Making Forsythia (and Other Shrubs) Bloom
As always, the generous Grump is here to answer your most perplexing garden questions. Here's one about forsythia from Pam Nichols:
Hi! I have three large forsythia that are about 20 years old. They are in full sun and have not been trimmed in a couple of years. The last two years they have had no blooms (well, maybe three flowers in all). What am I doing or not doing wrong? Help!
If your shrubs are 20 years old, I would say they are prime candidates for a technique called renewal pruning. This involves in cutting 1/3 of the oldest, woodiest canes to the ground every year for three years. Do this immediately after the forsythia finishes blooming. This will remove the old, tired growth and promote new, vigorous growth with lots of flowers. Don't prune in summer, fall, or winter or you'll cut off flower buds for next spring. In 3 years, you'll have brand new shrubs.
Some people advocate renewing shrubs like forsythia (also called yellow bells) by cutting them completely to the ground after they bloom. I think this is a little drastic, unless your shrubs are very overgrown.
Renewal pruning can also be used to rejuvenate the following shrubs:
1. Beauty bush (Kolkwitizia amabilis)
2. Fuzzy deutzia (Deutzia scabra)
3. Dwarf flowering almond (Prunus glandulosa)
4. Japanese kerria (Kerria japonica)
5. Lilac (Syringa sp.)
6. Spirea (many kinds)
7. Weigela (Weigela florida)
8. Winter honeysuckle (Lonicera fragrantissima)
9. Mockorange (Philadelphus sp.)