How to Determine a Plant's Water Needs (Part 2)

In the first article, we discussed how to determine a plant's water needs starting from an ideal environment. The amounts and frequencies of watering depend on the plant's growth stage (germination, young plant, adult), as well as its water needs category (low, moderate, semi-abundant, abundant). However, these recommendations applied under theoretical conditions: 19°C temperature, 60% humidity, and moderate wind at 5 km/h. In reality, numerous external factors influence the quantity and frequency of watering required for a plant. In this article, we will explore these influences to help you adjust your practices daily.

Rain is the main factor to consider when adjusting the amount of water provided if you are growing plants outdoors. A simple rule: 1 mm of rain equals 1 liter of water per square meter. Example: If your plant needs 10 L/m² and 1 mm of rain fell within 24 hours, you will only need to provide 9 L of water. A rain gauge or local weather forecasts can help you measure this rainfall. Using rainwater to water your garden is the best practice to save water consumption. Additional benefit: Plants prefer rainwater due to its lack of lime and slightly acidic pH.

Unsurprisingly, temperature is the first factor that comes to mind. Heat accelerates evaporation and increases the plant's water consumption:

• Between 24°C and 29°C: Subtract 1 day from the theoretical interval between waterings.
• Between 29°C and 35°C: Subtract 2 days.
• Above 35°C: Subtract 3 days.

Example: On January 1st, you need to water a basil plant with a theoretical watering frequency of 7 days, which sets the next watering date to January 8th. If the temperature is 26°C on January 3rd, the next watering date will be January 7th.

Mulching helps limit water evaporation and retain soil moisture. Without mulching, you need to water 25% more frequently.

Soil composition influences water retention, either positively or negatively. A "normal" soil is humus-rich, relatively dark, moderately draining, and moderately compact. Compared to this baseline, other soil types include:

• Sandy soil: Loose, very light soil that cannot be compacted. Water infiltrates and evaporates very quickly; you need to water 50% more often.
• Calcareous soil: Dusty to the touch. Water evaporates slightly faster; you need to water 20% more often.
• Clay soil: Heavy, highly compact soil that can be shaped into a ball when wet. It retains more water; you need to water 20% less often.

In reality, soils are never composed of just one type; they are mixes. By observing and touching your soil, you can estimate its category. Professionals can use agricultural labs to analyze soil composition accurately.

The plant's location affects how quickly water evaporates due to factors like wind, sun exposure, and soil depth:

• Greenhouse or outdoor: Follow the base watering frequency.
• Indoor: Plants grown indoors require watering 50% less frequently.

Example: A basil plant grown outdoors has a watering frequency of 7 days, whereas indoors, it would be 11 days.

Ambient humidity also affects watering frequency:

• Below 30%: Subtract 1 day from the theoretical interval.
• Above 80%: Add 1 day to the theoretical interval.

Considering these external factors is essential to adjust watering practices to meet your plants' real needs. A well-watered plant is a healthy plant capable of thriving.

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