The Gardeners Almanac

The place to find out what to do in the garden this week
My Back Garden

Dahlias


Related Links

Week Numbers

Aphids

Basal Cuttings

Caterpillars

Coldframe

Compost

Division

Fertiliser

Frost

Fungicide

Heating

Mulching

Nodal Cuttings

Planting out

Pricking out

Red Spider mites

Seed sowing

Show Schedule

Slideshow

Soil pH

Thrips

Watering

Overview:

Collection of Cactus & Decorative dahlias

Modern Dahlias

The modern Dahlia is defined as a half hardy tuberous, herbaceous perennial, and is a member of the compositae species, which also includes the Chrysanthemum, Helianthus (Sunflower), and Zinnia.

They are classed as dicotyledons, of which there at least 20 species, many of which have now been hybridised to give the modern garden dahlia.


Origins:

Dahlias are native to the mountain regions of Mexico and Guatemala, where they were first cultivated by the ancient Aztecs who ruled this region.

Close up of stem

View of hollow stems

Tree displaying pink  single flowers

Tree in flower

Tree in leaf

Typical tree

During the 16th century the Spanish conquistadors arrived with their botanists, whose job it was to bring back plants from the New World for Spain.

These botanists discovered what we today call the Tree Dahlia, (D. imperialis)

These Tree Dahlias grew to over 6 metres in height (20+ft) and had hollow stems topped with open centered single blooms.

These hollow stems were often used to carry water, hence the Aztec name for the dahlia; "Acocotli" or water-cane.


Potato like tubers

Potato like Tubers
Development:

In the beginning; Dahlia seeds and tubers were distributed throughout Western Europe from stock grown at the Royal Botanical Gardens in Madrid, Spain.

The tubers of these plants were used as a substitute for potatoes but they never quite caught on.

Hybridization work in Europe began at the beginning of the 18th century.

Initially this work produced the first fully double forms, along with many new color combinations.


Characteristics:

Beds filled with Pink,Purple & Yellow flowers

Beds of Dahlias

The hybrids commonly grown as garden plants provide brightly coloured displays in most hues, (except blue) from midsummer to the first frosts!

This attribute makes them the ideal plants for summer displays in containers and traditional beds.

The shrubby growth ranges in height from 300 mm (12")to 2400 mm (8ft) subject to variety.

The pinnate shaped leaves are usually mid to dark green in colour although a few varieties produce bronze coloured foliage.


The flower heads can range in size from 50 mm to 300 mm in diameter (2"-12")

The majority of dahlia species do not produce scented flowers, and like most other species that do not produce scented flowers, they do not attract pollinating insects.

They can be grown in various types of soil, providing it is fertile and well drained.

A pH of around 6.6-7.0 is ideal.

If grown for exhibition or for cut flowers, they are best planted in rows in specially prepared beds.


Classification:

Dahlia types in the UK have been classified into groups and sizes by The National Dahlia Society (NDS) and are as follows;


 Crimson red variety

Hadrian's Midnight

Group 1 - Single-flowered Dahlias:

The bloom has a single ring of florets around a central disc.

The outer florets may vary in number from eight upwards, they may overlap, but must not assume double formation.

They should be equal in shape, size and formation and should radiate evenly and regularly away from the central disc in a single flat plane.

The outer edges of these florets may be rounded or pointed.


 White florets & yellow disc

Paso Doble
Group 2 - Anemone-flowered Dahlias:

This group has one or more outer rings of generally flattened ray florets surrounding a dense group of tubular florets, and shows no disc in the centre.

The outer ray florets should be equal in shape, size and formation flat, and regularly arranged around the central florets.

The blooms should be poised at an angle of 45 degrees on a stem which should be straight and proportionate to the size of the bloom.


Group 3 - Collerette Dahlias:

 Cream inner and outer florets with gold disc

Claire de Lune

The outer florets of Collerette dahlias may vary in number from eight upwards, they may overlap, but must not assume double formation.

They should be equal in shape, size and formation and should radiate evenly and regularly away from the central disc in a single flat plane.

The outer edges of these florets may be rounded or pointed.

The inner florets or collar of Collerettes should be uniform in size.

