Winter protection

Our November meeting kicked off with Amelia giving a short talk on winter protection. This is a hot topic if you’ll pardon the turn of phrase, the long hot summer coupled with a period of extremely cold weather has led to many experienced UK bonsai enthusiasts having trees that didn’t recover through the following spring.

Amelia offered some advice on how we can improve our approach to winter. Growers living in towns, cities or by the coast might have an easier time with conditions, but there is no right or wrong way, just many different ways, depending on your garden, types of tree species, conditions, weather, etc. The more options you have the better off you will be.

Reasons why winter protection is good:

• it extends the growing season
• reduces the chance of ill health and death
• allows roots to continue growing more strongly
• protects the roots and new buds
• protects the pot from damage.

Note: Some people do not use any protection for their trees over winter at all.

What bonsai trees do you need to protect? Native tree species grow outside so may not need protection, however, bonsai trees are modified trees kept in a pot so may need protection as roots are more vulnerable in a pot, especially shallow pots. Smaller trees are more vulnerable than larger trees.
Tree species with fleshy roots like Trident maple, Yew, Redwood and Gingko should be given a better level of protection as this fleshy root type is more susceptible to damage.

Options for winter protection for broadleaved trees:

• Well ventilated garage during dormancy is okay, as they do not photosynthesise
• Cold greenhouses, cold frames or small plastic greenhouses
• Polytunnel
• Under a bench protected by tarpaulin or fleece
• Under a covered area outside on benches
• Fleeces or shade netting to reduce frost damage and wind damage

Options for winter protection for conifers

• well-ventilated garage with LED grow lights
• cold greenhouse
• polytunnel
• under a covered area outside.

Cautions

• for all options allow for ventilation and air circulation to avoid the build-up of mould
• do not overcrowd trees in over-wintering positions, allow airflow around the trees.

Options for tropical or semi-tropical species

• warm or heated greenhouse
• conservatory
• inside a house on a windowsill.

Note: Most tropical trees should not be exposed to any temperature below 10c. If you provide your trees with the best possible conditions you will reap the rewards.

Other forms of protection

For those trees in your bonsai collection that are more sensitive to cold weather try to provide them with some winter protection from the frosts especially if they are in small pots. The smaller the tree the more vulnerable to winter weather damage they are.

Options for protection include:

• double potting placing on pot inside a larger pot
• if you are growing trees in aquatic baskets definitely double pot by placing the aquatic pot into a solid plastic pot to protect the roots
• pot protection with fleece, polystyrene, and wooden boxes.

Continue to maintain winter protection in whatever form you use and check your trees daily for effects of frost or snow.

Preparation for winter

Check all your bonsai trees for health to ensure that before they are all in good health before the change in weather as the stronger your trees are the better chances they have of coming through the winter in good health.

Ensure that before placing them under cover or into a cold greenhouse the leaves for broad leaves have been removed as dead leaves rot and can cause mould and fungi to build up and affect the buds for next spring.

Watering

Your bonsai trees will still need watering but not as often as their growth rate has slowed right down so the water uptake has also slowed down and the trees are going into winter dormancy.
At this time of year, if your trees are still outside, they will be getting watered naturally by the rainfall, morning dew and melting frosts. However, monitor the moisture level to ensure your bonsai trees are not water-logged as this can cause roots to rot.

If your bonsai trees are under cover check daily and water if required to keep the soil damp/moist but not soaking wet.

Any bonsai being kept indoors should be watered regularly but this will have slowed down from a daily watering. They will be losing moisture due to the higher temperatures inside drying out the soil and the tropical species still tend to grow during the winter period inside.

The ideal time to water is in the morning to allow the water time to dissipate and to reduce the surface water should the temperature cool down overnight or there is a frost.

Pruning

This is a great time of year to review your bonsai trees and check the structural image of the tree as well as the tracery and branch placement.

Broadleaved trees are especially easy as they have for most species lose their leaves and are dormant. You can remove unwanted structural branches but remember to cover the wounds with wound paste or sealant as the trees are dormant and will not be able to respond to the pruning as effectively. Otherwise, just mark the branch(es) to remove and wait till the Spring to remove them.

Trimming

This is not required for either broadleaved or coniferous trees over Winter.

However, you may still need to trim the new shoots off tropical trees that are growing indoors to control the growth and develop their ramification.

Insects and pests

Continue to check your bonsai trees over fully for pests, insects and diseases as some insects can overwinter on or in your trees like red spider mites, scale insect and if you have not applied a winter pesticide or removed unwanted pests before now it is a chance to do so.

Pests are also likely if the weather is mild so worth checking to avoid disappointment in the spring when buds or leaves fail to flush due to pest attack.

Typical pests still around include scale insects, mites, aphids and soil-borne larvae, vine weevils and beetles.

Moss and weed control

This is the time to carry out pre-winter preparation and remove any weed and moss issues, particularly around the trunks of bonsai trees prior to winter.

The aim is to have a clear surface of the soil for improved moisture movement as well as to prevent bark rot from wet moss.

You can use a range of techniques including a diluted vinegar application on the bark which you apply and then around a week or two later the moss is removed more easily with a light brush action (toothbrush works well).

Wiring

If the wire is present on your bonsai tree check whether it is still serving a purpose and is taught and not digging into the bark causing wire marks and re-set if necessary or tighten.

This is also a good time to wire Pines and Larches to set the branches over the winter and it is an easier time to wire certain species like broadleaved varieties as the leaves have fallen off.

Re-potting

If the weather is mild repotting can be undertaken but avoid early repotting if you have no protection for the tree like a cold frame or greenhouse if frosts are likely.

Repotting season runs for broadleaf trees once they have lost their leaves and gone into dormancy which is usually from around late October through to mid-March.

Repotting for Conifers can also occur during October through to late March but it can vary depending on the species and some are better in the Spring when the trees are starting to become more active again
Prepare for repotting by checking soil supplies and also that if you are looking to change the pot check you have the correct pot size to repot into.

Feeding

Generally feeding over winter is not required as broadleaves trees are dormant and without leaves.
You can still feed conifers or evergreens although they have slowed down on their growth during this period and they should be allowed a period of around 2 months to rest without feeding or reduced nitrogen feed only. 

Continue to lightly feed tropical trees that are kept indoors if they are still showing signs of growth to sustain them during this period.

Assessment

This is also a good time to photograph your bonsai trees and update their records for re-potting timings, feeding regimes, soil type etc as it is sometimes hard to keep track of what you do for each tree when you start increasing your collection.

Keeping a photographic record does come in very useful over the years to see the tree’s development and your progress.

Club raffle

Also, club members are invited to join a raffle, the donated tree is a Mugo pine and up for grabs later this year.