Lilac (Vulgaris) Single Trunk

Question

Help with tree; Lilac Vulgaris. Live in Oslo, Norway. Planted over 2 years before. Up to now only trimmed off flowers after fading with very light snipping of dead small branches. Thinking my options are for end of flowering this season: 1- leave as is do nothing at all. 2- cut way down after fall (shown with red lines on photo) 3- cut further down as I use to do with Crape Myrtles in Texas. It is missing fullness; I could be more patient for the fullness but it seems to need something more from me.

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Answer 1

When the top growth reaches the height you want, cut the tips to encourage branching. Continue to remove the suckers from the lower trunk.

https://www.gardeningknowhow.com/ornamental/shrubs/lilac/lilac-care-growing-and-planting-lilac-bush-plants.htm
https://www.gardeningknowhow.com/ornamental/shrubs/lilac/pruning-lilac-bushes-when-to-trim-lilac-bushes.htm