Day Length Plants

On May 23th, our little plot will officially receive 14 hours of daylight. And that will be true until July 21st. This little bit of information, strangely, matters quite a lot! Many plants are day-length sensitive. This means they initiate flowering when the day-length is 14 hours or more, or they flower when the days are shorter in length. To complicate things, some plants MUST have a certain day-length while others are simply affected by it as a preference. Some plants need only 10 hours of day-length, some prefer 12+ hours. Some will go ahead and flower anyway if the temperature is hot or cold enough even if the day-length is not what they prefer. Usually, there's some downside, like shorter stems or poorer quality. Plants can be picky :D


The rich colors of cosmos.

Some of the plants we grow which have day-length sensitivity are:


Sunflowers - many sunflowers prefer 14 hours. If you plant them after May in the Texas heat, chances are they'll bloom sooner for you than your seed packet indicates.


Asters - they are tricky. They like to grow during long-hour days and flower under short-days that are cool. This means if you can nurse them through our summer heat, you should get beautiful fall blooms.


Cosmos - they also prefer to flower under short-days but they like it hot. If you plant them after May, you'll only get bushy growth but wonderful flowers once August hits. Before that, you may get flowers now, on tiny plants barely grown.


Foxglove - they like longer days but cooler weather. This means if we have a hot spring, not so good. But a cool spring will give us a nice one-time harvest. Those lucky northern gardeners often get a second harvest because it's still cool for them as the days grow long. No such luck here!


Strawflower - they also like longer days but can handle warmer temperatures than the foxglove. Our strawflower is really really tall but refuses to bloom until the days get longer! We've been patiently waiting and while it looks like we'll get nice strawflower on super long stalks, we'll likely only get one harvest of it and not a secondary harvest. Since space is short, there's some question as to whether we'll give it space next year. :)


Strawflower waits to bloom until the days are long.

Stock - Likes to flower under long days but cool temperatures. That's really tricky here because the warm short-days often trigger flowering on really short plants. We're working with different fall plantings in the hoop house and different varieties to find the best approach.


Snapdragons - depending on the variety, they flower under shorter days and cooler temperatures but only after a long juvenile period. So we have to make sure to plant early enough before the spring heat comes!


Statice - it likes to grow during long-days and flower during short days and warm weather. That's a super big problem here :) Unless you grow it for more than a year, or (like we did) grow the seedlings under a grow light early in the year to give it long-days and then transplant it outside during our short-day, cooler weather early spring. So I guess you could say we tricked it. *snicker* But it's giving us flower stems! We'll see if it noticed our trickery and how tall and pretty the flowers are. We're not sure yet who'll get the last word.


Basil - even if you were to grow these in the warmest heated greenhouse, unless they get 10 hours of daylight, they will not like you (honestly, sometimes I feel that way also :) Thankfully, we're now at 12+ hours of daylight and they're super happy.


Rudbeckia (black-eyed susan) grows vegetatively during short days and flowers when days are long. If you plant it out too late, it will bloom on very short stems because the day length is long.


All of this can make your head spin a little. But in another life, I was a data-analyst so I'm a nerd that likes all of this :) It might be a bit much for a home gardener to worry about but if you've ever wondered why some of these plants were being a little weird for you, this post might explain why. If you're really nerdy, like me, and want to read some technical data on these and other day-length sensitive plants, this study by Purdue is excellent: https://www.extension.purdue.edu/extmedia/HO/HO-249-W.pdf


Happy Growing!

Sarah Jo

Everbloom Fields


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