6' is pretty small for a lilac - how has it gotten "out of hand"? Generally,
pruning lilacs should occur immediately after blooming so as not to disrupt
next year's bloom cycle. Rejuvenation pruning can be done during the dormant
season, but a 6' lilac is unlikely to require rejuvenation. How long has it
been planted? Most young lilacs take several years to establish and become
full. They generate new growth from the base (suckering stems) and that usually
takes several years of establishment before occurring. I would not recommend
you remove more than a third of it at any one time.
Boxwood doesn't have thorns - perhaps you mean barberry? They can be pruned
back hard, but suggest you wait until later in the season to do any extensive
pruning. Plants in full growth mode and the heat of summer tend to get over
stressed with drastic pruning at this time of year.
pam - gardengal
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