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I have just read about the effect of day length on blooming. This might be very useful to know in the garden. Here is what I learned.
Not surprisingly, plants have a biological clock that tells them when to start blooming. When we reach a certain number of daylight hours, a particular plant starts to bloom. I thought this internal clock triggered plants to bloom when days were getting longer, but in fact some plants are triggered to bloom when days are shorter. We all know about poinsettia that initiate flowers only when the days are short (shorter than 10 hours), but did you know that cosmos react in a similar way?
For instance, this explains why spinach bolts and goes to seed so fast after it has started growing. It is a long day plant, programmed to bloom when days get longer than 13 hours (early in the gardening year). It would be interesting to do an experiment where you cover them in later afternoon to see if it would slow bolting. I plan to try this experiment this summer.
I always thought that cosmos and morning glories bloomed better in our garden later in the season, because I planted them too late. It turns out they are short-day plants, and so they flower faster when days get shorter. Our season is too short to start them early enough in the spring. I must learn to expect them to bloom mostly in the second half of the summer!
It gets more complex: some plants have to have long or short days to bloom, while others prefer long or short days but will eventually bloom even if they do not get them. Here are some examples.
Have to have short days - hyacinth bean, African marigold
Prefer short days - cosmos, moonflower, morning glory (Ipomoea) and zinnia
Have to have long days - bachelor’s buttons (Centaurea), fuchsia, gazania, rose mallow
(Lavatera), lobelia, petunia 'Purple Wave', Rudbeckia hirta and sweet pea
Prefer long days - calendula, dianthus, pansy (Viola), snapdragon, sunflower
Finally some are not influenced by day length - amaranth, cleome, stock, mullein and tomatoes.
It is good to know. If you start lavatera or sweet peas too late, you cannot expect them to bloom profusely because long days are required to trigger blooming. The experiment with spinach described above should also work with zinnias. Because they prefer to flower when days are shorter, you could cover them in late afternoon to see if they would bloom earlier. It is worth a try.
I have just read about the effect of day length on blooming. This might be very useful to know in the garden. Here is what I learned.
Not surprisingly, plants have a biological clock that tells them when to start blooming. When we reach a certain number of daylight hours, a particular plant starts to bloom. I thought this internal clock triggered plants to bloom when days were getting longer, but in fact some plants are triggered to bloom when days are shorter. We all know about poinsettia that initiate flowers only when the days are short (shorter than 10 hours), but did you know that cosmos react in a similar way?
 |
| Cosmos |
For instance, this explains why spinach bolts and goes to seed so fast after it has started growing. It is a long day plant, programmed to bloom when days get longer than 13 hours (early in the gardening year). It would be interesting to do an experiment where you cover them in later afternoon to see if it would slow bolting. I plan to try this experiment this summer.
 |
| Morning Glory - a short-day plant |
I always thought that cosmos and morning glories bloomed better in our garden later in the season, because I planted them too late. It turns out they are short-day plants, and so they flower faster when days get shorter. Our season is too short to start them early enough in the spring. I must learn to expect them to bloom mostly in the second half of the summer!
It gets more complex: some plants have to have long or short days to bloom, while others prefer long or short days but will eventually bloom even if they do not get them. Here are some examples.
 |
| Rose Mallow - a long day plant |
Have to have short days - hyacinth bean, African marigold
Prefer short days - cosmos, moonflower, morning glory (Ipomoea) and zinnia
Have to have long days - bachelor’s buttons (Centaurea), fuchsia, gazania, rose mallow
(Lavatera), lobelia, petunia 'Purple Wave', Rudbeckia hirta and sweet pea
Prefer long days - calendula, dianthus, pansy (Viola), snapdragon, sunflower
 |
| Dianthus blooms more readily when days are long |
Finally some are not influenced by day length - amaranth, cleome, stock, mullein and tomatoes.
 |
| The blooming of mullein is not influenced by day length |
It is good to know. If you start lavatera or sweet peas too late, you cannot expect them to bloom profusely because long days are required to trigger blooming. The experiment with spinach described above should also work with zinnias. Because they prefer to flower when days are shorter, you could cover them in late afternoon to see if they would bloom earlier. It is worth a try.
I was aware of some of these, but I didn't know that Fuchsias need long days. Mine seem to bloom all season long--of course, I start with established plants, so that probably helps. I'm also surprised by Sunflowers because they seem to bloom more toward the end of the summer when the days are getting shorter. Great post!
ReplyDeleteIt would appear that sunflowers "prefer" long days. As for fuchsias and other plants that have to have long days, I suppose that once they have had them they are OK for a while (it is the blind leading the blind - I am just guessing).
