How Mounding Up Soil Can Prevent Root Rot and Waterlogging in Some Hardy Australian Plants

Gardening in semi-arid regions like outback Australia presents its own set of challenges, especially when dealing with heavy clay soils and erratic rainfall. Even drought-tolerant plants, designed to survive harsh conditions, can suffer if their roots are subjected to poor drainage. The biggest culprit? Root rot and waterlogging.

This guide will walk you through the mounding method, where plants are grown in raised soil mounds, which helps prevent waterlogging and encourages healthier root development. We’ll also compare this approach to the traditional method of digging down, which is akin to building a dam around the plant—trapping water like a reservoir and causing harmful effects in heavy clay soils. We’ll highlight the unique challenges posed by clay-rich soils, the telltale signs of waterlogging in plants like Eremophilas, Acacias, and Saltbush, and explain how top-dressing mounds can keep your garden thriving long-term. Additionally, we will discuss the role of Poinciana trees and how you can expand your range of trees in this challenging environment.

Digging Down vs Mounding Up: Two Approaches to Planting

For years, gardeners in dry climates have dug down and built raised walls around their plants to trap precious water during dry spells. This method works well in sandy soils, but in clay-rich soils, it can lead to problems, much like building a dam across a river traps water.

Digging Down and Building Walls: The Dam Effect

When you dig down into clay soil and build walls around the plant, you’re essentially creating a mini dam. Just like a dam holds water back in a reservoir, the depression in the soil traps water around the plant. This causes:

1. Waterlogging: Clay soils have poor drainage, so water accumulates in the basin, turning the area around the plant’s roots into a swamp.

2. Root Rot: Prolonged exposure to moisture allows harmful fungi and bacteria to thrive, leading to root rot.

3. Compaction: Heavy clay soils compact easily under pressure, limiting the flow of air and nutrients to the roots.

4. Nutrient Deficiency: Waterlogged roots are unable to absorb essential nutrients, stunting the plant’s growth.

Building a dam around a plant might seem like a way to conserve water, but in reality, it often traps too much water, suffocating the plant’s roots just like a dam can flood the land it holds back.

The Mounding-Up Method: A Solution to Prevent Waterlogging

Instead of digging down and creating a water trap, the mounding-up method raises the root zone above the natural ground level. Think of it like diverting water away from the plant, rather than damming it up.

How Mounding Up Helps

1. Improved Drainage: By elevating the plant, water flows away from the roots rather than pooling around them. This prevents waterlogging, especially in heavy clay soils.

2. Oxygen for Roots: The looser, raised soil provides better airflow around the roots, allowing them to breathe and absorb oxygen more effectively.

3. Protection from Compaction: Since the roots are above the compacted clay, they have space to grow in the looser, aerated mound, free from the constraints of dense, waterlogged soil.

4. Access to Nutrients: Organic matter mixed into the mound improves soil structure and provides an easy-to-access source of nutrients for growing plants.

Water Flow: The Dam vs the Mound

The Dam: Digging Down and Building Walls

When you dig down and build a barrier around the plant, it’s just like constructing a dam. Water, unable to escape, pools in the depression. This may seem like a good way to conserve water, but in clay soils, it can drown your plant’s roots. Much like a dam traps excess water, the roots are submerged, depriving them of oxygen and leading to:

Waterlogging: Roots are starved of oxygen due to standing water.

Root Rot: Waterlogged soil becomes a breeding ground for harmful bacteria and fungi.

Compaction: Over time, the weight of water can compact clay soils, reducing their ability to drain or let roots grow freely.

The Mound: Elevating the Plant

In contrast, mounding up diverts water away from the plant’s root zone, like letting water flow around a hill instead of collecting behind a dam. Excess water runs off the sides of the mound, keeping the roots dry and well-aerated. This creates a healthier environment for plant growth, as water drains freely, and the soil remains loose and full of oxygen.

Signs of Waterlogging in Hardy Australian Plants

Even drought-tolerant plants like Acacias, Eremophilas, and Saltbush can fall victim to waterlogging, particularly in compacted, clay-heavy soils. Here are some signs to watch for:

Yellowing Leaves: When roots can’t absorb nutrients, leaves turn yellow, indicating the plant is starved of essential elements.

Wilting or Drooping: Despite wet soil, the plant may wilt as waterlogged roots struggle to function.

Stunted Growth: Plants suffering from waterlogging may not grow as quickly, or at all.

Root Rot: A surefire sign of overwatering, rotted roots are brown or black and mushy.

Dieback: Hardy shrubs may suffer dieback on the tips of branches as their roots fail to supply enough nutrients and moisture.

Leaf Drop: Some waterlogged plants shed leaves to reduce the demand on their roots.

