Deciduous Pre-Bonsai Care Sheet

General

This care sheet is meant to provide the basics of day-to-day deciduous bonsai care. Bonsai training is not the focus, and is only briefly touched on. Additionally, this care sheet focuses on cold hardy temperate deciduous trees that lose their leaves due to decreased temperature and photoperiod. Because cold hardy deciduous trees are native to much of earth, it is not possible provide concrete numbers for every species tolerance to temperature, humidity, sunlight, etc. Instead, the bonsai artist must research the requirements of their particular tree and use this care sheet for guiding principles that can be used for cultivating pre-bonsai. Also note that this care sheet is geared towards pre-bonsai in development, not refined mature bonsai. Bonsai and pre-bonsai are used interchangeably in this sheet, but in both cases we are referring to pre-bonsai in development. Below, we begin a fundamental dive into the care of deciduous bonsai.

Sunlight
Generally, deciduous trees enjoy a minimum of 5 hours of full sun during the growing season, however there are drastic differences between the sunlight requirements of different species. Deciduous tree species are broadly broken down into a few categories that have different light requirements. For whatever category the species of a particular bonsai fits into, it is best to provide it with the top end of its light requirements if possible. By providing ample light, it keeps the internodes (space between the buds) small, and the leaves small. It is important to note that the leaf morphology is driven by the light levels it was formed in. For this reason, it is important to not allow foliage to fully develop in the shade and then move your tree to full sun, as the shade leaves can be destroyed by the full sun. Below, the light requirements of the deciduous tree categories are broken down. These are meant to be used as guidelines because not all sun is equal depending on elevation, humidity, wind, and the time of day full sun is achieved. Optimal photosynthesis occurs between 65 – 85 F, so morning sun is always best if possible. If your full sun is achieved in the afternoon, you may need to provide light filtering as the afternoon sun occurs at higher ambient temperatures. The key is research, experimentation, and learning from other local growers.

Pioneer: These are trees that grow quickly in nature after an area is cleared by some event (fire, wind, avalanche). These trees prefer more light, 6+ hours a day, as they evolved to thrive in areas where there is little competition for sunlight. Pioneer species are many, but two well known examples are Birch and Aspen.

Fruiting: These are trees that generally produce very showy flowers and very fleshy fruits. These trees have evolved to mostly live on forest edges where they are exposed to full sun similar to the pioneer species. Fruiting trees do best with 6+ hours of full sun. Examples of trees in this category are apple, hawthorn, and plums.

Climax: These are trees that evolved to initially grow in the understory of pioneer species but eventually out compete them. These trees are more shade tolerant and flexible with their light requirements than the pioneers and fruiting trees. In general they can do well with 4+ hours of direct sun, and bright shade the rest of the day. However, they can also thrive in 6+ hours of full sun. The increased shade tolerance of climax species makes them more suitable for growing areas with less sun, but more sun should be provided if possible to reduce internode length and leaf size. Well known examples for climax species are many maples and beech.

Understory: These are trees that have evolved to live their whole life cycle in mostly bright shade under larger trees. These trees can thrive with even less light than the climax species, some performing well with 3+ hours of direct sun and bright shade the rest of the day. Examples of understory trees are red buds and many species of maple such as Japanese maples and vine maples. This category can be tricky as some can withstand much more sunlight, while others cannot, so species research if important.

Temperature
The individual species should be consulted for temperature tolerance ranges, and the season should also be observed for what temperatures are appropriate. Some species are more tolerant to heat, while others are more tolerant of cold. The ideal temperature range for photosynthesis is between 65 to 85 F, but some species can withstand much hotter temperatures without damage. It is more difficult to bracket ideal winter temperatures due to species tolerance differences. Some species can thrive in a wide range of temperatures, while others can only withstand a narrower band of temperatures. The season also has a strong influence on the temperature tolerance. The season are broken down below. 

Spring: Deciduous trees are vulnerable to extreme temperature in spring. A late freeze in spring can damage or even destroy tender spring leaves. Excessive heat can also damage the tender spring growth as the leaves have not yet formed their protective waxy cuticle and are prone to increased water loss. During spring, protect your tree from late hard freezes by bringing them onto a covered porch, inside a garage, or similar protected area. If there is an early spring time heatwave, the best thing to do is to shade the tree until the heat passes and ensure the soil is always moist.

Summer: Cold snaps are no longer a concern, and the leaves have formed their cuticle so are more protected from hot temperatures. On extremely hot days for the species, it is again best to provide extra shade and ensure that the soil does not dry out. If your schedule allows, extra watering on very hot days can help by the evaporative cooling it provides. In general, most deciduous trees will not need any protection until temperatures reach the high 90’s F for extended periods of time (days). However, this is highly species dependent so it is important to conduct species research.

Fall: This is a transition time for deciduous trees as they are storing sugars and nutrients for next year, and undergoing physiological changes that increase cold hardiness for winter. Early fall freezes are when the tree’s leaves have not started changing colors yet. These freezes are dangerous to a deciduous tree because the leaves may be destroyed and the tree has not had time to store much energy for the next spring, and also has not had time to develop cold hardiness. If an early fall freeze is expected, it is best to move your tree to a protected area. After the leaves have undergone their color change, this is less of a concern.

