I have a large perennial garden, with my favorite
bearded iris having a prominent role. However, so do the iris borer
who each summer decimate my iris. I dig them up, cut out the worms
and replant. Bingo, by August the following summer, the iris get
attacked again. Is there no preventative on the market to kill the
borers before they kill the iris? - S.C. -
No preventives, only controls. If disposing of borer grubs you
find in the roots is the extent of your effort, you need to do
Here's the borer's life cycle and how to interfere.
Winter: eggs wait on old iris debris. So, destroy all old
foliage each fall.
April - early May, when new iris foliage is 5-6" tall: eggs
hatch, tiny larvae chew on and tunnel into leaves. They're
vulnerable before they enter the leaf. Dust or spray iris with an
insecticide every two weeks during this time. Use diatomaceous
earth, horticultural oil, paraffin-based Sunspray, or a contact
insecticide such as Malathion.
July: larvae move into roots as they become large pinkish-white
grubs. Dispose of those you find in the roots. Others have moved
out into the soil, formed a pupa, and are about to become adults.
Either sift the soil around the plant to remove these, or rotate
the iris to new locations. Carolyn Study at Valley Nursery says
they've never had enough borers to worry about, though they've
grown iris for many years to sell along with other perennials,
annuals and produce. The reason for this is probably constant
movement: when the iris are divided, they're frequently moved,
sometimes almost a quarter mile away.
August - September: pupae in the soil mature. Grey, 1-1/4" adult
moths emerge, lay eggs in clusters on iris foliage and iris debris.
This is why crop rotation pays off. Though adults can fly, they may
not find the new location, and their chances of being intercepted
and eaten by predators is much greater than if they only had to fly
a few inches.
Double-up possibility while iris take a break
Bottom line: clean the iris bed well in late fall, and move your
iris around, even take them out of that bed for a time. Letting go
of favorites is hard, but change can be rewarding. Try unusual
bulbs such as quamash (Camassia cusickii, blue;
Camassia leichtlinii, blue or white) close planted with
late-emerging, late-blooming balloon flower (Platycodon
grandiflorus) or low and leafy sea kale. Quamash will fill the
iris time slot, late season balloon flower is a bonus.
More about iris borer... with
I need your help as soon as possible with my clematis
vines. I have them on either side of the garage, each one on a
trellis. Last year they bloomed beautifully with an abundance of
blooms and buds and suddenly as the weather warmed up they withered
and died. This year I moved the trellises a foot away from the
garage thinking the heat on the garage is killing them. Same thing
happened - loads of flowers and buds, but then hot weather and they
all withered. Do clematis hate the full sun? The heat? E.S.,
You're dealing with clematis wilt, a widespread problem that
mystifies many. May is the time to deal with this fungus disease,
so tickle this column to your spring calendar!
Clematis wilt fungi enter the plant through nicks or breaks in
the vine, reproduce, and clog up the stem's circulation. Branch
tips wilt as a result and then within a week the whole vine above
the point of entry blackens and dies.
Look closely at a stem that died. If it was killed by wilt,
you'll probably find discolored (rotted) spots where the fungus
invaded, just below the lowest pair of wilted leaves. The injury
through which the fungus entered may have been caused by wind
rocking the plant, a slug chewing the stem, or a gardener tying up
the vine. Wilt fungi are particularly active when it's very humid,
as the weather often is in July when the clematis is just coming
into full bloom.
If the clematis is well established before the fungus strikes,
it usually has enough energy left to produce new stems from below
the point of attack. New shoots may escape attack and so keep the
plant alive into the next year.
The fungus spores can lay in wait a long time, on fallen leaves
from past victims and even on unrelated plant species. So replacing
the vine is not a solution but a set-up in which the new plant is
To cure this problem you'll have to accept the fact that the
fungus does exist within your yard and your soil, and that you
can't eradicate it. Concentrate on making your vines inhospitable
to fungi, through good sanitation and the use of a copper based
fungicide such as Bordeaux mix.
Remove all dead and dying leaves and stems -- cut the stems
below the lowest discolored point, which may be below ground level.
Hot compost the debris or send it away from your garden. This
reduces the number of spores waiting to infect the clematis.
Then coat the stems and lower foliage of the vine regularly with
a fungicide to prevent new infection. Follow the fungicide label
Fungicide use has its drawbacks: it can kill or prevent the
growth of beneficial fungi that normally keep certain insects in
check or break down dead organic matter. Its residue may also repel
some birds and beneficial insects. So use this and all fungicides
only where needed - on the afflicted plant.
The late, great Christopher Lloyd wrote the book
Clematis (1989, Capability's Books, P.O. Box 114, Deer
Park, Wisconsin, 54007). It's a worthwhile investment for anyone
seriously interested in clematis. Mr. Lloyd spent a lifetime
growing clematis of every type and we're grateful that he wrote to
share what he learned.
Originally published in two parts, 9/3/94 and 9/10/94