Why you can’t grow cool-climate plants in hot climates
Since moving to Deep South Texas 4 years ago I've come to realize that many plants I used to love growing in the cool mild maritime climate of the SF bay area are impossible to grow where I live. This is not just because of the high daytime heat. It's not as simple as that. Specifically, it is the high heat during the night (and those warm nights are a direct result of the humidity) that causes cool-climate and cool-season plants to eventually die here. That's a bummer for somebody who loves plants from places like cloud forests of Central America, the Páramo of Ecuador, Alpine plants from the Rockies or Southern Andes, etc. This phenomenon is also quite fascinating however, and goes far to explain why growing certain plant species or even entire clades (evolutionary groups) of plants is so impossible outdoors in certain climates. It all has to do with metabolism and something called compensation point.
Like animals, plants respire. This means they burn sugars to create new tissue as well as the chemical compounds they use to defend themselves against fungi & insects, as well as simply to conduct daily metabolic processes required for survival.
“Compensation Point” refers to the light intensity or CO₂ concentration at which the rate of photosynthesis exactly equals the rate of respiration in a plant. At this point, there is no net gain or loss of carbon—the CO₂ absorbed during photosynthesis is equal to the CO₂ released during respiration, and the oxygen produced matches the oxygen consumed.
For a plant to grow (not just survive), it must operate above its compensation point. It must produce more than it consumes. That is, photosynthesis must exceed respiration enough to generate a net gain of carbon to -create new tissues
- put some energy in storage reserves in the form of starch and sugar
-produce defense chemicals (alkaloids, toxins, etc) and mechanical defense (spines, thorns, hairs, wax)
- produce reproductive tissues like pollen, nectar, flowers and fruits/seeds
Below the compensation point, and operating at a carbon deficit, the plant burns stored energy reserves and eventually dies. Along the way it will decline in health and will gradually become weaker and more prone to insect infestations and fungi.
What do warm nights do to cool season plants and why?
Respiration (which burns sugars for energy) increases exponentially with temperature (roughly doubling every 10°C rise). Essentially what this means is that plant metabolism is dependent on temperature, something I've repeated many times in videos. In areas where the nights remain warm (because the air is humid and contains water vapor…which is slow to let go of heat), the plant’s metabolism will run at an accelerated rate at night.
So essentially, warm nights mean the plant burns more stored carbohydrates (sugars, starch) just to maintain basic metabolism. This leads to a reduced net carbon gain in a cool-season or cool-climate plant that has spent the past few million years evolving in a place like the Páramo of Central America, or the high Andes, or the rocky mountains. As we stated before, the plant eventually dies, and it is probably a long protracted death along the way, where the plant might be infected by scale and mealy bugs and not have the metabolic energy to produce defenses against them.
A similar case exists with Tomatoes : Hot nights (>75°F / 24°C) cause excessive respiration, burning through the day’s limited photosynthetic gains. Even worse, high humidity reduces transpirational cooling, since the air is not as readily pulling moisture out of leaf stomata. This only raises leaf temps further. Why do tomatoes have this limitations and why do they behave this way? When we look at where the wild ancestors of tomatoes evolved, it starts to make sense.
The wild ancestors of tomatoes - a number of species such as Solanum pennellii, Solanum peruvianum, Solanum chilense, and Solanum pimpinellifolium - are native to very high elevations above 8,000’ in the Northern Andes, near the Peru-Chile Border. They grow in a climate unlike any that can be found in North America - namely high, dry elevations at low latitudes (18° South) - this means they are exposed to cooler temperatures (because of the elevation) than one might think they would tolerate at such a low latitude, while still being hit with a very intense amount of solar radiation and light (which they not only tolerate, but need. Tomatoes are full-sun plants, as most people know). I encountered Solanum pennellii growing in a drainage sandwiched between rocky, almost barren slopes that were dotted with massive Browningia candelaris cacti at 8,000’ (2440 meters) in Northern Chile. It was remarkable to see these little herbs - their leaves were covered in glandular trichomes (sticky hairs) that looked and smelled just like those of modern tomatoes. The ancestry was obvious.
But it is not just warm nights that cause certain plants to decline. If some plants do not get enough light, this also affects the compensation point. They are not able to produce enough carbohydrates via photosynthesis to create the defensive compounds they need nor the reproductive structures (which are highly metabolically costly) they need to reproduce and produce fruits and seeds.
This reminds me of a question I got a month ago from someone who had a cactus for 4 years that they were growing on a partially shaded window sill. They moved apartments and when they arrived at their new place, they put the plant in a sunny south-facing bay window which got light from above as well as from the side. Within a week or two the cactus bloomed. When the plant was placed in a location where it got more light, it exceeded its compensation point and since the plant was now producing an abundant amount of carbohydrates compared to the paucity of them that it was producing before, it was now able to devote some of that energy to the production of flowers.
The case with compensation point and the struggle of trying to grow cool climate plants in very hot humid environments is exactly the opposite for tropical plants and warm-climate cacti (Note that many high Andean cacti from South America operate at cooler temperatures than most Mexican cacti)... When hot-climate plants are attempted to be grown in temperatures that only reach the high 70s at most, their metabolism drastically slows down and they are not able to produce chemicals to defend themselves nor especially to produce new tissue and new growth.
Further, different plants have evolved their own variations of the photosynthetic process in order to cope with some of the extremes in the regions where they are native. Specifically, CAM photosynthetis, C3 photosynthesis and C4 photosynthesis are issues at play here.
In cacti from warm climates especially, CAM photosynthesis does not operate very efficiently below 85° f. CAM photosynthesis is a process whereby plants open their stomata and take in CO2 at night - when temperatures are much cooler - to avoid releasing too much H20 through those same stomata. CAM plants absorb CO2 at night and store it in the form of malic acid to be used for photosynthesis during the day when the sun is out and light can power the photosynthetic process.
Also playing a role in all of this are C3 and C4 photosynthesis. We won’t get into this now, except to say that C4 photosynthesis is far more efficient at hot temperatures (thus why many plants - especially hot-season grasses - from warm climates use it. They do this with the help of something they evolved called “Kranz Anatomy”, and Kranz Anatomy has evolved independently in plants many different times throughout history) and C3 photosynthesis is more efficient at cooler, milder temperatures. Whether C3 or C4 photosynthesis is more efficient - and why it evolved in the first place - has to do with a negative phenomenon called photorespiration. In short, photorespiration is the process whereby an essential photosynthetic enzyme called Rubisco. Rubisco is the most abundant enzyme on Earth and is supposed to fix CO₂ into sugars. At high temperatures, however, Rubisco bonds with oxygen, instead, which produces a toxic byproduct (phosphoglycolate) instead of sugars. Rubisco’s affinity for oxygen at high temperatures is its flaw, and some think that since Rubisco evolved 3 billion years ago, when O2 levels were low and CO2 levels were high, its affinity for oxygen wasn’t a flaw until “recently” (speaking in terms of the geologic time scale, of course.
Overall, compensation point and a plant’s ability to grow in a given climate is a balance between the light available, the carbon dioxide available, and the temperature (and how it corresponds to the plant’s metabolism, which is dependent primarily on what kind of climate it evolved in over the past few million years.
