A short-day plant is one that flowers when the daily dark period is longer than a critical threshold. The name is a historical accident: experiments in the 1920s varied day length, but it later became clear that the plant measures the uninterrupted night. "Long-night plant" is the more accurate description. Chrysanthemums, poinsettias, soybeans and photoperiod cannabis are all short-day plants.
In practice
Outdoors, photoperiod cannabis flowers in late summer as nights lengthen, earlier at higher latitudes and in cultivars from northern origins. Indoors, growers trigger it by giving a long, dark night: classically 12/12, and for many cultivars 13/11 or more light still works.
The critical day length varies widely by cultivar; hemp studies found flowering triggered anywhere from about 13 to 15 hours of light depending on genotype (Zhang et al. 2021). That's why GenetixBay encourages growers to test their cultivar.
A short night break (light in the middle of the dark period) can hold short-day plants vegetative. That's how mums are kept in veg commercially, and why light leaks matter.
Don't confuse with
- Long-day plant: flowers when nights are short (e.g. spinach).
- Day-neutral plant: flowers by age, like autoflower cannabis.
- Photoperiod: the general day-length response, not the type.
The daily length of light versus dark; photoperiod cannabis flowers when nights grow long enough, rather than at a fixed age.12/12 is the classic flowering schedule for photoperiod cannabis: 12 hours of light and 12 hours of uninterrupted darkness, which triggers and sustains flowering.13/11 is a flowering schedule of 13 hours light and 11 hours dark; GenetixBay endorses it, as most cultivars still flower while gaining extra daily light.Hemp is cannabis grown for fibre, seed or other uses and, in many laws, defined as Cannabis sativa with no more than 0.3% THC by dry weight.The genotype is the set of alleles a plant actually carries, either at one locus or across its whole genome, as opposed to the traits you can see.A light leak is unwanted light entering the grow space during the dark period; in flower it can delay or disrupt flowering in photoperiod cannabis.A day-neutral cannabis plant that starts flowering by age, usually a few weeks after germination, regardless of light schedule; the trait is mainly recessive.A family of plant photoreceptors that sense red and far-red light, measuring night length for flowering and shade for stem elongation.The reproductive phase of the cannabis life cycle, from the switch to flowering until harvest, during which the plant stretches, builds buds and ripens resin.