They should be symetrical and not less than one third of the length of the outer florets, even in colour and formation.

The central disc should be flat and circular, and should not contain more than two rows of pollen-bearing stamens.

The blooms should be poised at an angle of 45 degrees on a stem l be straight and proportionate to the size of the bloom.


Group 4 - Waterlily Dahlias:

A Waterlily dahlia should be fully double, and the face view should be circular in outline and regular in arrangement.

It should have a firm, circular, closed centre which should be proportionate to the size of the bloom.

The depth should be approximately half the diameter of the bloom.

Waterlily dahlias should be poised at an angle of not less than 45 degrees to the stem, and the stem should be straight and of a length and thickness proportionate to the size of the bloom.

This group is further divided into classes as shown below;

Fern Irene-Yellow wl
Group 4c

This category has medium sized flowers which are usually between 152mm and 203 mm (6"-8")in diameter.

Nepos-Blends of Pink
Group 4d

Small sized flowers which are usually between 102mm and 152mm (4"-6")in diameter.

Jean Fairs-Pink variety
Group 4e

Miniature sized flowers not usually exceeding 102mm (4")in diameter.


Group 5 - Decorative Dahlias:

The ideal bloom of a double-flowered dahlia should be symmetrical in all respects, and the outline should be perfectly circular.

It should have a firm, circular, closed centre which should be proportionate to the size of the flower.

The bloom should be "full", i.e. it should, without overcrowding, have sufficient florets to prevent gaps in formation and outline, and to give depth to the bloom which should be approximately two-thirds, or more, of the diameter.

The flowers should be poised at an angle of not less than 45 degrees to the stem.

The stem should be straight and of a length and thickness proportionate to the size of the bloom.

The general formation of blooms and their florets should correspond to the standards laid down for that particular group of dahlia.

This group is divided into five sub-groups that relate to their size.

Sir Alf Ramsey-Pink variety
Group 5a - Giant.

This group have giant sized flowers which are usually over 254mm (10")in diameter.

Kenora Valentine-Red variety
Group 5b - Large.

Have large sized flowers which are usually between 203mm and 254 mm (8"-10")in diameter.

Brian R - Purple variety
Group 5c - Medium.

Have medium sized flowers which are usually between 152mm and 203 mm (6"-8")in diameter.

Primrose Diane-Primrose petals
Group 5d - Small.

Have smaller sized flowers which are usually between 102mm and 152 mm (4"-6")in diameter.

Andrea Lawson-White petals  edges tipped with purple
Group 5e - Miniature

Have miniature sized flowers not usually exceeding 102mm (4") in diameter.


Group 6 - Ball Dahlias

All ball dahlias should be ball-shaped, but the tendency towards flatness on the face of the larger cultivars may be tolerated and a reasonable latitude may be allowed.

The florets should be symmetrically arranged and should dress back to the stem to complete the ball shape of the bloom.

The florets should be compact and dense at the centre.

The flowers should be poised at an angle of not less than 45 degrees to the stem.

The stems should be straight and of a length and thickness proportionate to the size of the bloom.

Wsterton Folly - Purple variety
Group 6a - Small.

Has small sized flowers which are usually between 102mm and 152 mm (4"-6")in diameter.

Barbarry  Primrose Hall - Primrose petals
Group 6b-Miniature

This group has miniature sized flowers that are usually between 52mm and 102mm (2"-4") in diameter.


Frank Holmes-Purple pom

Frank Holmes

Group 7 - Pompon Dahlias:

A Pompon dahlia should have fully double spherical blooms of miniature size,(usually not exceeding 52mm (2") in diameter)

The florets should be involute for the whole of their length (longitudinal axis) be evenly, and symmetrically arranged throughout the bloom and should dress back fully to the stem.

Blooms should face upwards on a straight, firm stem.


 

Group 8: Cactus Dahlias:

The ideal bloom of a cactus dahlia should be symmetrical in all respects, and the outline should be perfectly circular.

It should have a firm, circular, closed centre which should be proportionate to the size of the flower.

The bloom should be 'full' i.e. it should, without overcrowding, have sufficient florets to prevent gaps in formation and outline, and to give depth to the bloom which should be approximately two-thirds, or more, of the diameter.

