Greenhouses and dahlias

tomlinson04August 14, 2014

Hi there!

Sorry for what I am sure will turn in to a lengthy post, but I am in desperate need for some information about greenhouses.

I have 65 tubers ordered for 2015 from Swan Island Dahlias, and plan on buying two small stand up greenhouses with four shelves each to start my tubers growing before planting out. I have requested my tubers be sent March 5 the first date available because they can take up to a month in customs, so there is a chance I will receive them between March 5 - April 15. I want to get them going as soon as possible because I plan to take several cuttings and want the tubers to still have time to bloom.

Should I place the greenhouses outdoors south facing and keep them warm? (Also how might I keep them warm?) or should I put them in a covered area or in my house somewhere in a room with lots of light? Do they need supplementary light regardless? Should they be more humid or dry for dahlias and how do you control this factor in a small greenhouse? I would like to put them outside as I don't have the hugest amount of space but have a well lighted room I can put them in if I have to.

If it helps, I am in Zone 7 in Canada, in Nanaimo on Vancouver Island.

Thankyou in advance if you are reading this and have some suggestions!! :)


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First, there is an excellent nursery in Canada called Ferncliff and they have been selling dahlias for many decades. If you order from them you will not pay for the inspection certificate and the higher postage.

If you are not trained in green house procedures, you may have tough time with the dahlias. It is much easier to sprout tubers under florescent lights(or compact florescent) inside your house. 70 degrees is ideal for sprouting dahlias. Greenhouses have huge swings in temps from cold to very hot and dahlias do not generally like that.
Here is a link to an article on how to take cuttings indoors.:

    Bookmark   August 16, 2014 at 6:12PM
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Thanks so much for the response I will check the link out for sure!
Inside it is! :)

I am planning on also ordering some tubers from ferncliffe with some $ I have put aside for my cut flower venture; but I found swan island had more of what I was after! Thankyou very much for the suggestion it is helpful for a beginner!

    Bookmark   August 16, 2014 at 8:10PM
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This was my first year growing dahlias. I live in Washington State and I have TONS of slugs and I didn't want them getting eaten. So I got some of my dahlias started in March and others in April/June. I have a pretty large garden and a 6x10 greenhouse, so it not huge but plenty of space. It is facing south. I have 6 grow lights (those that cost about $100 each so an investment) that I start all my plants under. That is what I did with my dahlias until they started to sprout. I moved the first batch to the greenhouse before my last frost of the year but they did fine, no issues at all. I have the Solexx ($400 plus to cover my greenhouse, I built the frame with 2x4's which was cheap) panels up so not sure if that is why. My point is it was an investment but you are going to be growing other things then its not that bad.

    Bookmark   August 17, 2014 at 10:04PM
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Four foot fluorescent shop lights from walmart do great in pairs. No need for expensive grow lights unless you are growing medicinal drugs.

I use a cold frame over a raised bed in May to harden the plants off before planting in the ground.

    Bookmark   August 20, 2014 at 7:23PM
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There are grow lights marketed for VEGGIES too and not very expensive but great quality. So if you are interested you can find them at Wal-mart, Kroger, some nurseries sell them, online suppliers, and etc. So the comment about them being expensive and useless unless you are growing medicinal drugs is ridiculous. Plus budget is not the same for everyone, find what is your best fit! I would get other opinions obviously, maybe talk to folks at your local nurseries! Or heck there are folks that Im sure don't even bother with all this and just use a window with good lighting!

Good luck with whatever you decide!

    Bookmark   August 21, 2014 at 12:51AM
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"So the comment about them being expensive and useless unless you are growing medicinal drugs is ridiculous."

Who said anything about useless? I'd love a complete metal halide and high pressure sodium light setup with a separate ballast and fan/exhaust system, and came really close to buying that setup, but the return on investment is pretty low considering that dahlias grow just fine under inexpensive tube fluorescent lights. As you mentioned, some folk start their dahlias in windows, though they tend to get too leggy and can cause the plants to flop once planted.

Didn't mean to knock your setup, Ichorica. In fact, I envy it. The medicinal drug comment is based on the fact that the best informational sites I found about the above lighting setups were all about growing pot. I started getting worried that 'Big Brother' would alert the police about my internet search into nefarious subjects, as it is definitely not legal in my state!

