Spring is in full flow, albeit some frosts in some areas with still fairly cold nights but hopefully all your bonsai trees have come through the winter and are coping with the cold nights here in the UK. Here is a quick generic guide to bonsai in May but for individual species check out some of the other species-specific posts.
In the UK it has been a fairly dry winter so continue to check your trees daily and water as required but start to increase your watering with the buds now pushing and leaves opening. Water in the mornings ideally to avoid the colder temperatures at night. Spray any repotted trees to ensure they do not dry out. It has been a fairly dry start so ensure to water well as the leaves are developing and particularly flowering trees.
You should now have completed your spring repotting for most species so keep an eye on the trees your have repotted.
For any trees that you are not repotting still remove the top surface and re-top dress to remove any crusting on the surface to improve airflow and drainage down through the soil. Plus clean the pots of the trees if they have got green algae forming on them.
For broadleaved trees, start feeding any trees that are now out in leaf that you did not repot while do not feed those repotted in April / May yet, wait till the end of May. Start increasing the feeding of evergreen trees as they start to show more signs of stronger growth.
Tropical trees and Evergreens can continue to be fed with a low nitrogen feed to maintain health and vigor at the onset of spring.
Feed flowering trees that have not been repotted yet like Forsythia, Chamomiles, Cherry, Akebia etc.
Ideal feeding at this time is liquid feed as the watering regime has picked up and you can feed with each watering, and the general rule is little and often.
Species to avoid pruning in Spring due to potential for sap bleeding would be Acer, Betula, Carpinus, Carya, Juglans, Magnolia, Morus, Populus and Tilia.
Remove unwanted branches and look for dead branches now that you can see the new buds swelling and clean out any dead branches before the leaves fully flush. Prune back any branches that are too long to a suitable live bud pointing in the right direction. Seal any large wounds with cut paste.
Younger trees that are being developed can have unwanted branches removed and for older trees check that callus wood is forming on old wounds and if necessary re-cut or carve to promote callus wood formation.
This is the time to de-candle or cut in half the candles on 2 needle pines.
Now that the leaves are present continue to maintain the shape through regular trimming and remove the terminal shoot in some species to aid with creating a more compact form. Continue to prune tropical trees to maintain their shape and trim the new shoots.
Where too much growth is expected and buds are well advanced, removal of unwanted future twigs may be anticipated by pinching out buds now. Where density is required of foliage, pinch or remove the terminal bud to promote side buds to develop. This is also a good opportunity to check buds to see if they are viable.
Leaf cutting or full defoliation is applicable for some species like maples but ensure that they are vigorous and healthy before you carry out a full defoliation and it may be more suitable to do a section at a time.
This is not applicable at present unless any trees are being repotted but for the majority of species they should already have been done.
Insects and pests
Check for the newly emerging bugs and pests like green and black fly, red spider mite, scale insect, mealy bug as well as vine weevil beetles. With the onset of warmer weather insect pests can start to emerge and are more likely to appear on the new shoots and leaves.
Watch out for the on set of mildew and prune out any damaged leaves or treat any infestations if found.
Moss and weed control
Continue to keep this in check and treat or remove unwanted moss and remove any newly forming weeds.
Wiring can be started on some of the larger branches.
Continue to protect from late frosts as it is still not very warm outside and ensure to harden off leaves before moving your trees out from a cold greenhouse.
Continue to update your bonsai records and photograph your trees to record their progress.
This is something everyone should do each time they use their tools but often it is not easy to do depending on how often you are using your tools. However, it is good practice to clean and sharpen your tools after use and this is a good time to go through your tool kits and clean them up before the start of the pruning season.
If you have trees out in display areas check that they are not drying out or getting too much sun and ensure they have shade if required as the temperatures during the day currently are fairly hot whist the nights are cold.
Also check for wind protection and stability of trees
This is the time to take conifer cuttings.