The ray florets are usually pointed, the majority narrow and revolute for 50% or more of their length (longitudinal axis) and either straight or incurving.

The flowers should be poised at an angle of not less than 45 degrees to the stem.

The stem should be straight and of a length and thickness proportionate to the size of the bloom.

The general formation of blooms and their florets should correspond to the standards laid down for that particular group of dahlia.


Polar Sight - White petals
Group 8a - Giant

Have giant sized flowers which are usually over 254mm (10") in diameter.

 

Sulphur Springs - Cream petals
Group 8b - Large

Have large sized flowers which are usually between 203mm and 254 mm (8"-10")in diameter.

Amber Banker - Orange petals
Group 8c - Medium

Have medium sized flowers which are usually between 152mm and 203 mm (6"-8")in diameter.

Lavender Line - Purple petals
Group 8d - Small

Have small sized flowers which are usually between 102mm and 152 mm (4"-6")in diameter.

Weston Miss - Yellow petals
Group 8e - Miniature

Have miniature sized flowers not usually exceeding 102mm (4")in diameter.


Group 9 - Semi-Cactus Dahlias:

The ideal bloom of a semi-cactus dahlia should be symmetrical in all respects, and the outline should be perfectly circular.

It should have a firm, circular, closed centre which should be proportionate to the size of the flower.

The bloom should be "full", i.e. it should, without overcrowding, have sufficient florets to prevent gaps in formation and outline, and to give depth to the bloom which should be approximately two-thirds, or more, of the diameter.

The ray florets are usually pointed, and revolute for more than 25% of their length and less than 50% of their length (longitudinal axis), broad at base and either straight or incurving.

The flowers should be poised at an angle of not less than 45 degrees to the stem.

The stem should be straight and of a length and thickness proportionate to the size of the bloom.

The general formation of blooms and their florets should correspond to the standards laid down for that particular group of dahlia.


Pink Jupiter - Deep pink petals
Group 9a - Giant.

Have giant sized flowers which are usually over 254mm (10")in diameter.

Kenora Challenger - White Petals
Group 9b - Large.

Have large sized flowers which are usually between 203mm and 254 mm (8"-10")in diameter.

Grenidor Pastelle - Pink Petals
Group 9c - Medium.

Have medium sized flowers which are usually between 152mm and 203 mm (6"-8")in diameter.

Trelyn Rhiannon - Red tipped yellow petals
Group 9d - Small.

Have small sized flowers which are usually between 102mm and 152 mm (4"-6")in diameter.

Weston Lolita - Red petals
Group 9e - Minature.

Have miniature sized flowers not usually exceeding 102mm (4")in diameter.


Tahoma Hope(orch)

 Honka - Pale yellow petals

Honka (Star)
Group 10 - Miscellaneous Dahlias:

This group is to cover for dahlias that do not fall into any of the foregoing groups, e.g. Star and Orchid-flowered dahlias.

Blooms in the exhibit shall be similar in size and formation.

Blooms should be poised at an angle of not less than 45 degrees on a stem which should be straight and proportionate to the size of the bloom.


Petals white at their base and tipped with purple

Anna Marie-Small

 Pink fimbriated petals

Normandie Frills- Medium
Fimbriated Dahlias:

A fimbriated dahlia should be a fully double bloom.

The petals should be split or notched, uniformly throughout the bloom, to create a fringed overall effect.

The petals may be flat, involute, revolute, straight, incurving or twisted.


Cut Flowers:

A colloection of mixed coloured small ball flowers

Staged for Effect

 A collection of Pink and White Colloette flowers

Staged for Effect

It is important that blooms be presented on firm wiry stems and with proportionate foliage that enhances the blooms.

Stems which are bent, weak, short jointed or out of proportion to the blooms comprise a serious fault.

Bright attractive colour and blends should be regarded as meritorious.


Set of Regulation Rings
Flower Sizes:

For exhibition purposes Judges use a set of NDS regulation rings to determine the diameter of each group.

To comply these rings when passed over the blooms must not touch any petals.

To do so would render the exhibit as N.A.S (not as schedule) and it would be disqualified!