    Bookmark   August 21, 2014 at 1:36AM
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Sorry to say this but...

This is my first year growing dahlias and I must thank this forum and its experienced members like Teddahlia, mandolls, CCVacation, morpheuspa, and many more individuals who spend their time to provide free advice to others.This is a true "community" that comes together just because we all have a common passion.

I don't think CCVacation's (don't know him/her personally) comments were ill-disposed. Please keep the spirit of this community in mind, before quick reactions. Thank you for your passion for dahlias and for your understanding.


    Bookmark   August 21, 2014 at 2:08AM
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Florescent lights of the cheapest quality do work fine. Someone gave me some full spectrum bulbs and I could see no difference. But remember, we are only talking about rooting cuttings. I have had difficulties getting a rooted cutting to grow tall so I could take some leaf cuttings. They grow OK but they grow more quickly out in the greenhouse(the heated one that is).

    Bookmark   August 21, 2014 at 12:40PM
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Mine are pretty similar to yours actually with full spectrum light bulbs as well. Not to say the ones that are less expensive won't work. I had a super cheap single bulb grow light that was placed over a small tray in my garage for years and it did just fine (for our veggies). I am no expert of dahlias so I can't speak on what will or want won't work (just what I did, which worked). But I have talked to people and they have shared their experience, which is why I also said for poster to get other advice. I am assuming there are different ways and people use/have different techniques. I do appreciate the advice of the experienced growers here and anyone that could give tips.

    Bookmark   August 21, 2014 at 2:22PM
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I am digressing from the Greenhouse topic here, but Teddahlia, do you have any articles or step-by-step guide on how to do successful leaf cuttings on dahlias?

    Bookmark   August 22, 2014 at 4:25AM
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Dahliafever, he had posted this link at the top of this thread, but here it is again in clickable form.

I have this bookmarked and notated in several places, as this is the best tutorial on the subject I've read on the subject. I've tried multiple techniques from other online articles and local master growers, and all work to some degree, but Ted's method takes out unnecessary steps and works extremely well with minimal fuss.

Here is a link that might be useful: How to take dahlia cuttings

    Bookmark   August 22, 2014 at 8:35AM
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CCvacation, thank you; I was scrolling down on my phone and missed the link earlier... Never heard of the "kitchen bag" technique and will be sure to use it next year!

Ted, thanks for your article; it is very informative, much appreciated.

A question to all the experts:
This is my first year growing dahlias. Planted about 100 (mostly tubers) dahlias on or around May 17th. My location is in southern Wisconsin, an hour north of Chicago. As of today, only 9 plants have blooms, but almost 60-65 others have (lots of) buds on them. I lost roughly 25% to overwatering (didn't know what I was doing!) or to poor performance due to too much shade.

Next year, I am planning to start indoors, so that the blooms would start early. My goal is to plant 2 foot plants outdoors in mid May next year. The question to you are these - how early could I start them indoors? Is it possible to raise 2 ft plants inhouse (without setting up a greenhouse)?

    Bookmark   August 22, 2014 at 11:27AM
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To grow early dahlias, you do not want 2 foot tall plants but rather plants about 12-18 inches tall. You would start taking cuttings in March and the goal would be to have the plants ready to go into the ground about May 20th or so in your area(but watch for killing frost warnings). I leave all our cuttings in the 2.5 inch pots in the greenhouse and grow them tall using 20-20-20 greenhouse fertilizer(Jack's or Plantex). You can bury the root ball slightly deeper when you plant. 12-18 inch tall plants will start growing immediately and will bloom many weeks earlier than tuber plants. However, they do not grow well in shade. Dahlias want full sun. You get lots of flooding rains there that we do not get. It is hard on tubers that are just starting to sprout. I think plants from cuttings may be just bit more hardy but standing water kills all dahlias in a day or two especially in warm weather. One cannot grow plants to the 12-18 inches without a greenhouse. You can root them under florescent lights but they do not grow well after that. I move the rooted cuttings to a heated greenhouse about 20 days after taking the cutting. About 11 days to root the cutting and another 9 or so days to grow some roots to survive in greenhouse.

    Bookmark   August 22, 2014 at 4:26PM
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Hi Ted,
Thanks for the detailed response. Much appreciated.


    Bookmark   August 25, 2014 at 6:18PM
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