My weeping Japanese maple has been thriving at our cottage in Parry Sound for 4 years, transplanted from my Toronto garden. This spring it almost completely died back except for new growth at the bottom. What caused it and what should I do? Thanks!
If you are saying that in the spring, the existing branches did not leaf out or grow, and the only growth is coming from the root area, then what you are describing sounds like winter kill. According to Farm North, Parry Sound, in zone 4b, experiences more freeze/thaw events and more winter kill compared with other areas in the surrounding district — and certainly more than Toronto. You are fortunate that the tree survived as long as it has.
To try to save it, prune off dead material and encourage new growth by adding some compost on the soil around it (not up against the trunk) and making sure you water it during drought conditions. According to Landscape Ontario, “Once established, Japanese maples may be lightly fertilized only in the early spring (April) with 4-12-8 fertilizer or 15-30-15 water soluble mixture.”
Apply a thick layer of mulch, up to 4 inches (10 cm.) over the root area to protect the roots from winter damage. Water well before freeze up. You may lose some branches to winter kill again, but as with this spring, the roots may survive and new growth appear.
However, if you are saying that the tree did leaf out and start normal growth in the spring, but died back later, this indicates a fungal disease.
Japanese maples are susceptible to fungal pathogens in damp soil, such as Verticillium. The symptoms of Verticillium are leaves that turn brown, grey or yellow or curl inwards. Branches may drop all their leaves.
Pruning will slow the spread of infection but will not cure it, as the fungus is soil-borne and enters the tree via its roots. Bag affected branches and leaves and dispose in the garbage. (Do not compost them or put out for yard waste collection.) Next year, the disease will probably return, in which case, you are best to remove the tree.
For more information on Verticillium wilt, see https://extension.psu.edu/pests/plant-diseases/all-fact-sheets/verticillium-wilt-in-trees
And for good general information on growing Japanese maples, see our Gardening Guide: https://www.torontomastergardeners.ca/gardeningguides/growing-japanese-maples-a-toronto-master-gardeners-guide/
Good luck keeping your tree going!