Dual Classification:

Some varieties of dahlias can have a dual classification.

These are varieties that are borderline in size or form, and as a result of this, they can exhibited in more than one size group.


Dahlia cultivation:
Week 2-3:

Check over dormant Dahlia tubers, remove any rotten parts, and dust with sulphur powder.


Week 4-6:

Tubers showing signs of new basal growth

Tubers on Hotbed

 Tubers set up i trays of compost

Saved Tubers

Set saved tubers on the top of boxes/trays filled with a 50-50 mix of compost and sharp sand.

Water in with a mix of fungicide* and water, mixed to manufactures instructions.

* Use of a fungicide is optional!

Place containers on a hot bed set to give a bottom heat of 18°C (65°F) and a surrounding air temperature circa 10°C (50°F) to encourage basal growth for cuttings.

Normally, it takes around two to three weeks for basal growth to appear, and a further four or five weeks, to produce shoots that are suitable for cuttings.


Week 12 / 13:

Take 50-60mm long cuttings if required.

Remove bottom leaves and insert 30-40mm apart in a mixture of 50-50 (by volume) peat and grit sand (dipping cutting in rooting powder/ liquid is optional)

On completion; water cuttings in, with a fungicide* solution.

Mist spray (daily) with a fungicide* solution to prevent dehydration and damping off

* Use of a fungicide is optional!

Inserting cutting into a tray of compost covered in silver sand

Inserting cutting into Compost

 Cutting ready for rooting

Prepared Cutting

Reducing the cutting to length

Trim cutting below node

Severing cutting from tuber with a craft knife

Sever Cutting

 

 

Circa Week 17:

Depending upon available light, the cuttings should take around two to three weeks to root.

When the tip of the cutting looks a healthy shade of green suggesting that it has rooted, prick out into 70mm pots filled with potting compost.

Place pots in a well lit position but out of direct sunlight.


 A tuber slpit into two parts

Divided Tuber
Week 18:

Place cuttings in a cold frame to harden off.

If more stock is required lift tubers from trays and divide them into pieces ensuring that each piece has at least one eye (growing point) on it.

Allow the wounds on the divisions to cauterise (dry) before planting them out.


Week 19 / 20:

Raking in fertilser

Rake in Fertiliser

 General fertiliser on bed

Fertiliser spread on prepared bed

If not already done, prepare beds for planting out by digging in manure or garden compost.

Leave the bed roughly dug so that frost can break up the soil.

Check soil pH and adjust if required, aim for 6.5-7.0


A few days prior to planting out rake in a general fertilizer at 125g/sq m (4oz/sq yd).


 Plants planted and supported

Planted Out
Week 22 / 23:

When all fear of late frost has passed, remove the plants from their pots and plant out in pre-prepared beds and borders.

Set them 600mm (24”) apart supporting each with a 1200mm (4’) cane/stake.

To avoid any damage to the roots, position the supports in the hole prior to planting.

Tie the plant loosely to the support at regular intervals as the plant grows taller.


Pot up any surplus plants into 125mm (5") pots and grow on as pot tubers in the cold frame.

 New growing points emerging from soil

Growth from an early planting

These will form the basis for the following year’s cuttings.


For an early start; one can plant out dormant tubers about six weeks before the last frosts, however; plants in leaf should not be planted until all danger of frost has passed.

If the new shoots emerge above ground level from early planted tubers and there is still a risk of frost, protect them with agro-fleece or cloches.


Week 25/27:

With plants grown for general use, remove the growing tip of each plant to encourage bushier growth.

If growing for exhibition, limit the number of shoots on each plant to around three or four (subject to variety)

For example giants should only have two or three shoots whereas miniatures can usually withstand six to eight shoots.

Basically it all amounts to the size of the blooms required.

To produce high-quality blooms, disbud each shoot to one bud.

Dahlias require copious amounts of water during their growing season.


Week 26/28:

As flower buds develop, extra potash in the liquid fertilizer will promote strong stems and good flower colour.

It can be advantageous at this time to mulch the beds with straw, grass cuttings or spent mushroom compost to conserve moisture.

 Close up of earwig

Earwig

Do not place the mulch too close to the base of the plant as this may encourage stem rot.