DeleteAnd I will just do a happy dance when the days are long enough for anemone, the first bulb I have that comes up.
ReplyDeleteI think that like you, we are all ready for a "happy dance".
DeleteInteresting post, Alan! I've also learned that some plants benefit from longer days and cooler summers (towards north) in a matter of growth and overall appearance (especially perennials). For example monarda which tends to grow bigger bloom for longer in higher latitudes.
ReplyDeleteRegards to sweet peas I just grew them for the first time last year and I had a biggest flush of blooms in late september-october, after a little break in august. But they bloomed well in high summer too. Which reminds me that it is time to sow them again next month!
You are right, many plants dislike hot summers. At my parents where summers are much cooler, many things flower much longer and some (delphiniums, aconites) are "happier".
DeleteIt's interesting about cosmos as I thought like you that they flowered later as I planted them a bit later. Once triggered ny lengthening days do those plants stop flowering when days shorten or do they continue/
ReplyDeleteI don't know but my experience would be that some that require long days continue to bloom as days get shorter but at a slower rate.
DeleteWhat I find most useful is that for instance I never had much success with sweet peas but I can see now it is because I started them too late. By the time they were ready to blooms days were getting too short. I always thought it was because my soil was too poor.
I don't know about cosmos but some light sensitive short day chrysanthemums not only need short days to initiate flower buds they need continuing short days to flower. A consequence of this is some varieties 'stop' themselves with a bud in the short days of Spring and as the days lengthen the bud aborts and the shoot branches, subsequently initiating and flowering in Autumn!
DeleteThen again Summer flowering chrysanthemums are sensitive to temperature and stage of development and not day length.
You know what a bore I am Sue!
Very interesting post Alain.
Very interesting Roger. Chrysanthemum is certainly a good example. It seems to me that some need even shorter days than others. Is this the case? I have one that starts blooming so late that I was thinking of getting rid of it (because by the time it blooms it is getting too cold and it freezes).
DeleteSuch an interesting post which made me thinking about the flowers in my garden. I knew of course about the Morning Glories and when I think of Cosmos, yes indeed, but Fuchsias they start flowering with the long days but they go on and on till rather short days, sometimes even in November.
ReplyDeleteAnyway a great post and I shall pay more attention to this phenomenon.
All I read is that professional growers get Fuchsias to bloom earlier with artificial light because they are long day plant. I suppose once they get going they continue even when days are short.
DeleteIt's so interesting that plants have their own 'brain' for knowing exactly what they need to grow and bloom. I always think their blooms just depend on heat and moisture and usually forget about day length. Thanks for the reminder. :o)
ReplyDeleteIsn't it? You appreciate them even better. It is amazing how much adaptation has been necessary to create plants for all conditions - shade, dry spot, etc..
DeleteVery interesting! I always thought of Zinnias as being later bloomers because they need warmth, I never considered that they also need shorter days.
ReplyDeleteI had never thought of day length for garden plants either. I think I will plant some zinnias close together in the vegetable garden and put a cardboard box over them in late afternoon. I will plant some I won't cover and see if there is a difference.
DeleteHello Alain, that's very interesting, but all too much to keep in my head. It's not something that's going to appear on a plant label too. I'll just have to wait for the results of your experiments this season - particularly with the spinach, which is a prime candidate for bolting.
ReplyDeleteFascinating stuff Alain, commercial pot-mums are grown using a combination of variety and day length to control production. As an example a 10 week short variety planted 1st March requires lighting for 3 weeks to get the plants established and this is then replaced by shading to initiate buds and flower development until the buds show colour. The market date in this case would be 31st May, 10 weeks from the start of shading.
ReplyDeleteInteresting. I noticed that most of the information on this topic was meant for growers. I still find it strange to see blooms on mums in late May.
DeleteWhat you write certainly makes common sense, and yet I hadn't really considered the length of daylight as a factor. I think your spinach experiment will be interesting. Temperature surely plays a role in causing spinach to bolt. Shortening the daylight hours may not be enough to slow it going to seed. At any rate, it should be interesting to experiment.
ReplyDeleteAs you say, it will be interesting to see. I think I will try it both with zinnias and spinach with control groups.
DeleteA fascinating post, I have learnt something. I didn' t realise the importance of the length of daylight. I always imagined that it was to do with warmth. Now I know I can' t imagine why it never occurred to me.
ReplyDeleteI never gave the topic much thought either. I suppose that in fact every plant is influenced by the amount of light as well as the heat and cold.
ReplyDeleteJust beautiful flowers ! Cosmos is one of my favorite flowers !
ReplyDeleteGreetings