Greetings and good afternoon.
We are writing about two mature and overgrown lilac trees in our back yard in the west end of downtown Toronto. The lilacs seem to have at one time been cared for but have clearly been neglected over the past several years and are not happy or well at all. We would like to help support their flourishing again!
We have been reading about pruning lilacs but are unsure what approach to take – should we prune them down to stumps or do a less invasive prune? Or perhaps something else? Please advise. We’re also wondering at what time of year, with what tools.
“Rejuvenation pruning” is the term that is used for a severe pruning of old, overgrown plants such as the common Lilac, Syringa vulgaris. This should be done in late winter or early spring. “Renewal pruning” which is a less severe method taking two to four years, involves cutting back by one-third each year and is the other choice with your mature and overgrown trees. The best time for renewal pruning is during dormancy – before growth begins in the early spring..
In terms of equipment, here is an article by Mark Cullen who says that a good pair of loppers and a pruning saw are the essentials. To this, I would add a good pair of secateurs: https://www.thestar.com/life/homes/outdoor_living/2012/09/07/mark_cullens_pruning_primer_part_i.html
This article in Fine Gardening gives some clear information (including how to identify whether your lilac has been grafted) and especially mentions the importance of soil health: once you have cut your lilac back you should be sure to add some organic amendment such as compost to the soil around it: https://www.finegardening.com/article/pruning-lilacs
Finally, here is a link to a previous Toronto Master Gardeners post which also contains useful information about rejuvenation pruning of overgrown lilacs: https://www.torontomastergardeners.ca/askagardener/pruning-lilac/