If using composted grass cuttings as mulch ensure that they have not been treated with a selective weedkiller.

Protect your flowers from earwigs;

Insert bamboo canes alongside your plants, these can double up as plant support.

Place an upturned flowerpot/s filled with newspaper or straw on top of the cane/s, in the hope that earwigs will hide inside these pots, then daily empty out the pots and squash any earwigs that you find.


Week 29 / 30:

Give plants a high potash feed to stiffen the stems and enhance the colour of the blooms.

Thin out the number of branches on each plant and remove secondary buds. (see note above)

This will give better quality long stemmed flowers.

Tie the remaining branches to sturdy stakes set three to a plant.


Week 30-32:

If the plan is to exhibit, it is beneficial to erect some form of canopy over the blooms, to prevent them getting damaged by the weather.


Week 40 / 44:

 Tubers ready for boxing up  prior to winter storage

Boxed up ready for
winter storage

Lift out tubers once the foliage has been blackened by the first frost.

In the event there has been no frosts to blacken the foliage, leave the plants in place and lift from say late October to mid-November.

Place them upside down for a few weeks to ensure that no moisture remains in the stems and leaves.

Drilling a hole through the base of the stem with a screwdriver or similar, will enable any water lodged at the point where the base of the stem meets the tuber to drain away.

This will help to prevent moisture building up that could cause rot.

Remember to label named varieties!

Do this by pushing a length of plastic-coated wire through the hole in the plant label and tying it to, or through the tuber.

When the tubers are quite dry, remove the dried soil and trim off the thin water roots and any damaged parts of the tuber and store them in a cool frost free area until required.

Check periodically for signs of decay, remove any rot that may appear.


Bedding Dahlias:

Buff coloured seedlin

Single Variety

 Pink seedling

Semi-double Variety

Bedding dahlias, sometimes known as Dwarf bedding dahlias will thrive in most types of well-drained soil, that has been enriched with well rotted manure or compost.

These types are generally grown either by taking cuttings or dividing the tubers of named cultivars.

Alternatively, they can be grown annually from seed.


They grow to around 750mm (30") tall and should be planted out 400mm (16") apart in a sheltered sunny border.

The double, semi- double or single flowers appear from late July to November or the first frosts.

The flowers are generally around 75mm (3") in diameter and come in many pastel shades.

Regularly water throughout the summer months, and feed weekly with a high potash feed from bud formation.

Aftercare for bedding dahlias is basically the same as for disbud dahlias in terms of location, feeding and watering.


Week 2-3:

Check over dormant Dahlia tubers, remove any rotten parts, and dust with sulphur powder.


Week 5-6:

Tubers showing signs of life

Tubers on Hotbed

 tubers set up in tray of compost

Saved Tubers

Set up saved tubers on the top of boxes/trays filled with a 50-50 mix of compost and sharp sand.

Water in with a mix of fungicide* and water, mixed to manufactures instructions.

* Use of a fungicide is optional!

Place containers on a hot bed set to give a bottom heat of 18°C (65°F) and a surrounding air temperature of 10°C (50°F) to encourage basal growth for cuttings.

Normally, it takes around two to three weeks for basal growth to appear and a further four or five weeks to produce shoots that are suitable for cuttings.


Week 11:

Sow seed in pots or trays of seed compost and germinate at 20°C (68°F)

Germination should take around six or seven days.


Week 12 onwards:

Take 50-60mm long cuttings if required.

Remove bottom leaves and insert 30-40mm apart in a mixture of 50-50 (by volume) peat and grit sand (dipping cutting in rooting powder/liquid is optional)

On completion, water cuttings in with a fungicide solution*.

Mist spray (daily) with a fungicide* solution to prevent dehydration and damping off.

* Use of a fungicide is optional!

Inserting cutting into a tray of compost covered in silver sand

Inserting cutting into Compost

 Cutting ready for rooting

Prepared Cutting

Reducing the cutting to length

Trim cutting below node

Severing cutting from tuber with a craft knife

Sever Cutting

 

Week 13-14:

When seedlings are large enough to handle, prick out into 75mm (3") pots of potting compost.

