A Brief Cultural Guide to Growing
Dahlias
There can be few more spectacular
or exciting flowers for you to grow in your garden than the dahlia.
For most people their first point of contact with the dahlia is
either at an enthusiast’s garden or at one of the many flower
shows held throughout the country during the late summer. Their
initial impression is usually that they must be far too difficult
for them to grow. Nothing could be farther from the truth. Dahlias
are easy to grow and can provide a wonderful display from early
summer until struck down by the frost.
The dahlia has an interesting history. The first tubers are reported
to have arrived in Europe in 1789 having been sent over to Madrid
by the Spanish settlers in Mexico. Andreas DahI, a Swedish botanist,
after whom the dahlia is named, regarded it as a vegetable rather
than a garden flower, but interest soon switched from the edible
tubers to the blooms when the first varieties with large double
flowers were bred in Belgium. Interest increased dramatically
and it was not long before the dahlia was being grown in many
European countries including Great Britain.
In 1881 the National Dahlia Society was formed and serves as a
‘parent’ organisation for the many dahlia societies
that have been formed since that time. The Classified Directory
and Judging Rules published biannually by the National Dahlia
Society forms the basis for dahlia shows and exhibitions held
throughout the land.
SOIL AND SITE FOR DAHLIAS
For most people there is little or no choice as to where they
can grow their dahlias, but if you are fortunate enough to have
some control over this, dahlias seem to prefer a medium to heavy
loam. They do not like to become waterlogged and should wherever
possible be sited in a position where they will receive maximum
sun. Dahlias seem to prefer a bed to themselves but if you cannot
afford them this luxury do not be afraid to try a few plants in
your beds or borders mixed with other plants. They will not disappoint
you.
In most cases it is advisable to dig your ground in the autumn
and leave it rough. Well rotted farmyard manure or home produced
compost can be incorporated at this time to enrich the soil and
improve its condition. Many soils can be improved by the addition
of lime and this is best applied after the rough digging to allow
the winter rains to wash it in. It is generally recommended that
lime and manure should not be added at the same time. Keen gardeners
carry out periodic checks to find out the pH of their soil. The
pH scale is a means of measuring the acid-alkaline balance of
the soil. The neutral point on the scale is 7 and dahlias prefer
a pH of just under 7, that means slightly acid.
MANURES AND FERTILIZERS FOR DAHLIAS
Organic and inorganic substances are added to the soil to increase
its fertility. Organics are derived from animal matter e.g. excrement,
bones, blood and hair or dead plant remains. Inorganics are of
mineral origin either from natural sources or manufactured. Bulky
organic substances of animal origin, often mixed with straw, as
in farmyard manure, are always known as manure, while bulky plant
remains are known as compost. Both substances are best dug into
the ground in order that bacterial action can break them down
further into humus and release plant foods.
Inorganic substances and concentrated organic ones like dried
blood and bone meal are known as fertilizers. These are best applied
to the soil surface and then raked in so that the rains may carry
them down into the soil.
There are numerous ‘liquid feeds’ on the market nowadays
and they are generally quick acting. Always apply with care and
do not exceed makers’ recommendations. By giving a little
bit extra you will probably do more harm than good.
Manures and fertilisers supply plants with essential nutrients,
the main ones being nitrogen, potash and phosphates referred to
as N.P.K. on fertilizer packs or bottles. Nitrogen promotes the
growth of leaves and stems. Potash enhances colour while phosphate
is a constituent of protoplasm and is essential for the movement
of food in the plant. Other minerals are needed for healthy growth,
but only in very small amounts and are usually present in average,
fertile soils.
DAHLIA TYPES AND CLASSIFICATIONS
Dahlias may be divided into two main groups. Border dahlias are
named cultivars propagated by cuttings. They are mostly tall growing
and double flowered. Bedding dahlias may be single or double.
they are usually grown from seed and do not generally grow taller
than 18 inches.
In the National Dahlia Society’s Classified Directory, already
referred to, dahlias are classified into ten groups and the respective
group indicates the type of blooms which the plants will bear.
The groups are:— Single-flowered; Anemone flowered; Collerette;
Waterlily; Decorative; Ball; Pompon; Cactus; Semi-cactus and Miscellaneous.
There are further divisions by size — giant flowered which
are usually over 10 inches in diameter large flowered usually
between 8 and 10 inches — medium flowered usually between
6 and 8 inches — small flowered usually between 4 and 6 inches
and miniature flowered usually under 4inches. The blooms of Pompon
dahlias usually do not exceed 2 inches.
PROPAGATION OF DAHLIAS
There are four main methods of propagation.
Field tubers are the roots from the previous years
crop and may be planted directly into the garden during the month
of May. They should be covered to a depth of at least four inches
so that by the time the growing tips break through the surface
of the soil all risk of frost should be past. Large tubers can
be split to give additional plants provided each part of the tuber
has an ‘eye’ from which new growth emerges. Field tubers
are available from recognised dahlia nurserymen and from some
garden centres.
Pot tubers are the roots of a plant which has
been grown within the confines of a small pot during the previous
season. They too can be planted directly into the soil as field
tubers but are more often used to provide cuttings which are then
rooted and then planted out. Pot tubers can be bought from dahlia
nurserymen or garden centres.
Rooted Cuttings is the most common source of
increasing stock and is perhaps the best method for the beginner
to get started. Tubers are started into growth in a heated greenhouse
in early spring. When the young shoots are about 3inches in length
they are cut off, trimmed and placed in a rooting medium of compost
or sharp sand and peat. They should be sprayed occasionally with
water to prevent them flagging and they should be shaded from
bright sunshine. Bottom heat is required for them to root and
the most common method is to place them on a bed of sharp sand
heated by a soil warming cable.
Cuttings will normally root in about 2/3 weeks if conditions
are right. After rooting they should be potted up into small pots
of a good quality compost and grown on. After a further two weeks
they will require to be moved up to a bigger pot with fresh compost.
Towards the end of May they should be moved to a cold frame and
gradually hardened off prior to planting out during early June.
Seeds Bedding dahlias are usually grown from
seed. Varieties of seeds are generally available from garden centres
or can be obtained by mail order from seedsmen. Sow in gentle
heat (55-60 degrees F.) in March. Prick out and grow on in boxes
or small pots of compost. Harden off in May and plant out in June.
Flowering should be from late July until struck down by frost.
PLANTING OUT DAHLIAS
In Scotland dahlias should not be planted out before the first
week of June. Late frosts will cause considerable damage to your
young plants and many good plants have been lost because the gardener
has been too hasty in planting out. Bedding dahlias do not require
to be staked but the taller border varieties should be staked
as soon as they are planted to prevent them being broken by the
wind. Bedding varieties may be planted about 15 to 18 inches apart
while the larger border varieties should be planted no closer
than 24 inches apart with the giants being spaced about 36 inches
apart.
You will require to protect your young plants from slugs and it
is good practice to sprinkle slug pellets containing metaldehyde
around your plants while they are in the cold frame and at planting
out time. From time to time you will require to sprinkle fresh
pellets, particularly during a wet spell.