Frosty Beans: Unexpected extremes in a mild climate

by | Jul 5, 2023

Climate change is causing problems on our farms… droughts, floods, heat domes, fires, pests. 2023 is projected to be the first year that global mean surface temperatures exceed 1.5 C of warming. It may be getting warmer overall, but the transition to warmer conditions is not a smooth one. Push a system off equilibrium as we are doing with greenhouse gas emissions, and you get extremes, circumstances outside of the normal envelope around the equilibrium. And paradoxically these extremes vary outside the norm in both directions: warmer and colder, wetter and drier etc. This tendency brings my bean trial beds into the story.

In early June, 2023 on the Saanich Peninsula north of Victoria my trial beds experienced a damaging frost. The local farming folk whose family have lived on the land I use for part of my trails for 150 years had not heard of such a thing. We have a moderate climate, and the site is only a couple of kilometres from the ocean. Nor could they believe that a frost had occurred until I showed them the damage to the bean plants. The day had been warm and sunny, though night-time air temperatures above ground (typically measured at 1.5 m or so) were cool but certainly not freezing. The evidence however was unambiguous; damaged curled and even killed leaves were clearly visible on several varieties and indeed at least three species of beans (Figure 1). The field had experienced a radiation frost, typical of inland continental climates and of early spring on Vancouver Island, but not in June.

Figure 1: Yellow and brown shriveled leaves of common bean Phaseolus vulgaris var.Calypso, Brentwood Bay BC, damaged by frost June 3,2023. Photo Richard Hebda.

Radiation Frost

Radiation frost occurs under a clear sky and calm dry atmosphere with wind less than 8 km/jour (5 mph). Often the preceding day is warm and sunny, and the ground absorbs the sun’s heat and warms up. During the night, however this absorbed heat is radiated back into the atmosphere at longer wavelengths and literally cools the ground and near-ground leaf surfaces. A temperature inversion develops just above the soil and temperatures near the surface drop below freezing. The thickness of this surface cold zone depends on the length of the interval of longwave radiation and on whether or not the heavy cold air flows down slope where it may pool in frost pockets. My guess is that flat surfaces such as the field where my beans are growing cannot shed the cold air and the frost zone grows in thickness until warming in the morning. This phenomenon is called a temperature inversion because normally air cools with height above ground. In the case of radiation frost, heat is lost at the ground surface and the air warms with height above ground. Often there is a marked boundary between cool air at the surface and the warm air immediately above.

I could see that night temperatures were generally getting cold in our area from a weather station in the field at my own property. We were under the influence of a stable, dry cool air mass, unusual for us at this time of the year. Dawn temperatures (usually the coldest time of day) at 1.5m above ground reached as low as 3.2 C early in the morning ranging to 3.7C and 3.8C several nights in a row (Figure 2). And the nights were illuminated by a beautiful crystal-clear full moon; the air was still at night. From past experience, these are conditions when frosts are possible but not so late in spring.

Figure 2: Daily temperature graphs (blue line) June 1 to June 9,2023 for the Heggs Climate station 2 km south of Hebda’s bean trial fields. Data provided by Christopher Hebda. The vertical axis shows degrees Celsius. The lower green line shows the Dew Point temperature. The lowest dawn temperature occured on June 3. Note the 20-25C daily range in temperature during clear days. Cloudy skies arrived on the 8th of June and daily high and low temperatures differed by just a few degrees.

Like many gardeners I try to get a quick start on my crops. My early sown beans had shown excellent growth through all of May, but our daily temperatures had been exceptionally warm in BC, setting records over wide areas. At home, I did see unusual colours on some of the leaves, such as browning of the youngest leaves for example on the Mexican pole bean Flor de Mayo (Figure 3). I thought this might be a bean rust or some other malady. In my trial other fields all seemed well. Potatoes and beans were thriving, the days were warm.

Figure 3. Browning of young bean leaves of common bean variety Flor de Mayo in response to cool night temperatures in early June, Saanich, BC. Photo Richard Hebda June 2023

I came into the field mid morning June 5 and was dismayed to see beans with shriveled, even blackened leaves (Figure 1). This damage was widespread along the rows affecting varieties of common bean (Phaseolus vulgaris) such as Calypso, runner bean (Phaseolus coccineus) var. Handsome Johnny from Poland and lima bean (Phaseolus lunatus) var. Red Hopi. Newly sprouted seedlings with a small single pair of leaves seemed not affected. The stems of the damaged plants apparently resisted frost too.  There was no sign of damage on any adjacent potato plants nor were the leaves of young melon plants protected by row cloth affected either. The daily high had reached 23.1 C just 2km from the field at our home weather station. Even closer to the trial fields a station at a local school less 0.7km away but slightly closer to the seashore the minimum temperature at dawn of June 3 was about 6C and the previous day high was above 20C.

Figure 4. Common bean var. Calypso sproutlings showing new growth from stems with leaves previously killed by radiation frost (brown leaf at centre). Photo by Richard Hebda two weeks after damaging frost June 3, 2023, Brentwood Bay BC.

The good news is that despite significant damage to the leaves of several bean varieties and species growth has resumed from the stem including its apex. Nevertheless, the lesson is obvious, despite warming temperatures overall, it is the extremes in the field that we have to be concerned about. As far as young beans go, beautiful sunny late spring days do not protect the plants from frost. Large young leaves will be damaged.

There are several ways to avoid this damage, one of them being planting later than early to mid May. Young sprouts with only two small true leaves seem to have been undamaged and continued to grow. Some varieties such as Blue Jay (see article on this website) catch up to plants sown in mid May even if planted a month later in mid June. For small plots the bean rows can be covered in white row cloth. The melon leaves adjacent to a damaged bean row survived unharmed. Another option may be to plant on a upper slope from which the cold air drains and no inversion forms. Rather the cold air accumulates in a pool on the lower slope. There may also be frost resistant varieties. We hope that our citizen scientists bean growers across Canada make note of radiation frosts and their impact on different varieties especially noting the survivors. Climate Change is here: we maybe warming up but extremes including record cold events and unusual atmospheric conditions will challenge food production everywhere even in the most benign climates.