Grow on at a minimum temperature of 10°C (50°F) for two or three weeks then place them in a frost free coldframe to harden off until planting out time.


Week 15 onwards:

Depending upon available light, the cuttings should take around two to three weeks to root.

When the tip of the cutting looks a healthy shade of green suggesting that it has rooted, prick out into 70mm pots filled with potting compost.

Place pots in a well lit position but out of direct sunlight.


 A tuber slpit into two parts

Divided Tuber
Week 18:

Place cuttings in a cold frame to harden off.

If more stock is required lift tubers from trays and divide them into pieces ensuring that each piece has at least one 'eye' (growing point) on it.

Allow the wounds on the divisions to cauterise (dry) before planting them out.


 Plants planted and supported

Planted Out
Week 22:

Plant out 400mm (16") apart when all fear of late frost has passed.

In exposed areas insert a 1200mm (48") cane in the planting hole prior to planting to avoid damaging the forming tuber if done at a later date.


Week 28:

 Close up of earwig

Earwig

Protect your flowers from earwigs.

Insert bamboo canes alongside your plants, these can double up as plant support.

Place an upturned flowerpot/s filled with newspaper or straw on top of the cane/s, in the hope that earwigs will hide inside these pots, the daily empty out the pots and squash any earwigs that you find.


Week 30/32:

Give plants a high potash feed to stiffen the stems and enhance the colour of the blooms.

Thin out the number of branches on each plant and remove secondary buds, this will give better quality long stemmed flowers.

Tie the remaining branches to sturdy stakes set three to a plant.


Week 40 onwards:

Lift out tubers once the foliage has been blackened by the first frost, but if this does not happen, lift from late October to mid-November.

Remember to label varieties you would like to save for next year.

Do this by pushing a length of plastic-coated wire through the hole in the plant label and tying it to, or through the tuber.

Drilling a hole through the base of the stem with a screwdriver or similar will enable any water lodged at the point where the base of the stem meets the tuber to drain away.

This will help to prevent moisture building up that could cause rot.

When the tubers are quite dry, remove the dried soil and trim off the thin water roots and any damaged parts of the tuber and store them in a cool frost free area until required.

Check periodically for signs of decay, remove any rot that may appear.


Exhibiting:

Judging:

The judging of dahlias is not and cannot be an exact science, however the reading of The National Dahlia Societies Classified Directory and Judging Rules is a good starting point to determine what is required.

This is a fairly comprehensive document and if used in full may be too elaborate for the organisation running the show, so it is recommended that such organisations produce a ‘Show Schedule’ that includes any specific rules they may want to apply.

This would be particularly relevant at say a small 'village show'

The production of such a schedule will ensure that the exhibitors know what is expected of them and the judges can judge the exhibits accordingly.


Staging of Exhibits:

An exhibit of dahlias should be arranged so that all the blooms face in the same direction, are clear of each other, and staged so that a pleasant and balanced exhibit is achieved.

All blooms should be staged with some dahlia foliage, preferably on the stem.

The foliage should be clean, healthy and undamaged.

It is recommended that names of all cultivars, in an exhibit are clearly and correctly stated.

An exception to this is; varieties grown from seed.

The use of blocks under vases to create a tiered effect is permitted.


Various Staging Combinations:

 VGurtla Twilight poms
One vase of 5 Pompons

 

5 pink Kiwi Gloria
One vase of Five Small Cactus

 

 Six Cream Moonlight
Two vases of Three Large Semi-Cactus

 

Ten Trelyn Kiwi
Two vases of Five Small Cactus

 

 9 Pink Grenidor Pastelle
Three vases of Three Medium Semi-Cactus

 

 two red and one pink Giant decs
Three Giant Decoratives (One bloom per vase)

 

 4 vases containg pink/whit & Yellow cactus or semi-cactus dahlias
Multi-Vase Class

These classes can be a combination of various groups of flowers. e.g. this one is Four vases of Three medium cactus.

 A collection of mixed coloured Dahlias
The Society Class

A multi-vase class to include Dahlias from Five Classification Groups staged for effect.

Medals with Dahlia plant engraving on them
A Siver and Bronze Medal

Generally these awards are subject to criteria laid down by The Show Committee.