Pruning spring flowering shrubs

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June 1 is the official cut off that marks the difference between a spring bloomer and a summer bloomer. Does it matter that you know? Yes, if you want to properly prune because pruning after June 1 could result in no blooms next year.
This spring was a great one for spring bloomers: lilacs, viburnums, azaleas, rhododendrons and many others were all able to do their thing without a major frost or freeze here at the farm. Many of our favorite spring bloomers, however, invariably grow out of control so the question arises, “when is the best time to prune?” If you want to maintain your spring blooms then pruning spring bloomers right after their blooms fade is the goal. The rule of thumb that gardeners live by when it comes to pruning blooming shrubs: if it blooms before June 1 it blooms on buds set last year; if it blooms after June 1 it blooms on new growth from the current season. Most spring flowering shrubs will have set all their buds for next year by this July.
Even if your shrub is still in good form a little pruning each year is not a bad idea because the pruning cuts stimulate new vigorous growth, which keeps everyone looking good without a major overhaul. If you have ever tried to tackle an old lilac or viburnum you will appreciate the wisdom here; over grown shrubs can be difficult to prune if they have gone too long without attention.
When pruning set a goal that includes reducing size, thinning, and removing the oldest, dead or pest damaged wood. Another rule of thumb is that we should only remove about 1/3 of a shrub at a time. All this pruning will encourage new, vigorous growth and will improve the overall appearance and health of an old shrub.
Summer blooming shrubs like Spirea and Weigelia can still be pruned, as well, although it is best to get this done in early spring. Pruning now will delay your summer bloom a little but the act of pruning will encourage new growth and bloom.
Evergreens like boxwood, taxus and holly are best pruned after the threat of frost has passed in spring so you can still tackle the chore now to reduce size and thin the shrubs. These evergreens will put out vigorous new growth after pruning so don’t be afraid to do some hard pruning on these if major rejuvenation is needed.  Your next opportunity to thin and prune these evergreens is in late summer, early enough for the new growth (that is always stimulated after making a pruning cut) to harden off before our first frost.
While the recommendation for fertilizing woody plants is to do so in the fall of the year, spring bloomers and evergreens benefit from a little fertilization in the spring as they push new growth, bloom and set buds. Most of these plants like a lower soil pH, as well, so some composted manure with a layer of pine straw mulch or a light application of a fertilizer formulated for acid-loving plants will help to maintain an agreeable soil pH. There are some good sea-based fertilizers for acid-loving plants that are formulated from lobster shell and seaweed, for example, if you are looking for an organic alternative.