If you want height, I would leave the stems/canes as they are and prune any cane (or part of a cane) that's "winter killed" in the Spring.

By the way, the height of 8ft. is an estimate only and achievable after a long time, not 4 years but maybe 10 years or more.

Having a long growing season can "expedite" that growth but if you are in Z5, I would expect your growing season to be short/shoter. Pruning the canes makes the plant start growing from ground zero all over again.

Have patience and make sure the soil has appropriate levels of minerals (do a soil test now and then), is somewhat acidic and is as evenly moist as possible.

You know, I would kiddingly subtract 1 year from your shrubs' 4 y/o age to take into account the bad care that they got at the nursery.

;o)

Luis

Luis is probably right about the care at the nursery contributing to your slow growth.
I started my hedge from cuttings 4 years ago and they grew to over 6 feet this year. Keep in mind, I am in a warmer zone than you. I also fertilized with alfalfa tea (high nitrogen) once a month from April through August their first 2 summers. This year I stopped in June as I didn't want them to get too leggy (which they did anyway). They were good until a strong storm in mid August had them drooping to the ground. They are recovering now. This is the trade off I made by using the high nitrogen fertilizer. Now that they are near the height I want, I plan to prune only on the top growth next year; once in April and sparingly in June. I hope this will keep them from getting too leggy again. I will fertilize once or twice but not anymore that that. If they shape up the way I like them next year, I will maybe do minimum pruning in the following years. I have found that too much pruning of the paniculatas lead to the leggy, droopy growth. But sometimes it is necessary.
I hope my experience will help you with your hedge.
I do love the limelights, don't you?

Thanks so much for the advice! Will do. Also good to know about the time factor. Considering how puny and 70% dead they were, should be happy they made it through the first winter :) I do love them. Amazing how huge the flowers get. Especially considering their rather short stature.

I am a serious limelight lover. I planted a hedge in my yard the summer of 2008, for beauty and privacy. I was laid off in 2010, sitting home, bored, I purchased Cottage Farms Bud-N-Flower Booster. What happened next was just ridiculous. I applied this fertilizer to all my plants and they exploded! Height! Big heavy blooms! People were driving by my house taking pictures. The only down side was the big blooms made the plant droop after it rained which didn't look nice at all. So I stopped with the fertilizer. But it taught me to make the most of my growing season. My Advice, fertilize, according to directions.....don't cut them back - give the canes time to get thick and strong to support the blooms. Dead head in the spring. And give them time. I switched to Miracle Grow shake and feed, cause I needed something quick and that Cottage Farms stuff was crazy!

My limelights hit 9.5 feet this year! 6 years later.

Hi DD,

Did you use Bud-n-Booster carefree or bud-n-booster for acid loving plant? I wanna try it next year :)

DD, that's amazing!!!! I'm new to LL and mine didn't bloom at all this year. It's probabaly 2 yrs old. So maybe I'll try this next year!!! Ha!
Elce

But if blooms grow on new wood, why would one leave old stems at all? I'm totally confused. Hope someone can help me.

Pat Zone 5/6 SE Michigan

The terms do not mean what you are probably thinking.

"Blooms on old wood" applies to hydrangeas like Mopheads and Oakleaf hydrangeas- hydrangeas of these types produce invisible flower buds on last year's growth (around July-August-September) and these open in the Spring. Last year's growth is what is meant by "old wood". If my old-wood mophead develops a new stem this Spring, I would not expect bloomage from it until Spring 2016.

"Blooms on new wood" applies to hydrangeas like Limelight and Annabelle- hydrangeas of these two types produce invisible flower buds this year (around May-July) and these invisible flower buds then open as soon as they are ready (in a month+ or so maybe? meaning you are not waiting all winter long for example). The stems that produce blooms may be brand new ones from this year. Or they could be stems from last year that leafed out this year.

New stems, by the way, will not be as strong and as older ones. They will not support big heavy blooms as well so, you could see new stems with large blooms bending. Older stems get stronger as time passes and can support bigr/heavy blooms better.