Heatwaves and Bonsai: my Mykorrhizae is dying. What to do to ensure Recovery

An how to guide: 

Having spent over three decades working with bonsai trees across diverse climates—from the scorching summers of southern Europe to the cooler regions of the north—I’ve witnessed firsthand the challenges bonsai trees face during heatwaves. My experience includes eight years in a large nursery focusing on pre-bonsai material production, dealing with container-grown, field-grown, and collected yamadori, and running my own nursery since the late 1980s. One of the most critical aspects of bonsai health, particularly in extreme heat, is the maintenance of a healthy mycorrhizal network.

Understanding Mycorrhizae: Endomycorrhizae vs. Ectomycorrhizae

Mycorrhizae are fungi that form a symbiotic relationship with plant roots, extending the root system’s reach and enhancing water and nutrient uptake. There are two primary types of mycorrhizae relevant to bonsai care:

• Endomycorrhizae (Arbuscular Mycorrhizae): These fungi penetrate the root cells and are most common in deciduous trees and many conifers. They are crucial for phosphorus absorption and thrive within the root structure, forming a network that increases the tree’s ability to withstand environmental stresses.
• Ectomycorrhizae: These fungi form a sheath around the root and are typically found in pine, oak, and beech species. They do not penetrate the root cells but instead create a dense network around the roots, which helps in nutrient and water absorption, particularly in nutrient-poor soils.

Both types of mycorrhizae are essential for the health of bonsai trees, but they respond differently to environmental stresses like heatwaves.

Heatwaves and Small Bonsai Containers: A Perfect Storm

Small bonsai containers, particularly those made of ceramic, can exacerbate the effects of heatwaves. Here’s why:

• Heat Absorption: Ceramic pots absorb and retain heat more than plastic or organic containers, raising the soil temperature within the pot. During a heatwave, the soil in small pots can easily exceed 40°C (104°F), a critical threshold that severely impacts mycorrhizal activity.
• Surrounding Irradiation: The immediate environment around the bonsai also contributes to the heat stress. Surfaces like concrete or gravel can reflect and amplify the heat, further increasing the temperature around the bonsai.
• Reduced Soil Volume: Small containers have less soil, meaning they dry out faster and have less buffering capacity against temperature fluctuations. This leads to rapid soil heating, which can quickly stress the mycorrhizal network and the tree itself.

Mitigating Heat Stress:

• Shade and Microclimate Management: To protect your bonsai from excessive heat, move the tree to a shaded area during the hottest parts of the day. Placing the bonsai on wet gravel, without allowing the pot to sink, can help create a cooler microclimate through evaporation, lowering the surrounding temperature and reducing the stress on the mycorrhizae.
• Watering with Care: Use rainwater or distilled water to avoid adding salts that could stress the already vulnerable mycorrhizae. Regular, moderate watering will help maintain soil moisture without oversaturating the roots, which is crucial for maintaining mycorrhizal health.

Recognizing Heatwave-Induced Mycorrhizae Stress

Recognizing the signs of mycorrhizae stress during a heatwave is critical:

• Sudden Decline in Water Absorption: A healthy mycorrhizal network increases water absorption, so a sudden drop in water uptake is a strong indicator that the fungi are under stress.
• Wilting Despite Adequate Watering: If the tree continues to wilt despite regular watering, it suggests that the mycorrhizae are no longer effectively assisting in water absorption.
• Yellowing or Browning Leaves: Nutrient uptake is compromised when mycorrhizae are stressed. This often manifests as yellowing or browning at the edges of the leaves.

Advanced Techniques for Rebuilding Mycorrhizae Populations

When you notice signs of mycorrhizal stress, prompt action is required. Here’s an advanced approach to rebuilding mycorrhizal populations using young plants of the same species:

1. Prepare the Substrate:
• Remove the Tree from Its Pot: Gently unpot your bonsai to inspect the roots. Remove any dead or damaged roots to prevent further stress.
• Amend the Soil: Incorporate fresh bonsai soil that promotes mycorrhizal growth. Adding organic matter, such as compost or decomposed bark, helps provide a food source for the fungi.
2. Inoculate Mycorrhizae from Same-Species Seedlings:
• Select Seedlings of the Same Species: Since mycorrhizal fungi are often species-specific, it’s essential to use seedlings from the same species as your bonsai.
• Harvest and Repopulate: Carefully uproot the seedlings, ensuring that the fine, white mycorrhizal filaments remain intact around the roots. Chop the seedlings at the nebari level (just above the root flare) and mix these roots directly into the substrate around your bonsai. This method boosts mycorrhizal activity by introducing a dense population of fungi directly where it’s needed most.
3. Enhance Mycorrhizal Activity with Sugary Water:
• Why Sugary Water? The addition of a weak sugar solution (1-2% sugar in water) can stimulate mycorrhizal growth by providing an immediate energy source for the fungi. This can be particularly effective in re-establishing the mycorrhizal network quickly.
• Application: After inoculating the soil with mycorrhizae, water the bonsai with the sugary solution. This encourages rapid fungal colonization and enhances their symbiotic relationship with the tree roots.
4. Replant and Monitor:
• Replant the Bonsai: Place the bonsai in a temporary, wider pot with the amended substrate. around the original rootball. Ensure the roots are in close contact with the mycorrhizagently get rid of them with a clean cut, without touching healthy root areas. 
• Monitor for Improvement: Over the following weeks, observe the bonsai for signs of recovery—improved water uptake, healthier leaf color, and new growth are positive indicators that the mycorrhizal network is re-establishing itself.
5. Ongoing Care:
• Regular Inoculation: Consider using commercial mycorrhizal inoculants during repotting or soil refreshing to maintain a robust mycorrhizal population.
• Manage Microclimate: Continue to manage the microclimate around the bonsai by shading and using wet gravel to lower the surrounding temperature, particularly during heatwaves.

 

 

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