66 Basic Perennial Garden PlantinG & Maintenance
and borers, and columbine and leafminer are frequent companions. If you can’t
eradicate a problem completely, ask what can be done to reduce the problem, or
consider how you can alter your attitude to accept some of it.DISEASE PROBLEMS
Stem and root rots can occur in certain perennial species where soil conditions
are too moist. Damaged plants should be removed, and soil drainage improved.
Perennials troubled with different leaf spots should have the affected parts
pruned off if they are a problem.
Peonies in particular can have a host of problems. They look tatty and start to
decline with the heat of August. This is because new eyes, or dormant growth
buds, start to form at this time, taking the strength from the foliar portion of the
plant. Klehm Nursery, famous American peony growers, recommends that
gardeners not prune the plants until after the first of September in zones 3, 4, and
upper zone 5; until the end of September in the rest of zone 5; and until after early
October in zones 6, 7, or below. Resist pruning back even if the plants look ugly,
so they can continue to photosynthesize, which will usually mean healthier
plants the following year.Botrytis and Phytophthora
The red peonies, as well as the more fragrant forms, are always more susceptible
to disease. Klehm Nursery explains that this is because the red peonies produce
more carbohydrates (evident by sticky sugars on the buds). Botrytis, an airborne
disease, sticks to these buds more easily. Paeonia tenuifolia ‘Flore Plena’ (double
fernleaf peony), which is red, is very susceptible to disease, so it is recommended
that this form be deadheaded immediately after flowering to prevent the entry of
disease from the mushy dead flower into the soft stem. Soil should also be kept
well drained. If peonies are infected with disease, prune off infected parts and
remove any debris from the base of the plants. All foliage and debris from around
the plants must be removed each autumn and destroyed, not composted, to
remove the possible sources of the Botrytis.
Phytophthora is one of the most common causes of root and stem base decay
in a wide range of trees and shrubs, and is often characterized by yellow or sparse
foliage, wilting, and branch dieback.Powdery mildew
Powdery mildew can rob a plant of water and nutrients and cause leaf yellowing
and even distortion of the plant, though usually only in extreme cases. Let’s start
by selecting resistant plants. I know of more than 20 mildew-resistant Phlox
forms, in a variety of flower colors, heights, and bloom times, and I’m sure there
are many more; a fair number of mildew-resistant beebalm (Monarda didyma)
cultivars are also available. Some of these phlox and beebalms might not be
totally free of disease, and again, much depends on the weather conditions, but I
think we can live with a light dusting or a spot or two—the plants definitely can.
Phlox and monarda both can be cut to the ground if mildew is a problem. They
will usually put up fresh, clean growth later in the season if provided with
sufficient moisture and will not miss a stride the following year. When cutting
down infected plants, it’s a good idea to disinfect your pruners by dipping them
in a 10 percent bleach solution (1 part bleach to 9 parts water) or at least wash
them with soap and water after cutting to reduce the chance of continued spread.
Powdery mildew is a unique fungus in that its life cycle is not encouraged by
free water but rather by high humidity and lack of air circulation, as well as