Hi Tom,

What do you mean by prune them back to a reasonable size? How many inches or feet do you need to remove?

I would recommend taking off a little at a time and I would wait until late winter/early spring. I would prune the tips of the branches, making sure to not take off all of the green growth. Taking off some of the growth should allow more light to reach inside the shrubs and force the plants to release hormones that might stimulate latent buds to pop along the branches. I would check on the plants regularly and see if new growth is coming from inside the shrubs, closer to the trunk. If I did see new growth, I would again prune small amounts up until June (maybe taking off 6 inches?). Again, allowing more light and air in, and hopefully stimulating growth.

However, if after the first prune in late winter, I did not see latent buds pop in April or May, I would consider replacing. 

On the other hand, if you think you are going to have to replace, you could try cutting them down very hard. You might kill them, you might not. If not and they start to grow, you need to prune them regularly to keep them within your parameters. If you kill them, that's okay, too. You might be able to replace with something you like even better. 

When pruning, you should use hand clippers or a small pruning saw. You should not use gas powered hedge shears. 

Feel free to write back and ask more questions.

Lindsey K. Kerr, MS, MHP (she, her, hers)
Consumer Horticulture Educator
Michigan State University Extension