How to Determine a Plant's Water Needs?

Watering plants is a crucial step for successful cultivation and maintenance. Understanding their needs helps avoid overwatering or water stress. But many people ask daily: how much water does a specific plant need? How often should it be watered? The most common answer is often: "it depends." While true, this lacks precision. Let's try to answer these questions concretely in this article.

Definition

Water needs depend on two factors: quantity and watering frequency. A plant's water needs are not constant and evolve throughout its lifecycle. Regarding watering, we can distinguish three main stages:
  • Germination: a critical establishment phase where the plant needs consistently moist soil.
  • Young plant: a phase where water needs gradually increase until adulthood.
  • Adult: a phase where water needs stabilize until harvest or the plant's end of life.
These amounts and watering frequencies assume that the plant is growing in an ideal environment, meaning without stress. Generally, this corresponds to: a temperature of 19°C, 60% humidity, and a moderate wind of 5 km/h. Here, we'll consider the theoretical water needs of plants and later address how external factors (temperature, humidity, wind, soil type, mulching, etc.) influence them.

Water Needs During Germination

During germination, it is crucial to keep the soil slightly moist at all times without over-saturating it. Two methods can help achieve this:
With a sprayer: Ideal for small quantities of seeds. Surface humidification on one square meter uses about 0.1 liters of water. This requires very frequent watering: approximately every two days.
By capillarity: Ideal for reducing watering frequency, as water rises to the roots from a reservoir.

Water Needs of Young Plants

Between the end of germination and adulthood, water needs gradually increase. Watering should be done at the base of the plant, ideally in the morning. You can estimate a plant's needs by applying a percentage of its adult water requirements, increasing as the plant ages. For example, consider a tomato plant that reaches adulthood in 70 days: at 35 days, it will require 50% of the water needed by an adult tomato plant. Conversely, the watering frequency decreases as the plant matures.

Water Needs at Adulthood

It is at adulthood that water needs are precisely calculated.
First, plants can be categorized by their water requirements:
  • Low: 7.5 liters/m².
  • Moderate: 15 liters/m².
  • Semi-abundant: 20 liters/m².
  • Abundant: 25 liters/m².
The watering frequency varies depending on the plant's ability to tolerate dry soil, ranging from every 5 days to up to 30 days for some plants like cacti.
To calculate the individual needs of a plant (not per square meter), divide the water quantity by the number of plants that can occupy one square meter.
Example: A tomato plant falls into the semi-abundant water category, needing 20 L/m² every 7 days. Each plant occupies 0.5 square meters, given its spacing of 50 cm x 100 cm. Thus, there are 1 / (0.5 * 1) = 2 plants/m². The water needed for one tomato plant is therefore: 20 / 2 = 10 liters.

Conclusion

In summary, understanding a plant's water needs involves adjusting the amount and frequency of watering according to its stage of development, while taking into account both theoretical (ideal conditions) and environmental factors.

=> In the next article, we'll discuss how external parameters influence watering.
=> You can find the theoretical needs of over 200 plant species in our wiki.

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