At all times of the year, we see questions on gardening/landscaping sites with regard to ‘when do I prune my flowering shrubs?’
The issue, people, revolves around — does the plant flower(that is, set buds which then open up) on ‘old wood’ or on ‘new wood’. Now, this is actually a lot more complicated than it looks, frankly, because you can’t give a blanket statement on this. But in general:
Old Wood: You have a shrub. It is now October or November. The growth that took place THIS YEAR, is now ‘old wood’. It’s brown on the outside/green on the inside.
New Wood: You have a shrub. It is now March. The growth that WILL TAKE PLACE this season in April, May, June, July, August, September is NEW WOOD. It will be green, a little bit ‘bendy’ or even floppy early in the spring. It gets firmer later in the season and by August/September/October, it turns brown on the outside, is quite firm. At that point – voila! It’s ‘old wood’.
Let’s look at some shrubs from the garden here at Chez Siberia that show ‘old wood’ flower buds:
This is a rhododendron. Rhododendrons and azaleas both show this ‘flower on old wood’ situation – see the flower buds in the middle of the sprays of leaves? Right now, they are protected by leaf ‘scales’ but in the spring, provided that they do not get blasted by really hard freezes during the winter, they will open up. This particular rhodie got blasted by horrific (minus 27 degrees F) freezes last year. We got no flowers in the spring – not to worry, I pruned out the dead stuff and the bush put on new growth, made new buds. I’m thinking seriously about putting burlap around this for the winter.
This is an Andromeda shrub – see all the strings of little flowers? Again – these set this year and will open in the spring: old wood.
This is a Star Magnolia – those fuzzy things which look like pussy willow catkins are the flowers – the magnolia uses a the same protective device for the flowers – the fuzzy scales. If they get super frozen, we will have no flowers in the spring.
Now onto ‘new wood’. This is a Rose of Sharon. Because of the horrific freezes we had last winter, in the spring, a lot of it was frankly dead. So, I made a ruthless pruning – you can see the cuts in the middle of the picture. Literally all I had left of the bush was a few ‘sticks’ like that. See all that new growth coming out of those ‘sticks’? That bush in the late summer/early autumn was literally covered in flowers. This is a bush that a) loves to be pruned and b) sets flower buds on ‘new wood’.
And finally, we go back to the photo at the top. These are three hydrangeas. Don’t ask me what the varieties are – the tall one is probably a ‘pee gee’ hydrangea. The two at the side are mophead types. These are ‘new wood’ flowering hydrangeas. If we don’t cut them way back in the late fall/winter, then they will not set flowers the next year. I learned this a couple of years ago and though it sort of kills me to go after them with the pruners and loppers, I do love the flowers. 
As you can see, I basically cut the small ones to half their size – their full size is about as tall as I am – these are 2.5 to 3′ tall and will set a lot of new growth in the spring and summer. The huge ‘pee gee’ in the center will get the same treatment – that thing is taller than even my husband is so, I’ll be taking that down to probably 4′.
Now, having said that, I have to admit that NOT ALL HYDRANGEAS FLOWER ON NEW WOOD. So, you need to take this on a case by case basis and ask when you buy one to put into your hard ‘new wood or old wood’ – the nursery will know.
Here are two lists of shrubs which bloom on new or old wood:
Old Wood — prune these in the spring, right after they flower.
Witch Hazel
Forsythia
Andromeda
Flowering quince
Kerria
Viburnams
Daphne
Azaleas/Rhododendrons
Purple leaf sand cherry
Pearl bush
Deutzia
Weigela
Mock Orange
Lilac
Ninebark
SOME hydrangeas
Some Spireas (Vanhoutte/bridal veil) – the ones that flower in the spring.
Some Roses — the old fashioned sort that only blossom for one period in the spring/summer
New Wood — prune these in late fall/winter/early spring.
Beautyberry
Blue beard (caryopteris)
Buddleia
Rose of Sharon
Hardy hibiscus
Smoke Bush
Some hydrangeas(annabelle and panicle types)
Some Spireas – summer-blooming varieties such as Neon flash and Gold Mound
Buttonbush
Clethra
Roses – the repeat blooming varieties
Hope this helps. Everyone enjoy your holidays!!
H
