This year we have certainly had a cold start to winter and it’s time to get going with the repotting of deciduous trees if you haven’t already started.
Some of the species you should be working on include crabapples, maples, elms, wisteria, liquidambars, flowering prunus of all types, deciduous conifers and ashes. Bear in mind with flowering bonsai, that any trimming you do to the tops of the trees will reduce the quantity of flowers you have in spring. Any severe pruning of the top should wait until after flowering has finished. It’s fine to prune the roots as long as you balance that with trimming back immediately after the spring flowering. Deciduous flowering trees should have plenty of energy in store to get them through the first few weeks of spring, even if they have been root pruned in the winter.
One of the first of the flowering ornamentals to burst its buds (around the same time as the flowering quinces) is Prunus mume (flowering apricot), so attend to these first if you have any in your collection. Boston ivy and Virginia creeper are also likely to come into leaf fairly early in the season. If you have a number of deciduous trees to work on, watch carefully for the swelling of leaf or flower buds and prioritize accordingly. Once the flowers or first leaves have opened, it is too late to repot for the year, and you will need to wait until next year.
Winter is a great time to pot up any deciduous group or forest plantings you have planned for. Make sure that all the trees you use are identical in species, and are similar in behavior (i.e. that they all lose/ don’t lose their leaves fully, all colour well in autumn etc.) or your group may not give you the year round effect you want. A greater impact can be achieved with ‘starters’ by grouping them together in a tray with interesting rocks, pebbles, mosses and/ or grassy plants.
I usually repot the majority of my native species (not figs of course) in July, as at this time of year they are less likely to suffer any setbacks because of the cold weather. Most native species are likely to be pretty dormant now but check that the tree is not in active growth at the time of repotting. There are other times of the year when natives can be successfully root pruned, as most species have several brief periods of ‘dormancy’ during the normal growing season, when there is no active new growth. I have found though, for me, that doing it in July gives me a high success rate and the repotting of some natives can be notoriously tricky to time correctly.
Keep clearing up any wayward weeds and fallen leaves, and continue vigilance with watering. Care needs to be taken not to overwater during winter, as this may lead to fungal root problems. Deciduous trees in particular, are much slower to dry out while they are in their dormant phase. Be aware though, that any recently repotted trees will have a reduced root mass, so care needs to be taken not to allow them to completely dry out.
Check and remove or rewire any of your trees that have been wired during the growing season. Winter is also a good time to check the outline of deciduous trees, while leaves are not blocking your view, and trim back as appropriate. Always make sure that you prune back to a spot just above a bud pointing in the direction where you want to see future growth, if possible.