cut it back whenever you so desire ... why do you need permission .... track down a stem .. along the trunk .. and if you see a few tiny leaves where a leaf is attached.. that is a dormant bud .. cut off what is above.. and that bud will become dominant.. and start growing.. go as low as you want .... every daisy i ever had.. had the dormant buds ... let me know if you see them .... this idea is for ANY plant ..... and on many you wont see them at all.. but they will be there .... if you dont see them.. and you have more than one plant.. cut one.. and give it a week.. and see what 'pops' .... and if you like what you see ... go to town on the others ..... and never forget.. you arent a true green thumb gardener.. until you have killed every plant in your yard 3 times .... and that is why you should learn propagation first.. lol ... ken PS: dont forget.. this time of year.. you have a mature root mass.. so cutting off the top.. should lead to explosive growth .. since there is no insult to the roots ... all other variables proper [no drought, etc] .... in other words.. there is plenty of time for a hair cut.. they should grow back faster than you can believe ....

Yes, if you want to cut it back go ahead. It will bloom later.

Lauren W: After last winter in Z5 failure of both the lilac and forsythia to bloom could well have been the result of extreme cold. Most lilacs are very cold resistant, but not all. Many forsythias will fail to bloom after significantly sub zero temperatures. Prune both shrubs in the way gardengal suggests above. Or do just the lilac that way and cut the forsythia back to manageable size.

You can still cut back - I've always cut back until beginning of July.