Growing dahlias as perennials

gweni_hetzelJuly 27, 2008

We live in the highlands of Papua New Guinea. Our elevation is 5,100 feet. Even though we are very near the equator, it is quite cool here year around, and our average temperature here is 12-22 degrees Farenheit.

We grow Dahlias here as Perennials. I would like to know what is the recommendation (for our kind of climate) for when to lift and divide the tubers?

My personal experience in the last 2 years is that after planting a tuber in well amended soil, it will put out a tall, first flush of flowers. When the flowers die, I cut it down to just above ground. I will then get a slightly smaller flush of flowers. I can do this a third time, and get an even smaller flush of flowers. All of this takes about 1 year. But then, if I do not dig up the tubers and divide them, it seems they no longer will produce stalks and flowers. In fact, a number of dahlia species grown in gardens here, have completely 'disappeared,' probably from not being divided and re-planted (?).

I have searched all over the internet, and cannot find a definitive suggestion as to what to do with Dahlias in a tropical climate. Some sites suggest that I would NEVER have to lift them.

Does anyone know the "rule" for dahlias in this situation?

Thank you for any help you can give!

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Hi Gweni, I hope you mean the temps are 12-22 degrees C instead of F. My first question is do you pinch out the growing tip when the plant is about 12" tall? That will give you many more laterals growing instead of just the main stalk and the plant will stay shorter. This will bush up the plant making it much more attractive. The plants should bloom continuously but only if you keep the spent blooms deadheaded. This is just a suggestion but I would try to pick a period during the year to let the plant rest for 1-3 months. This will help in the flower production during the next season. It sounds like you would have to dig them and keep them out of the ground to accomplish this rest and that brings a whole set of issues in keeping the tubers alive.
As when to dig and divide your tubers I would suggest you pick a time after they have been in the ground for at least 4-6 months which should be sufficient time for tuber development. The best time in that cycle is when you have new shoots just coming out of the ground then you can divide one shoot with at least one tuber.
Your tubers are disappearing from 1. Poor drainage when the annual rains come and they rot; or 2. Voles or other burrowing animals discover them and eat them. Dahlias can be planted in plastic pots and put in the ground without the blooms or plant being affected in quality and the burrowing animals can't get to them. Availability of plastic pots may not be very good for you but anything that will keep them at bay will work.

    Bookmark   July 28, 2008 at 6:17AM
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sturgeonguy(5a ON)

Firstly, I have never had a Dahlia variety that deserved to be cut back after its "first flush of flowers." All of mine continue to bloom. Perhaps the first thing to ask yourself is why you think cutting them "hard back" is a good idea. Doesn't sound like one to me. Perhaps such severe treatment explains why your not getting propagation?

You are in the ideal tropic zone for Dahlias, e.g. their native zone, albeit it perhaps much higher than they were intended. I have no advice pertaining to your altitude, besides the obvious...maybe its a big factor.

Honestly, from what you've written, I have to believe you have not given your Dahlias sufficent time to grow. Your "cut back after first bloom" concept is contrary to everything I think about Dahlias. I love Dahlias because they constantly product blooms, for a long time...but you say you haven't let them do this. Perhaps I'm misunderstanding you.

I'm not in a climate that would let them grow and grow and grow. But here, they bloom for 4+ months. I would never think of cutting them back after their first blooms, nor even pinching (pinching, I think, should be done before the first bloom.)

Have you let yours grow longer? Is there a reason you don't?

Tubers take time to grow, think of them like potatoes.


    Bookmark   July 29, 2008 at 12:03AM
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Sorry, I didn't express my method well.

In describing my method, let me zero in on my 6 foot dahlia plants. After I get some strong stalks (no, I don't pinch, but plan to from now on), I get profuse blooms from that plant for several months. I deadhead constantly. After the leaves start to wilt, and only a few flowers are being produced (the end of the growing season), I have cut down the stalk.

In a few months I get some more stalk growth, and more flower production (just not as great as the first time). It seems that the more conjested the tubers are below ground, the less flower production.

When I said some dahlias "disappear," what I meant is that some people have left them in the ground for years, without ever dividing the conjested tubers, and the plant stops producing above ground. Unless the gardener knows exactly where he/she planted it, it will seem to no longer exist.

