Topic of the week: Dahlia species and related Genera
Sun, 13 Jul 2003 15:59:40 PDT
Dear All,

Mary Sue Ittner asked me to write about Dahlia species and here is my personal 
experience about those I grew or grow.

In general I find it difficult to obtain true, non hybridized species, maybe 
this forum brings a welcome change to that problem...... Also I find that 
naming is often wrong.

All Dahlias are easily grown from seed, if treated like tomato seed i.e. given 
an early germination and then greenhouse treatment until they can be planted 
out after the last frosts are over. Many of them will flower the first year but 
not all of them. Some may flower the first year from seed but not reach their 
eventual height, this only in subsequent years. In cold winter countries the 
tubers will have to be dug up in autumn, usually after the first frost has 
destroyed growth. Tubers should be stored cold but frost free and should NOT be 
totally dry. They may survive total dryness but will suffer. I put mine in 
large plastic bags only one big tuber per bag. I leave the bag wide open until 
the surface of the tubers has become totally dry and then close it but not 
hermetically. I watch for condensating water inside the bag and if this occurs 
I open it again and let everything dry a bit more for some days. I have had no 
losses this way.
The emerging shoots in spring are in great danger of slug damage, even total 
loss is possible!

Now my experience with some species:

Dahlia coccinea var palmeri:
This is so far the best Dahlia species I grow: around 2m tall needing no 
staking even in my exposed garden it has finely dissected leaves which gives 
the whole plant an elegant and airy appearance. The flowers are single about 
the size of an ordinary Cosmos but held horizontally. The colour is a bright 
orange red. It starts flowering in August and becomes better when the weather 
becomes cooler and moister. It sets plenty of seed (which I have distributed 
several times) some of it comes up spontaneously in the garden in late spring. 
The tubers are not hardy, though. It combines beautifully with Asters and late 
flowering Salvias.
When digging up the tubers care must be taken because the tuber itself is 
attached to the crown via a surprisingly long thin root and thus can be easily 
Personally I find the ordinary Dahlia coccinea a less attractive plant than its 

Dahlia dissecta:
Only grown from seed this spring I cannot yet tell a lot: very finely dissected 
bright green leaves on so far low plants, no flowers yet but looks attractive. 
The whole plant has an almost succulent look, very nice and dainty so far but 
no buds as yet.

Dahlia imperialis:
This giant Dahlia must be beautiful in flower, but I have never seen any except 
on pictures. (large pink single flowers in large trusses, there is a white 
form, too) I did grow it for some years hoping for flowers...... but no, it was 
always killed by frost before. I later learned that D. imperialis is an extreme 
short day bloomer which means flower buds are only produced when the daily 
hours of light fall well below  12 hours. By the time the buds develop and the 
flowers open it is mid winter. I have heard that it may be possible to force 
this plant to flower earlier by wrapping it with black polythene sheets but 
have never tried this. (must be quite a job taking its size into account!) This 
is a very tall plant, up to 4 m in flower and would be a subject for a very 
large conservatory or the like in cold winter countries. It produces huge 
compound leaves on a straight thick canelike stem and is worthwile growing for 
its leaves and its tropical appearance alone. I have seen large plants in 
growth but not in flower in gardens on the Cote d'Azur in France where it will 
have the frost free winters to flower. Must be nice in California. My plants 
were dug up every autumn like ordinary Dahlias but I have stopped growing it.
There is a good picture in the same Phillips/Rix boook as well as a most 
tempting picture of Dahlia excelsa. Does anybody know anything about this 
plant? Does it behave the same way as Dahlia imperialis, i.e. late flowering? 
Is it available anywhere?
Does anybody know if there has been done some selection or breeding to produce 
a day-length-neutral, i.e. earlier flowering plant of the same stature?

Dahlia laciniata:
Same as with Dahlia dissecta, only grown from seed this spring. The leaves look 
like ordinary garden Dahlia leaves, slighty dark but not purple. The slugs had 
a good go at them but recovering, no flowers yet, will be  taller that D. 

Dahlia merckii: 
Often seen in English gardens in great beauty, I was not at all sucessful with 
it here in Germany. I assume that our more continental summers were too hot, 
the plant went into a strange kind of half dormancy during  hot weather and did 
not flower only until the weather became cool and moist in autumn but then was 
soon cut down by the first frost. I do not grow it any more. Roger Phillips and 
Martyn Rix state in their excellent book on Conservatory plants that it comes 
form Mexico and grows at an altitude around 1600m. It is not that high and not 
that cool a climate there, so I do not know...... Does anybody else have 
similar or different experience?

Related Genera:

There are other genera that look similar to Dahlias and are also often 
tuberous: Cosmos (often tuberous) and Hidalgoa (not tuberous, the "Climbing 
The most common tuberous Cosmos probably is C. atrosanguineus. I have 
so far never managed to restart the tubers in spring. An attractive plant with 
smallish chocolate coloured and chocolate scented flowers, a bit straggly and 
prone to mildew in moist weather. All plants available are micro-propagated and 
sterile, apparently this plant is extinct in the wild. Did anybody ever have 
seed from this plant?
There are other tuberous Cosmos species but they are not in cultivation: do 
prove me wrong, please....
In Bolivia I collected seed of a dwarf large flowered tuberous Cosmos of which 
I have two small seedlings...... It grew quite high around 3000m in 'potato 
growing climate' I can't tell much more yet but will keep you informed.
No personal experience and never seen a plant of Hidalgoa, is it an attractive 
garden plant?

So far my (limited) experience with Dahlia species, I thought I will give you a 
follow-up of this topic in early autumn when I can (hopefully) say more about 
my novelties. 

Greetings from Germany, Uli

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