About this time each year many dahlia growers decide to give it a go at growing dahlia seedlings or multiplying their stock by doing cuttings. By May, many of them will be rather disappointed with the results.
For a number of years I too struggled to grow good dahlia transplants from seed. They would come up, get reachy and flop over. I would pinch them back severely, they would struggle on and in May I would plant survivors that most people might consider discarding.
Eventually, I promised myself that I would raise my own good seedlings and I began a study of the subject. The key to my eventual success was proper lighting and this also proved to be a key to my later success in propagating dahlias from cuttings.
I did most of my research on the web and this is what I think I learned about plant lighting in general and dahlia lighting in particular:
1) Plants need light from both ends of the spectrum and reflect the middle range (green).
2) The red and far-red end of the spectrum helps plants mature, flower, set seed and fruit.
3) The blue end of the spectrum helps suppress the reaching (elongation) tendency in seedlings for 8-10 weeks.
4) Cool White fluorescent tubes emit more blue than red and should be the lighting of choice for seed starting.
5) Light intensity is very important and should be in the range of 1000-3000 lumens per square foot depending on the plant.
6) Light intensity of 2000 lumens per square foot is probably the minimum for good dahlia growth.
Since my practices of using commercial seed starting mix, bottom heating for germination and clear plastic tray covers to maintain humidity seemed appropriate, I concentrated on correcting my
apparent lighting problem and this is what I did for less than $100.
1) I purchased a 4-shelf, 4ftx1.5ft steel shelving unit at a flea market.
2) I attached TWO, 2-tube fluorescent light fixtures to the bottoms of EACH of the three upper shelves.
3) I purchased and
installed 12 GE Residential fluorescent tubes, which have replaced the
traditional low priced Cool White shop lights. Advertised light output is
3150 lumens per tube x 4 = 12,600 lumens over each 6 square foot shelf or
2100 lumens per square foot.
4) I started raising stout and sturdy seedlings.
I have also noticed that Home Depot sells a Phillips, ALTO series Utility tube with a rated light output of 3200 lumens. For the record, most fluorescent shop lights, plant lights and bulk tubes emit light in the 1800-2200 lumen intensity range. You need 3000 lumens or more per tube for good dahlia growth.
So, if you decide to do seedlings or cuttings this year you might find it to your advantage to make sure that you are providing lighting with an intensity somewhere close to the levels indicated above.
When I first started raising good seedlings I set the timers for a daily light duration (photoperiod) of 18hrs because someone told me to. It seemed to work for seedlings and when I started doing cuttings it worked for them too.
Later, I learned that growing dahlias under half day (12hr) or shorter photoperiods would induce tuber formation. Thinking that this might give tubers a head start and possibly result in more, bigger or better tubers, I switched to 10hr days and gave it a try. The result was that I grew the cutest little tubers on my seedlings and cuttings, and I saved a lot of electricity, but I really didn’t notice much other difference between the 18hr seedlings and the 10hr seedlings.
Then, a few years ago, shortly after I first shared this article on the internet, I received an email referring me to an article called “Success With Cuttings” which had appeared in the March 2004 Bulletin of the American Dahlia Society”. This article by Bill McClaren, one of the Guru’s of North American dahlia growing, changed my mind again about day length.
Although this article deals with cuttings, it also contains an excellent discussion of the effects of day length on growing both cuttings and seedlings. Like many things in nature either too much or too little of anything can result in adverse results and it suggests that a day length of at least 14hrs might be optimum.
Well, I like to try things so I reset the timers for 14hr photoperiods and when it came time to plant out I had fewer baby tubers and more feeder roots on both the cuttings and the seedlings. Then that fall when I dug it seemed like many of the cuttings and seedlings had root clumps that were well organized and some of them could actually be divided into separate tubers.
At this point I still don’t know that I know anything for sure about day length, but I think 14hr photoperiods might be a good place to start, if you want to see how varying day length might work in your growing environment.
Fluorescent tubes, unlike incandescent bulbs do not experience a sudden end of life. Instead, they tend to fail, one by one, over an extended period of time. The ‘Lamp Mortality’ table below graphs survival rates over time and you might note that the advertised ‘Tube Life’ is the point where 50% of the tubes have failed.
Failure is generally pretty obvious when we notice that a tube is emitting a markedly dimmer level of light. Then, when we replace the tube we generally notice quite a bit of tube end darkening and we worry about whether we have been light starving our plants and for how long.
So when should one replace fluorescent tubes?
The ‘Lumen Maintenance‘ chart below graphs the remaining light output for survivors as a per cent of original light output over time. What this tells us is that contrary to conventional opinion, light intensity in the T-12 fluorescent tubes that most of us use remains very high (>90%) until near end of life.
So, I guess the solution to the replacement dilemma is to watch for flickering light or darkening tube ends and replace the expiring tubes at your convenience. You might also want to use a permanent marker to date each tube as you install it. Then on long living tubes you may want to just retire them as a group as they pass their estimated half life or some other milestone.
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