Written by Spirekassen 26 Jan 2023 11:12

Long-day plants and short-day plants

It is the beginning of a new year! We live in a region, where the difference between the seasons is big. Right now, it is winter, and, in a few months, spring is upon us, and the cultivation of plants is speeding up. Both the UK, Germany, and Denmark have a great variation in the number of daylight hours during summer and winter. The more northern you live, the darker the winter half-year is. It is not only us humans who are affected by the length of the day – plants are as well. All green plants use sunlight in their photosynthesis to form energy into growth. The sunlight also has an importance for other elements.


The length of the day matters

The length of the day has great importance to the development of plants and the ability to form flowers. Almost all plants can be divided into three categories. All plants, both vegetables and flowers, can be divided into these three categories.

  • Short-day plants
  • Long-day plants
  • Neutral-day plants

Some plants only bloom when the night is long, and the day is short. Other plants do it the other way around. The last group of plants do not care about the length of the day or night. How exact the length of the day should be, depends on the origin of the plant. If the plant is from around the Equator, the plants can measure the length of the day almost to the minute. Plants who originate further away from the Equator, for example the polar circle, are not as precise.


Good to know

The terms ‘short-day plants’, ‘long-day plants’ and ‘neutral-day plants’ are actually a little misleading. The plants always measure the length of the night.  The measurements happen in a plant protein called Phytochrome. Phytochrome is a sort of pigment system with plants. The pigment system plays a crucial part in anything from sprouting, vegetation, and flowering.

Long-day plants

Every plant below requires a night shorter than the day. Every single species has their own light/darkness combination. Here are a few examples:

  • Dill
  • Fuchsia
  • Potato
  • Sweet pea
  • Snapdragon
  • Pink
  • Radish
  • Salad (some sorts)
  • Spinach (some sorts)
  • Pea
  • Onion (some sorts)
Short-day plants

The mentioned species below all need a night longer than the day. Every single species has their own light/darkness combination. Here are a few examples:

  • Onion (certain species)
  • Poinsettia
  • Begonia
  • Freesia
  • Chrysanthemum
  • Dahlia
  • Strawberry
  • Sage
  • Violet
  • Spinach (some sorts)
  • Salad (some sorts)
Neutral-day plants

The list down below is a list of classical greenhouse plants. In principle, the species can be cultivated all year around because they will bloom, and form fruits no matter how dark or light it is.

  • Tomato
  • Chili
  • Pepper
  • Cucumber
  • Zinnia
Why is this relevant?

It can be quite helpful to know a little about plants and what special needs they have BEFORE you buy them. With a little knowledge you can easily avoid the classical beginner mistakes.

Example 1

You bought a seed for an onion in the south of Europe. You tried sowing them, but the onion won’t evolve. The reason is simple. The correct development of an onion only happens when the combination of night and day is just right. We here in northern Europe do not have the same day and night variation as in the south of Europe. Therefore, you must choose onion seeds that are adapted to the climate zone of northern Europe.  

Example 2

If your wish is to grow salad before midsummer, you have to choose the sort of salad that needs increasing light. After midsummer choose a sort that needs decreasing light. If you do it the other way around, your salad plants will bolt and produce seeds. A salad that seeds is uneatable.

Example 3

The poinsettia is a Christmas flower in the north, but not in Mexico where it originates from. In Mexico it grows wild in the tropical forests. The beautiful colour on the bracts is shown in northern Europe when the length of the night is more than twelve hours, usually around Christmas. If you have your poinsettia for several years, it is absolutely necessary with at least twelve hours in complete darkness for several weeks before Christmas, for the colour to show.  

Example 4

Cucumber, chili, pepper, and tomato can be grown all year around if you have the right conditions for it. They are all neutral-day plants. That means the plants will bloom and fruit all year around.