In other words, when it is very cold (not freezing) or very hot, plants do not grow.
The relationship between Celsius and Fahrenheit degrees is:
 C = (F-32) / 1.8
F = (C x 1.8 ) + 32

 For many garden plants, the minimum, optimal and maximum temperatures are, with wide deviations, around:
Minimum: 8°C Optimal: 16°C
Maximum: 28°C

The minimum and maximum temperature limits (also the optimal) are different for each plant variety, as it has already been said. But different specimens of the same variety may have different limits, according to the conditions they are subjected to (example: too little or too much water).

The hobby gardener would be interested, in general, to know which plant varieties are better suited for his garden. However, he may also be interested to know if he could easily modify the conditions to which his plants are subjected and what results should he expected.

Plant growth  depends on the optimiza-tion of the following parameters, these being interrelated and, besides, having non-linear expressions:

Plant development or growth status: In other words, how large is the factory producing the plant’s growth chemicals. Here, we cannot act.

Efficient use of water: Roots should be able to absorb water at the required speed: Here we can act, watering adequately.

Stomata aperture: Stomata are fully open if there is enough light and water: Here we can act by creating an artificial damp micro-climate and also by watering adequately.

Light intensity: Excessive light (a different limit for each plant) reduces the photosynthesis. Here we can act by transferring our pots to a shady area or by creating several degrees of shade.

Too high temperatures damage the inter-nal substances of the plant, these being necessary for the photosynthesis. This is why many plants halt their growth in full summer in hot climates. Here we can act with regular water sprays (mists) to lower the ambient temperature.

Root temperatures: Especially for plants in containers but also for plants on the ground. Some plants are very sensitive to heat on the roots (example: Clematis). For plants on the ground we should provide some shade with other plants or with objects (stones, etc.) For plants in pots, we should put the standard black or brown pots inside white pots (two pots) and avoid putting them in direct sun . Do not use only one plain plastic white pot as these are translucent and roots dislike the light. The author suggests touching the pots with the hand to feel the high temperature they reach when they are hit by the sun. We also could introduce a thermometer on the substrate. Do not water the plants when the soil is hot. This causes a shock and helps the action of fungus, Fusarium, among others.

Availability of CO2, not too much, not too little: Here, we cannot act as hobby gardeners.
Summary of Possible Actions:
- Water adequately
- Regular sprays (mists)
- More shade for certain plants
Up to this moment we have reached some conclusions that were already known to us. Let us see if we can progress a little further.

What should not be done: to spray while the plant is hit by the sun. This one was and the burns are visible. According to data obtained in the field, when spraying a garden area in the shade, the ambient temperature repeatedly decreased, on several days, between 2 and 4 degrees Celsius, depending on the initial relative atmospheric hu-midity. These values are coincidental with the values obtained by manufacturers of air conditioners based on evaporation of water impregnating a filter through which air is forced to circulate.
Remember the three vital main plant functions:
- Photosynthesis
- Transpiration
- Respiration

Photosynthesis:
The main growth limiting factor is the reduced availability of substances produced by the plant because of a reduced photosynthesis activity due to high temperatures.

High temperatures may even partially damage the citoplasma coloid thus altering the structure of the chloroplasts (cells containing chlorophile). The damage may be unrecoverable on the exposed leaves and one will have to wait for the takeover of new leaves when temperatures are lower.

The most frequent causes for the reduction of the photosynthesis activity is the water or CO2 shortage. Water can and ought to be controlled by us. CO2 is not available in quantities optimal for the plant in the atmosphere. In fact, plants would do better with a little more. A lot more would damage them. The amount of CO2 in the garden air is not controlable by the hobby gardener.

Transpiration:
Transpiration is the expulsion of water vapor through the stomata (while these are open) with the dual purpose of refrigerating the plant and carrying, towards the leaves, the nutrients coming from the roots. These nutrients are involved in the later stages of photosynthesis, helping in the growth of the plants.
Consequently, for a better plant’s growth, the first to be checked is that the early stages of photosynthesis are optimized (with an adequate water management on our part) and that there is an intense transpiration for the purpose of lowering the temperature and carrying nutrients. The highest transpiration occurs when the stomata are open, so it depends on us to create water mists to open them fully.

Respiration:
It is operative 24 hours a day. Inside the plant several substances, mainly sugars, are burnt. This combustion needs O2 and releases CO2 and H2O. The intensity of respiration depends on temperature. The higher the temperature, the heavier the respiration of a plant. A plant is consuming day and night part of the substances created by the plant only during the day. So, it could be possible for a plant producing very limited amounts during the day, because the tem-perature is very high,  that consumption by respiration would be equal (or even higher) than the production.

Consumtion higher than production? Plant grows negatively (the plant consumes its own stored resources and finally dies).

The author deducts two possibilities in order to overcome the above problem:

Plants ought to consume less (save) - Nights ought to be much cooler than days so the plant will save during the nights, or Plants ought to produce more. Days need to be longer so the plant has longer production cycles and the total production exceeding the plant’s needs can be used for growth. The answer could be: artificial lighting.

Note:This fact is known by all involved in agricultural production. When the temperature difference between day and night is not large enough, the crops are reduced.

Possible Actions:
Same as deduced before:
- Water adequately
- Regular sprays (mists)
And something new added:
- Artificial lighting

Conclusion:
The best advice that can be given to a hobby gardener seeking plants for his garden could be the following two:
1. Forget about beautiful gardening books written in and for other countries and observe (your neighbours) to see what plants are most suited for your climate.
2. Water them adequately.
Besides the above, one may be willing to cultivate certain plants being on the limits (fringe) of their possibilities in the climate. Or it may happen that one may be willing to know what to do in the case of a heat wave.

Water management will be extremely im-portant, as previously seen, and possibly will be the only thing a gardener will be able to do. (He cannot install an air conditioner in the open air garden.)

"The author would also like to propose to experiment with a few plants subject to artificial lighting, something which he has never tried but intends to do in the future."

The author has never heard that artificial lighting could be used against the growth halt due to excessive heat. This is only a logical deduction from the thoughts contained on this page. Perhaps people with long experience cultivating in greenhouses would know…

 
A Word for Fuchsia Enthusiasts:
The fuchsia root system is prone to ac-quire a number of diseases caused by soil fungi. When the plant is subjected to stress during the high temperature season, growth is slowed down or practically halted. At the same time, the same high temperatures are favourable to the fungi, which are develop-ing very rapidly.

The gardener is compelled to water the plant generously, as this requires a lot of water. A wet soil also favours the development of fungi. Under these conditions the fungi “win” and the plant dies. Fuchsias require special care in areas where temperatures may go above 25 degrees Celsius:

- Keep plants in the shade.
- Beware of your garden fuchsias becom-ing infected with external agents.
- Use a potting substrate with little or no organic matter.
- Be mean with your frequent waterings and splendid with your mists.

Happy growing!