For many garden plants, the minimum, optimal and maximum temperatures are, with wide deviations, around:

• Minimum: 8 ºC
• Optimal: 16 ºC
• Maximum: 28 ºC

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 subject to (example: too little or too much water).

The hobby gardener would be interested, in general, to know which plant varieties will be 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 subject and what results should he expect.

We saw in another article (water and photosynthesis) that plant growth  depends on the optimization of the following parameters, these being interrelated and, besides, having non linear expressions:

• Plant size 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 microclimate 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 shades.
• Too high temperatures damage the internal substances of the plant, these being necessary for the photosynthesis. This and because having surpassed the optimal temperature 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 to avoid the sun hitting them. 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 would cause a shock and help the action of the fungus Fusarium, among others.
• Availability of CO2. Here, we cannot act (as hobby gardeners)

SUMMARY of POSSIBLE ACTIONS:

1. To water adequately
2. Regular sprays (mists)
3. 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.

Vital plant functions

Remember (water and photosynthesis) the three main 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 chlorophyll). 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 can be: water or CO2 shortage. Water can and ought to be controlled by us. CO2 is not available in quantities optimal for the plant on the atmosphere. In fact, plants would do better with a little more. A lot more would damage them. The amount of CO2 on the garden air is not controlable by the hobby gardener but the air inside a greenhouse must be ventilated (entry of some more CO2) specially after the inner air has been excessively purified by the plants.

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 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 temperature is very high,  that consumption by respiration would be equal (or even higher) than the production.

Consumption 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 (1) 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

(1) 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 deducted 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:

• Forget about beautiful gardening books written in and for other countries and observe (your neighbours) what are the plants most suited for your climate.
• 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 important, 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)

As an additional option that could be experimented with a few plants (not suggested for the whole garden), the author would like to propose artificial light during the night. He has not tried it yet but he intends to do it 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...