University of Vermont Extension
Department of Plant and Soil Science
Spring News
Article
DELIGHTFUL DAHLIAS
Dr. Leonard Perry, Extension
Professor
University of Vermont
We have the Aztecs
to thank for the dahlia, a tender bulb (actually a tuber) planted in
late
spring for gorgeous blooms in late summer.
A long-lasting cut flower, the dahlia makes a great addition to any
garden.
Dahlias range in color from white and
yellow to orange, pale pink, lavender, and red. Bloom size ranges
from half an inch to a
foot or more across. Flowers may be in
tight balls to very open, from single to double, with petals that
are flat,
curved, or rolled into tubes. Based on
flower type, the American Dahlia Society (www.dahlia.org) lists 20
classes.
The
dahlia, a relative of the daisy, was first cultivated by Aztec
botanists in
Mexico. In the early 1500s it was discovered by Spanish explorers
who brought
this tuberous plant back to Europe. Interestingly, they had the same
problem
with storage of the tubers as do many modern-day gardeners. The
genus Dahlia
gets its name from an 18th-century Swedish botanist, Andreas Dahl.
The
dahlia became a favorite in the gardens of working class Europeans
after being
distained by the upper class as being too flamboyant for their
carefully
manicured gardens. However, it gained prominence in the mid-1800s
after a
devastating blight wiped out the potato crop in France as it was
thought to be
a good substitute for this starchy vegetable. Unfortunately, it was
not, but it
soon became popular in gardens both for its flowers and interesting
foliage.
Although a perennial plant, in northern
climates dahlias don’t survive winter so are treated
as annuals. They are planted in the spring as soon as the soil has
warmed up
and after the last chance of frosts—about when you plant tomatoes
outside. They can be grown one year as annuals, or
tubers lifted and stored in the fall after the first hard frost and
the foliage
is killed.
Dahlias do best in a sunny spot with
light, fertile, well-drained soil. If you have heavy clay soil, work
in a two-
to four-inch layer of well-rotted manure or compost a few weeks
before
planting. Add fertilizer--about a quarter pound of a balanced
fertilizer like
5-10-5 per ten square feet of garden—once a month after plants start
growth, or
lightly sprinkle fertilizer around plants. Avoid high nitrogen
fertilizers
(those with a higher first number) as they will result in leaves at
the expense
of flowers. Use a general garden or
tomato fertilizer.
Wait until the danger of frost has past
before planting the tubers, unless you planted them in pots in early
spring.
Plant at least four to six inches deep, laying them horizontally on
their
sides, with roots down and buds facing upward. Cover with two inches
of soil,
adding more as shoots appear.
Space smaller varieties two to three
feet apart, larger ones three to four feet. Larger plants also will
require
support as they grow. To avoid damage to the roots later on, drive a
stake into
the ground now, a few inches from where you plan to plant each
tuber. As the
plants grow, tie the stalks to the stakes with double strands of
garden twine. Pinching
back the center shoot once three sets of leaves appear will keep
plants more
bushy.
Most
dahlias will bloom in late summer and early fall. Some of the newer
and bedding
varieties are relatively short, a foot or so high, and generally
bloom
repeatedly through the season. Pinch off spent blooms from these to
encourage
continual flowering. Once frost has blackened the foliage, and after
the tubers
have hardened in the soil for a week, it's time to dig them up and
store until
the following spring.
With a sharp knife, cut the stalks at a
height of about a foot. Then carefully dig up the clumps, taking
care not to
injure or spear the tubers. Trim the stalks to a few inches. Shake
off the
loose dirt and separate the tubers, allowing them to dry for only a
couple days
(or they will start to shrivel and dry too much).
Brush off the remaining dirt, then place
in a plastic bag in a box, or plastic box (to keep them from drying
out)
containing peat moss, wood shavings, coarse vermiculite, fairly dry
compost, or
similar. Cover, label and store in a dry, cool (non freezing) place.
The ideal
storage temperature is 40 degrees F. Check every few weeks to make
sure tubers
aren’t shriveling (add a little moisture) or staying too wet which
will cause
tubers to rot. If too wet, leave uncovered until the storage medium
dries out,
or replace it with drier material.
If you have large clumps of tubers,
individual ones can be separated off with a knife. Just make sure
to keep a piece of the crown
(the thickened stem where the tubers join together), which has the
future
growing points or eyes. Then wrap each
tuber with plastic wrap. Easiest,
especially if you are already growing tubers in large pots, is to
just bring
the pots into a non-freezing area for winter, keeping them dry.
More dahlia tips, cultivar
(cultivated variety) listings, resources and sources can be found
from the
American Dahlia Society. More information, as well as heirloom
cultivars, can be found at Old House Gardens
(www.oldhousegardens.com).
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