lowering the freeze point of internal fluids.

Nikolay Maximov read his work, then injected red cabbages with some sugars (glucose, glycerol) and some alcohols to show each could increase cold defenses. Sucrose is the primary sugar in the roots of sweet peas on limestone grasslands in central Europe in summer, according to Franz Hadacek and Günther Kraus.

Polish botanists noticed sweet pea’s relatively wide, coarse pairs of leaves have large, irregularly shaped cells which would give them greater tolerance for any ice crystals that might form than smaller cells would have. They also found summer leaves contain relatively high levels of salt and calcium. If maintained all year, the first would lower the freezing point, while the second would keep cell membranes flexible enough to expand and retract with potential ice.

The leaves look the same, summer or winter, mere dark presences hiding leguminous roots that put some nitrogen in the soil. However, the external matte appearance masks their strategies for survival as completely as the snow hides their existence from the drying sun.

Notes:
Fuller, Barry J., Nick Lane, and Erica E. Benson, Life in the Frozen State, 2004, describes work of Bengt Lindforrs and Nikolay Maximov.

Hadacek, Franz and Günther F. Kraus. "Plant Root Carbohydrates Affect Growth Behaviour of Endophytic Microfungi," FEMS Microbiology Ecology 41:161-170:2002.

Kartesz, John T. "Lathyrus latifolius L.," USDA National Resource Conservation Service, National Plant Data Center.

Koike, Yasuhiko, Tomoaki Inoue, Shigetoshi Suzuki, Haruzo Higuchi. "Effect of Photoperiod on the Flowering of Lathyrus latifolius L.," Journal of the Japanese Society for Horticultural Science 69:770-772:2000.

_____ N. Kouyama, A. Yoshii, S. Suzuki, and H. Imanishi. "Effect of Low Temperatures Applied to Seeds and Seedlings on Flowering of Lathyrus latifolius L," Horticultural Research 6:525-528:2007.

Krstic, Borivoj D., Pal P. Boza, Ljiljana S. Merkulov, Lana N. Krstic, Slobodanka P. Pajevic, Zivko S. Stankovic. "Morpholigcal, Anatomical, and Physiological Characteristics of Lathyrus latifolius L. (Fabaceae)," Matica Srpska Novi Sad 103:81-89:2002, in English.

Sammis, Ted. "Frost Depth in New Mexico and the United States," New Mexico Climate 6:4-6:2008.

Photograph: Dead and green perennial sweet pea leaves in light coming through crack between fence boards, 10 January 2009.