Winter: In general, if you live in USDA zone 8 or higher, you will only need to provide minimal winter protection to your trees and will likely not need to worry much except for a handful of days a year. Again, this is highly species dependent so do your research. In addition to species dependencies on winter hardiness, it also matters how far into the winter season it is, and what temperatures your tree has been exposed to recently. Some deciduous trees are extremely cold tolerant, while others can’t take much below freezing, so it is important to research your specific species. Winter cold hardiness also develops and tapers off on a bell curve. Trees are less cold tolerant in early and late winter than they are in mid winter. Additionally, trees that were recently exposed to fluctuating “high” temperatures conducive to root growth (greater than 42 F) will be less cold tolerant in their root system to very low temperatures. This means if you expect an extreme cold front within the week, you want to keep your trees as cold as possible until those low temperatures hit.

Trees in pots are much less cold tolerant than those planted in the ground. Research suggests that a potted tree is several USDA cold hardiness zones less cold tolerant than the published data which are determined for trees in the ground (we will be releasing some more information about this soon in preparation for our talk at the RMBS Colorado Springs meeting in December). This means that pre-bonsai planted in pots should be protected from strong winter weather much sooner than the USDA hardiness zone suggests. Methods of protection include placing the trees in a garage or other sheltered area over night, placing them on the ground and mulching around the pot, and blocking the line of sight of the tree from the sky. Temperature swings are also dangerous for deciduous trees in the winter. It is best to shield your trees from the direct sun in winter until you notice the buds starting to swell. This is because deciduous trees can move small amounts of water in winter, so a tree can desiccate itself when it is exposed to full sun, but the root ball is frozen.

Humidity
Most deciduous trees are very flexible in their humidity requirements. We have grown many different species in both humid and dry climates without any adverse effects. However, average humidities do influence the care you should provide. Very low humidity days can increase evapotranspiration in the tree and dry out pots much quicker. Very high humidity can reduce evapotranspiration to the point where it is easy to overwater your trees and cause root issues. 

One last note on humidity is that the size and number of stomata on the leaves is dependent on humidity in which the leaves were formed. This can cause issues for some bonsai growers because if you allow your tree to leaf out in a humid a environment  and then move it outdoors where it is dryer, the tree could drop its leaves and possibly be killed depending on overall health.  

Water
Most deciduous pre-bonsai will grow best if they stay moist and are only allowed to slightly dry in the upper level of the soil. Always ensure that the water supplied to your bonsai can penetrate into the root mass. Overtime, soil breakdown and root growth can cause a situation where the soil mass if very compacted and the water is only able to flow around the outside of the root mass. If this occurs, you can introduce aeration around the root ball by penetrating it with a sharp chopstick in many places and then rehydrate the tree by submerging it into water for a short period until it is rehydrated. 

There is a lot to be said about water quality and the growth of bonsai. In most cases, municipally supplied water sources are fine for your bonsai. Exceptions do exist and we do have customers that report having to adjust the pH or mineral content of their water for tree health. This is rare though and if issues exist with tree health the grower should first investigate more basic things such as light, over/under watering, and pests. One last note is that as a bonsai matures past the pre-bonsai stage and into a small container, watering needs and practices will change.

Soil/Pots/Fertilizer
For developing pre-bonsai, we recommend 50/50 perlite-potting soil as a starting point. If you are growing your trees in a more humid area, you may want to increase the perlite a bit. Likewise, drier climate growers may want to increase the potting soil a bit. 

We recommend wide pots around 6” deep for pre-bonsai development. The goal for deciduous bonsai is to create a flat and radial root system and a shallower pot helps achieve this goal while still allowing the tree adequate room for its root system so the trunk and branches may be grown out as necessary. As the tree develops, the depth and overall size may be reduced. 

For pre-bonsai at our nursery, we utilize Miracle Gro All Purpose Plant Food diluted by 50% during the growing season and fertilize every 1 – 2 weeks. This contains most of the nutrients and trace elements to keep your tree healthy. We do occasionally cycle in other fertilizers for short periods of time to make sure that we are filling in any holes in the trees’ nutrition not covered by the Miracle Gro.

Dormancy/Troubleshooting

Cold hardy deciduous trees must have a winter dormancy to thrive long-term. The length and severity of the dormancy is highly dependent on the species. Some southern species only need a photoperiod decrease and modest temperature drops in winter with few freezing days. Others require a much longer winter with many more freezing days. It is best to research individual species to determine their dormancy requirements.

Most problems with your bonsai will arise due to light, water, improperly timed work, or pests. Leggy growth or larger foliage can occur when not enough sunlight is applied. Foliage dropping can occur when moving a tree between areas with very different growing conditions, as discussed earlier. It is best to have the tree leaf out in the spring in an environment that is close to what the tree will experience in summer.

A lot of watering issues arise from soil breakdown and from the tree being rootbound. If a tree appears water stressed, but you are regularly and heavily watering it, investigate the soil mass. If you find an issue, aeration needs to be reintroduced to the soil, and the tree needs to be regularly submerged until the repotting season.

Timing of work is not meant to be a large topic in this care sheet, but it is mentioned here for completeness. Repotting and root work should be done right when the buds start to swell, almost never when the tree has opened leaves on it. The last pruning of the year should be around 6 weeks before the first frost in your area. This will give the post pruning flush of growth time to harden off prior to winter. Pruning too close to the onset of winter may cause the new flush of growth to be exposed to freezing temperatures and being subsequently destroyed weakening the tree for the next year.

Pests

Pests should be treated on a case-by-case basis. Look for pests when you have a lot of malformed growth, unexplained dew on leaves, or on weak trees suffering from other issues. Isolate trees with pest issues as best you can. We mostly use Pyrethrin and Neem oil for foliage pests. If your tree is suffering from dehydration that does not seem to be related to watering or soil issues, it could be a root dwelling pest such as root aphids or root mealy bugs. You can lightly investigate the soil to search for them. If present, you can apply a soil drench insecticide such as bifenthrin.