The Myth of "Rain is Enough": Why Your Newly Planted Trees and Shrubs

As a gardener, you've likely heard the phrase "rain is enough" when it comes to watering your plants. While rain can certainly provide some hydration, it's often not enough to sustain your newly planted trees and shrubs, especially in hot climates and during summer. In this article, we'll explore why rain alone may not be sufficient and provide tips on how to ensure your plants receive the water they need to thrive.

Understanding Water Absorption and Evaporation

When it rains, the water doesn't immediately soak into the soil. Instead, some of it may run off or evaporate quickly, especially on hot surfaces like soils covered with rock, non-mulched soil, or clay soils. If the soil has become hydrophobic, it can also affect water absorption. Even if the water does penetrate the soil, if the rain is light or less than an inch, it may not reach the roots of your plants.

The way plants interact with rainwater can also impact their water intake. For established plants, the canopy can act as an umbrella, directing rainwater to fall on the drip line (the area under the plant's outermost branches) rather than the roots. However, for newly planted trees, this can be a disadvantage, as the roots may not be extensive enough to benefit from the water that falls outside the planting area. Water absorption also depends on several factors, including soil type, temperature, and the plant's root system.

Water Absorption in Different Soils

Different soils have varying levels of water absorption, which can impact how much water your plants receive. For example:

- Sandy soils tend to drain quickly, allowing water to penetrate deep into the soil but potentially leaving the roots of plants parched.

- Clay soils, on the other hand, can retain water for longer periods, but may become waterlogged if too much water is applied.

- Loamy soils, which are a mix of sand, silt, and clay, tend to have good water-holding capacity and aeration, making them ideal for many plants.

Understanding the water absorption characteristics of your soil can help you tailor your watering schedule to meet the needs of your plants.

Evaporation: The Silent Water Thief

Evaporation is another factor that can reduce the effectiveness of rainwater. When the sun heats up the soil and plants, water evaporates quickly, leaving less moisture for the roots to absorb. This is especially true in hot climates and during summer, when evaporation rates are higher.

For instance, in Phoenix, Arizona, the average annual evaporation rate is around 100 inches (254 cm) per year, with rates exceeding 0.4 inches (1 cm) per day during peak summer months. If we receive 1 inch of rain, a significant portion of it may evaporate, leaving only a fraction for the plants to absorb. Let's say 0.2-0.3 inches (0.5-0.75 cm) of water evaporates, leaving only 0.7-0.8 inches (1.8-2 cm) of water available to the plants. However, if the soil is sloped or compacted, runoff may occur, further reducing the amount of water available to the plants. This also does not take into consideration of the soil has been amended, of other pants are having competition for resources, if there's materials like rocks, concrete or brick near by, working as a heat mass.

Microclimates and Watering

Microclimates can also play a significant role in determining the watering needs of your plants.

A partial shaded garden, for example, will have less evaporation than a fully exposed garden with full day sun. On the other hand, a full shade garden may receive less water with rain, but also experience less evaporation. Understanding the microclimate of your garden can help you adjust your watering schedule to meet the specific needs of your plants.

Different Plants, Different Watering Needs

Plants have diverse watering requirements, and it's crucial to comprehend these needs to ensure optimal care for your plants. For instance:

- Ferns and hydrangeas prefer moist soil and may require more frequent watering.

- Succulents and cacti, on the other hand, are prone to root rot if over watered and may require less frequent watering.

- Yucca plants prefer dry soil and can tolerate neglect, while plants like impatiens and coleus prefer consistently moist soil.

Measuring Water: The Inch Rule

So, how much water do your plants need? A good rule of thumb is to provide about 1 inch of water per week, either from rain or irrigation. However, this amount may vary depending on the plant's root ball and the climate. Newly planted trees and shrubs, for example, may require more frequent watering to establish a strong root system. I often get asked "so how much?", and even though I can tell you what the plant generally requires, I don't know your garden, type of soil, microclimates, terrain, or watering habits. Only you, as the owner of each garden, can take the time to understand and learn how each part of your garden works and how fast water is being absorbed and, per se, how often plants will need water.

Tips for Watering Newly Planted Trees and Shrubs

When planting new trees and shrubs, it's crucial to water them regularly to help them establish a strong root system. Here's a general guideline:

- First and second week: Water every day, especially in summer, when some plants may need daily watering depending on their type and size. For example, smaller plants or those in hot, dry climates may require more frequent watering.

- Third and fourth week: Water every other day

- After that: Water every 3 days until established

Seasonal adjustments are also important to consider:

- Spring: Water consistently, but be cautious not to overwater, since spring rainfall can vary unpredictably.

- Summer: Increase watering frequency, particularly during heatwaves or drought conditions. Newly planted trees and shrubs might require watering every 2-3 days, or even daily for certain plants, based on their type, size, and the climate.

- Fall: Maintain consistent watering, particularly during dry fall periods. Newly planted trees and shrubs may require watering until the ground becomes frozen.

- Winter: In regions experiencing cold winters, watering might be unnecessary when the ground is frozen. However, if the winter is dry or the plants are situated in a warm microclimate, watering may still be required.

Special Considerations for Potted Plants

When it comes to potted plants, it's always best to check the soil moisture by touching the soil prior to watering. This is because potted plants can dry out quickly, and overwatering can be detrimental. Stick your finger into the soil up to the first knuckle, and if the soil feels dry, it's time to water. If it's still moist, wait another day or two before checking again. This simple trick can help prevent overwatering and ensure your potted plants receive the right amount of moisture.

Remember: These are general guidelines, and the specific watering needs of your trees, shrubs, and potted plants will depend on factors like soil type, climate, and plant species. Be sure to monitor the soil moisture and adjust your watering schedule accordingly.

Checking Soil Moisture

To ensure your plants receive enough water, check the soil moisture by inserting your finger into the soil up to 3 inches deep. If the soil feels dry, it's time to water. Repeat this process in multiple areas around the plant, as soil moisture can vary.

Timing is Everything: Watering in the Morning

When watering your plants, it's best to do so early in the morning. This allows the plants to absorb the water throughout the day, reducing evaporation and runoff. Watering in the morning also helps prevent fungal diseases that thrive in moist conditions.

Conclusion

Although rain can offer some moisture, it usually isn't sufficient to support your newly planted trees and shrubs, particularly in hot climates and during the summer. By comprehending water absorption, soil temperature, and evaporation, you can ensure your plants get the necessary water to prosper. Measure water by the inch, adjust for the plant's root ball, take microclimates into account, and water early in the morning. By following these guidelines, you'll be on the path to cultivating healthy, thriving plants in your garden.

Key Points

Rain alone may not be enough to sustain your plants, especially in hot climates and during the summer.

Water absorption depends on soil type, temperature, and the plant's root system.

- Water absorption and effectiveness can be diminished by soil temperature and evaporation

- Measure water per inch, and adjust for the plant's root ball and climate.

- Regularly water newly planted trees and shrubs to help them develop a robust root system.

- Regularly monitor soil moisture and water in the early morning to reduce evaporation and runoff.

By Adriana Marsh