68 Basic Perennial Garden PlantinG & Maintenance
Rust
Rust diseases are rarely a problem with perennials. Improving air circulation
around the plants and growing plants in lean, well-draining soil can help reduce
the incidence of rust. If it does occur, the affected plant parts can be pruned off.PEST PROBLEMS
One hopes that perennial gardens are full of people and friendly creatures. It is
our duty as gardeners to minimize damage from pests by selecting resistant
plants, practicing sound cultural habits, accurately identifying any problems,
and then handling them with some tolerance and manual controls before
resorting to any form of chemical control that might have negative long-lasting
effects.Slugs
A limited number of perennial genera are troubled by slugs—Acanthus, Delphin-
ium, Ligularia, and Hosta come to mind. But because hostas are such a popular
perennial in many gardens, slugs seem to be a rather prevalent problem.
Hostas with a thicker substance to their foliage and those whose leaves do not
come in contact with the ground are usually less prone to slug damage, although
this varies depending on location. How favorable are the conditions to slugs to
begin with and how many of them are there? If a slug is hungry enough, I’m sure
even the most discerning slug (if there are any) would eat the most thick-skinned
hosta. (See the discussion of Hosta in the Encyclopedia of Perennials for
slug-resistant types.)
All the usual remedies can be used against slugs, including the beer bash
(stale beer in shallow containers) and salt shaker approach, although these are
not always practical from a contractor’s point of view. I use diatomaceous earth
(crushed sea diatoms) in clients’ gardens for Hosta and Ligularia plants that suffer
a good deal of damage. Apparently slugs don’t like to crawl over the rough
surface of the diatoms (such delicate creatures!), so a collar of sand or ashes
around the plants may have similar effect. Diatomaceous earth is helpful in
curbing the problem, but doesn’t eliminate it completely, and it needs frequent
replenishment. Van Wade, one of the premier hosta growers in the United States,
has found that mulching with pine needles helps reduce slug numbers. Copper
strips (sold as Snail-Barr) will give slugs a mild electric shock if they crawl over
them. I do not use any metaldehyde-based baits because of their toxicity to cats,
dogs, birds, and toads—all which live in the gardens. In recent years, iron
phosphate–based products like Sluggo have become popular weapons in the
battle against slugs and snails, and are safer to use around pets than
metaldehyde-based pesticides.
Fortunately I do not have many problems with slugs in my own gardens. This
may be due to several factors: my gardens are generally on the dry side, and a bit
of slug damage doesn’t bother me. It may also be because I have a fair number of
toads in my gardens, and toads are predaceous of slugs. I have heard that
delphinium and monkshood (Aconitum) have their own self-defense mechanism:
after the first set of leaves are lost to slugs, plants send out a second set that are
supposedly toxic to slugs. (Interestingly, both of these plants are poisonous to
humans if ingested.)Borers
When it comes to iris and borers, I feel the easiest solution is to avoid, or at least
limit, the use of bearded iris. I never use them in clients’ gardens, and the only