Chapter 47 Identifying and Treating Common Cultivation Problems

47.1 Soil Does Not Wet Properly

When soil refuses to take up water it usually means:

• Roots are packed too closely into the container,
• The soil has been compacted,
• Soil particles have broken down too finely, or
• Soil components have stopped absorbing water properly.

When trees become root-bound water does not penetrate and the soil becomes more difficult to keep moistened. Left long enough, near-permanent dry spots can develop. This reduces the effective volume of soil in the container (increasing water stress and making nutrients less available. Most trees slow down or stop growing and can show symptoms of nutrient deficiency like chlorosis.

This is a good example of why it is important to look beyond the obvious symptom. Feeding with chelated iron would be the appropriate treatment for chlorosis in a landscape shrub, but sometimes the problem is that there is not sufficient WETTABLE soil in the pot to support normal cation exchange. Feeding with iron is not going to solve the problem in this case; the dry spots in the soil need to be corrected too.

Wych elm being repotted. The soil is filled with roots and white growing tips are visible, but otherwise is in excellent condition. The soil is fully wetting and does not have any large empty voids. Photo by Jerry Norbury; link to original image.

A cherry being repotted. Larger void spaces have started forming just below the surface as smaller soil particles separate and sink in the root mass. Photo by Jerry Norbury; link to original image

A desert rose being repotted. This specimen was thoroughly watered just before being slid out of its pot, but the root mass still is dry. The roots have filled the available spaces to the point that water does not penetrate to the bottom of the soil mass. Link to original image

47.1.0.1 Symptoms

The way to diagnose this problem is to push an unsharpened pencil into the soil several times. If water now enters the soil more easily, the container is packed too tightly, and it is time to repot.

Soil compaction is less likely with bonsai because we repot frequently. Still it can happen if soil mix is packed into a pot too tightly while repotting. Soil mix should fill the spaces between roots completely, but be loose enough to push a finger into easily. I’ve found the easiest way to avoid compaction when repotting is to pour soil around the roots, pack in the obvious open spaces, then hold the based of the tree still while I bump the pot 2-3 times on the table or ground to settle the soil. I then water thoroughly TWICE, about 30 minutes apart. The water settles the soil around the roots without having to push it in by hand. Wait to fill in the remaining surface space until after the soil has been settled.

Soil particle breakdown can make a tree drop lower in a pot, and also can make soil wet poorly and prevent water penetration. Ironically breakdown sometimes makes the soil retain too much water. Particle breakdown occurs as organic matter decomposes. It also occurs when using akadama clay or similar soft inorganic materials. Vermiculite can become over-compacted as well, but is not used very often in bonsai soil mixes. To diagnose this, scoop a teaspon of soil from the container, preferably some soil from below the surface. Rub it between your fingers and a hard flat surface. If the particles break up into gritty mud or clay, the inorganic particle structure is breaking down. If the inorganic part is breaking down, there will be no large or harder pieces left, only fine soft or spongy bits of wood.

Hydrophobic dry-down is both harder to diagnose and to treat. Soil mix with >50% peat moss, coir, or very finely shredded bark needs to stay above a minimum moisture content. If soil moisture falls below a critical point the structure of the particles and fibers changes. They form hydrophobic masses that are extremely difficult to rewet. Older soil containing these materials is more likely to become un-wettable, but it also happens in fresh soil if trees experience drier than normal conditions for several days.

Soil exhibiting hydrophobic dry-down will not absorb water. Instead it beads on the soil surface. Link to original image

Dry-down usually starts in small pockets, so can be hard to diagnose without an electronic soil moisture meter. I usually discover it by accident when repotting neglected nursery stock. I lift out what I think is a well-watered tree and the soil falls away in a dusty cloud. I’ve seen this enough times that I have sworn off using peat moss in my soil mixes almost entirely. Coir is less susceptible and offers some other benefits that outweigh the risks.

A nursery grower showed me a trick for diagnosing dry-down that has worked well the few times I remembered to use it. Put a suspected tree in a bucket, and start adding water OUTSIDE of the pot. Run the water up to 1/2 inch from the lip of the pot, and wait 2-3 minutes. A pot with soil that still absorbs water properly will take in water through the drain holes and feel as wet as when you water the tree normally. If the soil is not wetting properly, it will float up out of the pot and spill into the bucket.

The recommended treatment for hydrophobic dry-down is to submerge a pot in water up to the lip then let it soak until the soil rehydrates. This takes longer than you might think. A 2- to 5-gallon pot might need to soak for 24 hours, fully submerged. Do not soak for longer; roots need air too.

Even after a long soak, I still find residual dry spots. Watering with a wetting agent after a 24-hour soak further reduces the hydrophobic barrier of the soil particles. Look in the garden chemicals section of a garden supply store for a wetting agent or surfactant that is safe for direct spraying on plants. Mix with water as directed and water 2-3 times at weekly intervals. Watch the tree closely for any signs that the surfactant is stressing the tree.

Some people claim dishwashing liquid works well, but it was not formulated for direct root contact, which is why I do not recommend it.

47.2 Soil Remains Too Wet

Roots need air as well as water to thrive; healthy soil has up to 50% open air spaces. Continuously saturated soil lacks air spaces so that roots suffocate. Even if the soil is not saturated to the point of drowning the roots, excess moisture creates conditions that favor root rot.