How Mounding Protects Roots

When plants are grown in a mound, their roots experience several benefits compared to being trapped in a waterlogged depression:

Oxygen Supply: Mounded soil is looser and provides better airflow, meaning roots can breathe and grow stronger.

Drainage: Water drains away from the elevated root zone, keeping moisture levels balanced and reducing the risk of waterlogging.

Freedom from Compaction: The mound prevents the soil from becoming compacted, giving roots more space to grow and explore.

Nutrient Access: By mixing organic matter into the mound, plants have easier access to nutrients as they spread into the loose, enriched soil.

Top-Dressing Mounded Gardens: Maximising Health and Growth

Mounding up your plants is a great start, but to maintain the benefits, top-dressing—adding a layer of organic matter or mulch to the surface—can further enhance soil health, moisture retention, and overall plant growth.

Why Top-Dressing Matters

Retains Moisture: Mulch or compost helps slow evaporation from the mound, keeping moisture levels consistent for the plant’s roots.

Nutrient Boost: As organic matter breaks down, it provides a slow release of essential nutrients into the soil.

Weed Suppression: A thick top-dressing layer can smother weeds, preventing them from stealing nutrients from your plants.

Erosion Control: Mulch or compost protects the mound from erosion, especially during heavy rains, helping it retain its shape and structure.

How to Top-Dress a Mounded Garden

Select Organic Matter: Use well-rotted compost, manure, or mulch like straw or bark to build a 5–10 cm layer over the mound.

Apply Evenly: Spread the top-dressing evenly, making sure not to pile it too close to the plant’s stems, which can encourage rot.

Lightly Water: After applying, lightly water the area to help settle the top-dressing into place.

Maintain Regularly: Reapply top-dressing as needed throughout the growing season to keep the mound fertile and protected.

Trees That Tolerate Waterlogging and Drought

While most trees struggle with both waterlogging and drought, a few hardy species have evolved to tolerate these extremes, making them well-suited for the outback environment. These trees can survive conditions that might prove fatal for other species, especially when the soil management practices of digging down and building dams are used. Here are some examples:

1. Melaleuca quinquenervia (Paperbark Tree): This tree thrives in wet, swampy areas and can withstand temporary waterlogging. However, it also has adaptations that enable it to survive drought conditions, making it a versatile choice for varied landscapes.

2. Eucalyptus camaldulensis (River Red Gum): Known for its resilience, the River Red Gum can tolerate periods of waterlogging as well as extended dry spells. It often grows near rivers and swamps, where its roots can access moisture.

3. Casuarina glauca (Swamp Sheoak): This species can handle both saline conditions and periods of waterlogging, and is often found in coastal areas and riverbanks. It can also withstand drought, making it an excellent option for challenging environments.

5. Ficus rubiginosa (Port Jackson Fig): This hardy tree can tolerate waterlogged soils and is also drought-resistant once established. Its aerial roots can help it absorb moisture from the air, making it an impressive survivor in tough conditions.

6. Poinciana (Delonix regia): Known for its striking red flowers, the Poinciana can handle waterlogged conditions, particularly in its early years. While it prefers well-drained soil, it is also capable of surviving brief periods of flooding. Once established, it thrives in dry conditions, making it a stunning addition to any garden in semi-arid climates.

Complementary Plants for Improving Soil Structure

When planting in heavy, compacted clay soils, consider incorporating a variety of complementary plants. These plants can improve soil structure, enhance drainage, and support the growth of your main trees. Some great choices include:

Deep-Rooted Grasses: Grasses like Kikuyu or Buffalo grass penetrate compacted soils, breaking up hardpan layers and improving aeration.

Legumes: Plants such as Lucerne (Alfalfa) and Bursaria spinosa (Sweet Bursaria) not only fix nitrogen into the soil, improving fertility, but their roots also help break up heavy clay.

Ground Covers: Low-growing species like Creeping Boobialla (Myoporum parvifolium) can help stabilise soil and reduce erosion while their roots create channels for air and water to reach deeper layers.

Herbs: Planting herbs such as Thyme, Oregano, and Sage can improve soil structure while providing culinary benefits. These plants thrive in well-drained soils and can tolerate a variety of conditions.

Native Shrubs: Incorporating species like Grevillea and Hakea can help improve the overall soil health by attracting beneficial insects and improving biodiversity.

Consider plants like radish, marigolds and rosella for breaking up compacted clay soils too.

Adopting the mounding-up method not only helps prevent root rot and waterlogging but also creates a sustainable, thriving garden even in the harshest outback conditions. This approach mimics natural water flow, allowing plants to access moisture without becoming waterlogged.

By mounding up soil around your plants and adding top-dressing, you create a nurturing environment that allows a broader range of trees to flourish. With careful selection of complementary plants to improve soil structure and regular maintenance, you can cultivate a vibrant garden that stands strong against the extremes of the outback climate, celebrating the resilience and beauty of Australia’s native flora.