I’ve been gardening for more than 30 years, but it’s still a new day when I trip over a problem and discover growing insights. First, the happy solution. A beautiful pot of annual Rio Series yellow portulaca (Portulaca oleracea) was the key to understanding unexpected problems on my front walk gallery of ornamental plants and tomatoes in containers. It was a few years since I had last looked at portulacas, and my goodness, what changes the hybridizers have wrought in these pretty, serviceable plants.
Older versions of portulaca look a bit hairy, crawly and primitive, as would be expected from a plant not far evolved from its purslane origins. A South American breeding program in Cartago, Costa Rica, took genes of species portulaca (P. oleracea, P. umbraticola and P. grandiflora) and produced the Rio Series (8 x 16 inches / 20 x 40 cm), a noticeably improved generation of colourful plants with stouter stems, an arching posture and svelte, succulent foliage. The large ruffled flowers (yellow, white, scarlet or orange, all with yellow centres) close at night, but stay open under overcast skies, unlike older portulacas that close on cloudy days.
Now, for the distressing problem. Recently, the removal of a large fern-leaf beech tree in the front garden created a very bright planting space. Some areas became too hot for certain plants, such as ‘Sum and Substance’ hosta, while a few, like perennial salvias, penstemons and Russian sage (Perovskia atriplicifolia), relished the increased light. I began moving new plants into these hot spots, looking for potential sunbathers that could form a new and permanent collection.
The brightest area is along the front pathway, where I put containers of tomatoes, knowing they would excel in the heat and sun. Unfortunately, I had forgotten a sharp lesson learned one year, when tomatoes in that location developed strange white blotches on their foliage and their growing tips stopped expanding. When they were moved to less intense light, the blotches disappeared and growth resumed. This time, the tomatoes had brown colouration creeping inward from the leaf edges, and some had markedly stunted growth. Both years, the changes in leaf colouration and retarded plant growth occurred quite rapidly.
The brown-leaved tomatoes were swiftly pulled back to parts of the path with less intense light, and I retreated to the computer for some Internet research. The American Horticultural Society’s heat zone map is a most illuminating resource (pun intended!) with information about how plants use light during climate change (see ahs.org/gardening-resources/gardening-maps/heat-zone-map).
Nowadays, we have more intense heat and sunlight than when my grandfather was growing tomatoes, and this directly affects how much is absorbed by plants, and what they do with these resources. And not all the effects are good. Full-spectrum light can enter plant foliage, but only rays with shorter wavelengths can exit. Longer-length light rays are trapped inside foliage, accumulating heat over successive sunny days with high temperatures.
Heat resistance and tolerance of environmental stress are related to the evolutionary location of a species. Some plants (presumably like tomatoes) begin to suffer physiological damage from heat accumulation at temperatures over 86°F (30°C). Others, like portulaca, can endure greater heat accumulation in their foliage and continue healthy growth. As foliage tissues accumulate heat, biological changes occur, such as the dispersal of chlorophyll and tissue discolouration. I learned that heat-stressed foliage will change from green to white or brown — and doesn’t that sound familiar?
Well, I guess all has been revealed. Tomatoes can’t take so much sun-generated heat, and portulaca doesn’t mind at all. It’s always something.