Yes, we do have an ideal climate here -- perfect for dahlias in many ways. But if we treat them like perennials, and never dig them up and divide them, then they produce less and less top growth, and eventually none at all.

I spoke to a local gardener yesterday (a national lady that has been growing spectacular dahlia gardens for many years now). She told me that she digs and divides her tubers 4-6 times a year to keep the stalks strong and the production of flowers high. So I guess I have my answer.

Thanks for your input, Gweni

    Bookmark   July 29, 2008 at 7:49AM
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sturgeonguy(5a ON)

FWIW, I cannot speak to your environment.

However, now that you've explained it the way you have, I think I understand what's happening.

When I was first introduced to Dahlias, a friend had a "bush" that they divided because it was too big. I got not a clump from a single plant, but a clump from a divided bush tuber.

I could be wrong, but I think most here grown single tubers (or cuttings as I did this year) and don't run into this (or at least if they do, its not talked about often.)

My friend thought to curtail her Dahlia by dividing it, the way I'd divide a hosta. She simply cut the "root ball" in half. For that variety of Dahlia, it worked.

When I got it, however, I didn't simply plant the clump. My research said to break it apart into individual tubers and plant each alone. They grew too.

So perhaps, and this is just wild speculation, you have varieties that cannot survive this way. I can see how a single plant would produce enough tubers that, if left undivided, could actually collapse in on themselves. Too many plants growing from a clump can, often, produce no plants. My recent research on "pot tubers" tells me you take the pot tuber clump and let one stem grown (ultimately.) To let too many grown would mean that none would grow really well (again, depending on the variety.)

Conversely, we know that we only need a single tuber to produce a great new plant.

Ergo, its not rocket science to take your experience and say, if you don't divide, you're going to have (or may well have) problems.

Now this is my first your growing pot tubers. My plan is to have a clump of tubers in a small pot at the end of the year. When I grow these on, I will let a single stem grow up. Anything outside of the 4" pot I will cut away. So I will neither have excess tubers (which if they rotted could very well spread to all of the tubers) nor too many stems (which if they all grew could mean none are getting enough to propagate new tubers.)

My suggestion would be to follow our (colder clime) procedure. When you cut the stalk back, let them stay in the ground for a week or two. The tubers should "eye up", or start to form new sprouts. Take the clump out and find one or two tubers that have the most eyes. Cut away everything else, and put them back into their same spot (discarding the excess, or planting it far enough away from the original so there's no chance of them intertwining.)

Dahlias were originally cultivated as a replacement for potatoes. If you don't "cull" them, perhaps you leave them without soil nutrients (because they'll be surrounded in tubers.) So "pare" them down. But given that every tuber can make you 10+ plants, give yourself the most prolific tubers when paring.

Of course the other thing to think of is soil composition. Given how they may produce a lot of tubers, they may well be taking way whatever nutrients they need to produce more (which could explain why they only last a couple of seasons.) I lived in Liberia, and there, they used to burn the rainforest to get enough nutrients in the soil to grown rice (hopefully you can see that that would only last a couple of seasons.)

Test, and amend. But reduce your growing mass (e.g. take away tubers) so it can be sustained.

Anyway, enough speculation...;-] Perhaps something in this will ring true. Again, I've no experience to help you with other than what we do in colder climes, but its what I would imagine will happen...;-]

Good luck, and I miss being in such climes...;-]


    Bookmark   July 29, 2008 at 11:37PM
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Restrict your shoots coming out of the ground to the strongest two or three and then reduce it to one by pinching them off at ground level. They will not grow back. This will restrict the number of tubers growing under the ground and you should only need to divide them once a year and flower production will remain at a high level.

Does JAARS fly into your location? I have a good friend who is a JARRS pilot.

    Bookmark   August 2, 2008 at 6:42AM
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I'm going to try some Dahalias here at about 7400 feet in the Colombian Andes. Temps sound about right, but we may be just a bit cooler. We also have a place at about 5500 feet (Medellin).

It's great to hear form someone else in a tropical montane environemnt. I'd like to stay in touch as things grow differently in tropical areas due mostly to minimal variation in temperature ranges and photoperiods.

Phil Bunch
Medellin/Piedras Blancas, Colombia

    Bookmark   August 16, 2008 at 2:08PM
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