The group of small ornamental shade trees lumped under the name Japanese maples, Acer palmatum and A. japonicum, and their many hybrids, are very popular with gardeners and plant enthusiasts. Most of the questions we receive about problems with Japanese maples are horticulturally related to poor growing conditions and maintenance rather than insects or diseases. The causes of these problems are usually root or trunk-related issues. So, let’s start with a look at the planting conditions Japanese maples need in order to thrive.
Planting and Care
Japanese maples generally grow slowly and mature to heights of 30-40 feet or less, especially for the dissectum types (18 feet) and the dwarfs (3-6 feet), over a 50 year period. The green forms can tolerate full sun, but most will prefer some afternoon shade, and this is especially true of all the colored and variegated forms. During hot summers, sunburn and bronzing of the foliage can occur even if adequately watered, and leaf scorch will definitely occur under drought conditions. So, think of the growing conditions that are best for rhododendrons and azaleas, with slightly acidic well-drained soils, and you will have the right conditions for Japanese maples.
These trees have shallow, very fine root systems that need a cool, evenly moist place to grow. Severe drought will kill roots from heat and desiccation. After establishment, the aerial parts of Japanese maples can survive freezes down to zero, however the roots can only survive to 14 F. So roots need protection in severe winters, especially if planted in above ground containers. A light covering of mulch will help to protect roots and conserve water.
Container grown plants need special care since their roots are even more vulnerable to drying and freeze damage. Containers may need added insulation, or relocation to more protected areas, during severe winter conditions. In-ground plants should be sited with these issues in mind.
Bare root and ball-and-burlap plants should only be planted during the dormant season. Container plants can be planted throughout the season provided their root balls are carefully spread and special attention is given to watering. Japanese maples are not heavy feeders so most don’t need fertilization unless they are grown in containers.
Japanese maples will benefit from thoughtful pruning and shaping during their lifespan. Major pruning should be done during the dormant period except from January to May when the sap is rising and bleeding may occur. Light pruning and removal of dead wood can be done anytime.
Pests, Diseases, and Problems
The most common question we receive is “why are my leaves turning brown along the edges?” This problem is referred to as leaf scorch. This symptom can be especially severe on dissected leaved cultivars since their leaves are very delicate and thin. The solution is planting where the tree will receive some afternoon shade to prevent heat damage and to provide a uniform water supply.
At noted above, roots can be damaged by heat and drought. Severe freezes also will kill shallow roots and the inner bark of the trunk. In such cases, the symptoms will progress from leaf scorch to leaf drop, and to branch, and trunk dieback. In some cases a good portion or maybe half of the tree may die along one side especially if prevailing winds come from that direction. Trees located near structures that retain or reflect heat, such as white walls, patios, and sidewalks can also damage foliage from overheating and droughty soils.
Many gardeners will jump to the diagnosis of a fungal root disease such as Verticillium wilt when they observe dieback, however this has not been proven to very common in Maryland soils even when submitted to the University of Maryland Plant Diagnostic Lab for confirmation.
A frequent question we receive is about spotting or damage to the leaves. This often is caused by rough handling of young plants, freeze damage to young leaf buds as well as physical bruising caused by windy conditions. In very wet spring weather, fungal diseases can infect the leaves. These include leaf spots, usually caused by Phyllosticta. Anthracnose diseases, which appear as irregular blotches anywhere on the leaves, are caused by three fungi, Colletotrichum, Discula, Aureobasidium. The solution again is proper siting of trees so that they receive moderate weather exposure and have good air circulation to reduce the likelihood of fungal infection.
Another common problem we hear of is dieback of the fine twiggy growth on the inside of the foliage canopy, especially on laceleaf cultivars. This is because of the dense leaf growth of the outside canopy that shades the inside branch growth. This dead twiggy growth should be removed to thin the canopy and allow for better air circulation and also to expose more of the interesting branch structure of the tree itself.
In total there are many Japanese maple cultivars for the garden landscape with numerous horticultural attributes that keep this tree in high demand and versatile in many landscape design situations. With a little care in planting and maintenance, Japanese maples can be beautiful additions to our gardens. One cautionary note: if you live near a natural area, some cultivars of Japanese maples have been observed to escape cultivation and seed into forested areas. Keep this in mind as you make new plant selections.
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