in the season, within reason. Why perennials respond differently is not com-
pletely understood. As is emphasized throughout the book, regional differences,
age and vigor of the plant, and weather conditions for the season, among other
factors, can affect the results. Photoperiod (day length) and the obligatory cold
period (vernalization) that many perennials require to induce flowering could
also be responsible for differing results of different pruning techniques and
different perennials.
An important thing to keep in mind is that often the more of a plant that is cut
off or the closer it is pruned to its normal flowering date, the greater the delay in
flowering. Also, the amount of delay is not directly related to the timing of the
pruning. In other words, a plant cut back on the 30th day of the month will not
necessarily have its flowering delayed 15 days longer than one cut back on the
15th day of the month. Also keep in mind the natural habit of the plant. Waiting
until later to cut something back may result in a rather odd-looking shape to the
plant. Plants that are cut back late may develop a tall single stem topped with a
multibranched head, creating an awkward look to certain perennials, reminis-
cent of the standard look often used for woody plants such as lilac or fuchsia.Summer-flowering perennials
It is well known that certain autumn-flowering perennials benefit from cutting
back early in the season to reduce the need for staking, but it is not as well known
that this can be done with several different species of summer-flowering
perennials as well. Summer-flowering phlox respond well to cutting back before
flowering and can be treated in a variety of different ways to reduce height and
delay flowering. Phlox maculata ‘Alpha’, for example, cut back by half when in
bud in early June may flower at 18 in. rather than at 2 1/2 ft., and about 2 weeks
later than usual. Heliopsis can also be pruned by half at the same time to flower
at 2 1/2 to 3 1/2 ft., rather than at 5 ft., with about a 1 1/2-week delay. Balloon flower
(Platycodon grandiflorus) tends to flop, but pruning it back by half in early June
can produce plants 1 to 2 ft. shorter than normal and with a 2- to 3-week delay in
flowering.
Preventing flowering altogether, rather than just delaying it, by shearing or
cutting off the flower buds may be desirable with certain perennials. I think of
this pruning as a form of disbudding, although technically the term doesn’t apply
(disbudding traditionally refers to removing surplus buds to promote production
of high-quality flowers or fruit)—but after all, it is removing the buds! This
pruning technique is utilized when plants are grown mainly for foliar display or
when the flowers are unsightly, distracting, or simply unwanted. Heuchera
micrantha ‘Palace Purple’ and Penstemon digitalis ‘Husker’s Red’ are a couple of
examples; I don’t want the flowers of these plants to compete with the attractive
foliage, so most of the time I cut off the buds as they set. When plants like
teucrium and others are grown for hedging purposes, the flowers are not desired.
Removing the flower buds is also effective in stopping the decline in vigor of
many silver-foliaged plants, particularly silvermound artemisia (Artemisia
schmidtiana ‘Nana’) or lavender cotton (Santolina chamaecyparissus), which
decline after flowering. The flowers on these 2 plants are not especially attractive
anyway, so why bother to keep them? Preventing flowering can mean several
shearings if new buds develop after the initial pruning.Autumn-flowering perennials
Many late-flowering plants benefit from being cut back before they bloom.
As with the summer-flowering plants that benefit from cutting back for height
control, the late bloomers treated in this way will have a fuller and more116 PruninG Perennials