Many trees planted in containers perform poorly. Some decline rapidly after planting, while others become established, grow and then decline. Symptoms include marginal necrosis of leaves, undersized leaves, leaf drop, slow growth, branch dieback and, in some cases, whole tree death.
What’s causing the problem? Although a number of agents can cause some or all of these symptoms (including salts, specific ion toxicity and root disease), the most common cause is water deficit. Trees in containers are highly prone to water-deficit injury because transpirational water loss can quickly deplete all available water in the container substrate. Without frequent replenishment of available water, injury symptoms appear and tree performance declines.
Although not all trees growing in containers are injured by water deficits, it is a common problem. It can be found in municipal, commercial and institutional settings, and virtually anywhere watering is minimized due to maintenance budget constraints. It is particularly a problem in locations that receive little or no summer rainfall, where trees must depend solely on irrigation to meet their needs.
Why do water deficits occur in trees planted in containers? Basically, it’s a matter of water supply and demand. Water is stored in the container substrate (supply) and lost from leaves (demand). When demand exceeds supply, water deficits occur.
Minimizing the potential for injury requires an understanding of factors that affect both supply and demand. Also, some means of measuring or otherwise assessing water status is needed to develop appropriate irrigation schedules. With the proper information, you can develop strategies to address this challenge.
Factors affecting water demand and supply
The potential for water deficit in container trees is linked to supply and demand. In short, where water supply is high and demand is low, a relatively low potential for water deficit occurs. Conversely, when supply is low and demand is high, water deficit potential is high.
Demand: Which factors promote water loss in trees? It’s a function of both plant and environmental factors. The amount of water lost via transpiration is closely related to the amount of leaf area. For plants of the same species in the same location, a plant with a greater leaf area will transpire a greater amount of water than a plant with a lesser leaf area. Trees with larger crowns will have a higher water demand than those with smaller crowns. Transpirational water loss rates can vary with species, however, so water loss from a species with a somewhat larger crown may be equivalent to that of another species with a smaller crown. Nonetheless, larger crowns generally mean higher water loss rates, or higher demand.
Transpirational water loss is driven by the evaporation of water from leaves. Factors that increase the potential for evaporation also increase the potential for water loss (rising air temperature, higher light intensity, greater wind speed, declining humidity). For example, trees in sunny locations have a higher potential for water loss than those in shaded locations, and trees in windy locations have a higher potential than those in protected locations. As the evaporative potential of a location increases, then transpirational water demand increases. Container trees on decks or rooftops commonly show water-deficit injury because the evaporative potential can be extremely high.
Supply: Supply is limited by the water-holding capacity of the substrate and the volume of the container. For many substrates, water-holding capacity is relatively low (compared with field soils). For example, in substrates based on the University of California mix (peat, sand and redwood sawdust in equal parts by volume) water content is 76 percent of total volume at saturation.
At container capacity (that is, the moisture content after excess water is allowed to drain following an irrigation), water content is about 60 percent. Because the unavailable water is approximately 24 percent, about 36 percent (60 – 24 = 36) of the total container volume is available water. In other words, the water available to the plant occupies a little more than one-third of the volume of the container. Certainly, substrates with other constituents can have lesser or greater amounts of available water.
Aside from water-holding characteristics, the substrate volume is a major limitation to tree development. Compared with field soils, containers generally have much less volume available for root development and, therefore, a substantially lesser amount of available water. In field soils with reasonably good physical and chemical properties, volume may be limited only by the genetic potential of the species. In containers, volume is always limited. Consequently, the potential for tree development is limited as well.
The combination of container volume and water-holding capacity determines water supply. Of the two factors, container volume has a greater influence on water supply. In cases where the substrate volume is small (small containers) and demand is large (sizable leaf area), the potential for water-deficit injury is relatively high.
Estimating water supply in a container
The amount of available water in a container can be calculated from the container volume and the water-retention characteristics of the substrate. Consider a container and substrate with the following specifications:
At container capacity, this combination of container volume and substrate will have approximately 73 gallons of water available to meet transpirational demand. If a tree planted in this container loses 25 gallons per day due to its crown size (leaf area) and the prevailing environmental conditions (temperature, wind, and humidity), then it will have a three-day water supply (approximately). If no water is supplied by rainfall, it will have to be irrigated at least every three days.
Consider the growth factor
As trees grow, leaf area increases, and an increase in transpirational water loss follows. Because water supply does not change, the greater demand (larger leaf area) causes an increase in water-deficit potential. To avoid injury, a commensurate increase in water supply is required, which means more frequent irrigation
Essentially, as the tree grows, irrigation frequency needs to increase. A tree that was watered adequately once a week when first planted may require irrigation three times a week after a year or two. Eventually, for large trees in small containers in environments with a high evaporative potential, irrigation may be needed every day. This creates a fragile situation where missing a single irrigation on a warm day could result in significant injury.
The management challenge
Although some trees in containers have a relatively low potential for water-deficit injury, it has been our observation that most have a moderate to high injury potential.
Many of the factors that increase injury potential have been discussed here: relatively large trees being planted in a small containers, sites with a high evaporative potential (hot and windy), container substrates with low water-holding capacity, infrequent irrigation and water-deficit-sensitive species being planted. It is often a combination of these factors that results in injury.
By knowing the key factors that affect water supply and demand, and by making appropriate adjustments in management practices, the potential for injury can be minimized.
Here are some practical considerations to keep in mind.
- Understand that trees in containers have a relatively high maintenance requirement (labor-intensive) and should not be used where budgets cannot support the level of required maintenance.
- If you have a choice between placing trees in containers or directly into the ground, it is generally best to plant them in the ground. Typically, the supply of available water in field soils is many times greater than that in containers. If the field soil is very poor quality, however, containers could be the better option.
- Select species carefully. In general, choose species that are highly tolerant of water deficits (drought-resistant) and small in stature. Avoid species that have a relatively high growth rate, are large at maturity and are sensitive to water deficit. For example, olive (Olea europaea) would be a much better choice for containers than horsechestnut (Aesculus hippocastanum).
- If possible, install an automatic irrigation system. This is not possible for containers in many locations, but, where practical, it will help ensure adequate water supply and reduce maintenance inputs. Keep in mind that irrigation systems can malfunction and should be monitored regularly.
- Select the largest container size possible for the tree to be planted.
- Match the species and container size. It is best to use small-stature trees in large-sized containers.
- Use container substrate that has a reasonably good water-holding capacity (50 to 60 percent by volume). Avoid mixes that are droughty (have a low water-holding capacity).
- For containers of similar volume, those that are wider than they are tall have a higher water-holding capacity than those that are taller than wide.
- Place containers in protected locations to reduce evaporative potential. Avoid sites that are particularly windy and/or hot.
- To minimize evaporation from the soil surface, add an organic mulch. This is particularly important for large containers with a substantial amount of surface area exposed to the atmosphere.
- Place container trees close to an easily accessible water source, with a water spigot and hose. Attempting to water container trees with buckets of water carried over even small distances is likely to lead to inadequate irrigation.
- Be prepared to water as needed. In some cases, that might be daily.
- Check plants frequently for signs of water-deficit injury. Adjust the irrigation schedule as needed.
- Do not add other plants to the container ( for example, annuals or ground covers). They will increase water demand.
Considering these factors in management plans for trees in containers will help reduce the potential for water-deficit injury. However, it is important to recognize that even with good planning, container trees represent a significant management challenge. If water demands cannot be met because of budget, logistic or labor limitations, then it would be best to consider alternatives to trees in containers.
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