47.2.0.1 Symptoms

Soil should never smell rotten. Foul smelling soil suggests root rot due to the soil not draining properly. Pillbugs, fungus gnats, and ants do not damage tree roots directly, but their presence suggests something may be decomposing.

When the soil surface stays too wet, the bark covering the surface roots can soften and start breaking down. Nebari roots exposed on the soil surface need to dry out a bit between waterings. Consider pulling some of the surface soil away from surface roots and replacing it with fine gravel that retains less water. Fine aquarium gravel works well, as does expanded shale.

47.3 Salt Crusts

Salt crust on irrigated soil. Link to original image

47.3.0.1 Symptoms

White or pale gritty deposits on the surface and edges of particles of soil at the surface of the pot. Crystals may be needle-like or flat. They are caused by fertilizer or minerals from hard water building up. The deposits are more likely to form when mineral-laden water stays in prolonged contact with the soil due to poor drainage. Visible mineral deposits have a salt level high enough to burn or kill roots, and should be flushed out of the soil.

47.3.0.2 Prevention and Treatment

Correct the underlying drainage problem, then flush out of the soil with low-salt water. If regular water supply is not suitable, collect rainwater or use store-bought distilled water.

If the root mass is not too tightly bound, shake loose soil free from the root mass then replace with fresh soil.

47.4 Fertilizer Burns

Example of fertilizer damage. Link to original image

47.4.0.1 Symptoms

Fertilizer burn is leaf scorching caused by over-fertilization, usually with too much nitrogen. It happens because the excess fertilizer causes ion imbalance, rupturing cells. The dead areas dry out and turn brown. High concentration fertilizers (10-10-10 or higher) are more likely to cause it.

Burned leaves will not recover.

Fertilizer burns are hard to distinguish from leaf scorch due to dry hot weather or other causes. Burning also looks very similar to potassium deficiency, as you can see below.

Potassium deficiency. Link to original image

47.4.0.2 Prevention and Treatment

Soak pots to flush out excess salts. Switch to using lower concentration fertilizers, diluting them further, or using slow-release organic fertilizers.

47.5 Chlorosis

Interveinal chlorosis of a sweetgum leaf. Link to original image

Chlorosis is less a condition, and more of a symptom for a range of conditions. Plants require larger quantities of nitrogen, phosphorus, and potassium (the NPK trio listed by numbers like 10–10–10 on fertilizer labels). Micronutrients are required in smaller amounts, but still are essential.

Fertilization is essential for healthy tree growth because it provides these nutrients consistently. Even with regular fertilization though, trees occasionally develop macro- and micronutrient deficiencies. Chlorosis often is part of the suite of symptoms of deficiencies, especially iron, magnesium, or zinc deficiency.

Chlorosis also can appear when:

• Plants are infected with a virus or other pathogen
• Soil pH is too far from optimal for the species
• Soil is waterlogged or compacted
• Trees are exposed to ozone, sulfur dioxide, or herbicide

47.5.0.1 Prevention and Treatment

Chlorosis is treated by solving the underlying problem. Consult the Micronutrients and Micronutrient Deficiencies page for details on how to recognize and treat micronutrient deficiencies.

47.6 Leaf Scorch

1. 2.

Examples of leaf scorch due to viral infection (1) or nutrient imbalance (2). Link to original image 1; image 2

47.6.0.1 Symptoms

Leaf scorch can be caused by sudden weather changes (particularly for unhardened trees) soil compaction, dry wind, transplant shock, nutrient deficiency, drought, salt toxicity, herbicide injury, or a variety of pathogens.

Affected plants may sometimes recover through watering (and fertilization if that was not the original cause). Light pruning may also help to reduce the water demand.

Scorch symptoms may differ between plant species, but it typically appears in July and August as a yellowing between leaf veins and along leaf margins, and a browning on the tips of leaves. Since these leaf parts are the last to be supplied with water from the roots, they are usually the first to be affected. Browning of dead tissue often appears without any previous yellowing, extending into the leaf between the veins. Entire leaves may curl and wither when leaf scorch is severe. Scorched leaves are usually abundant on the side of the plant most exposed to prevailing winds and strong sunlight. Leaves on the same branch often show similar symptoms but an entire plant may not be uniformly affected.

47.6.0.2 Prevention and Treatment

Scorch damage alone is insufficient to kill an otherwise healthy tree. Treatment focuses on removing damaged material and correcting the causes. Prune weak shoots and heavily damaged leaves. Avoiding frequent, light watering; water deeply and less frequently so trees build deeper roots. In high summer, provide mid-day shade for most or all trees, especially Japanese and other maples, elms, beech, ash, oak, birch, azaleas, viburbnum, hornbeams, tropicals, and most other deciduous trees.

47.7 Sun Scald and Frost Cracks

Sun scald (brown area) on the trunk of an ash tree. Link to original image

47.7.0.1 Symptoms

Long cracks or broken ares in the bark, usually on the trunk of a tree. It is most commonly seen on the southwest side of trees.

Sun scald occurs in winter and spring when bark freezes and thaws quickly in the sun. Warmth activates bark to grow, then the bark freezes again at night. Fluctuating winter temperatures can also cause frost cracks, which result from the expanding and contracting of the tree trunk.

47.7.0.2 Prevention and Treatment

Over-winter dormant trees in a shaded area to reduce the chances of a sudden temperature change. Keep an eye out for possible infections entering the fresh wound.