As we approach summer and flowers begin to bloom again, it may seem as though some types of plants follow an internal calendar that lets them know what time of year to produce flowers. That’s because they do – sort of. Plants have many different ways to sense their environment and use all of that information to tell what time of year it is. While plants may not have a sense of time, they sure seem to know how to keep track of it.
Different plants have different requirements for blooming – that’s why some start blooming in early spring while others wait until late summer. This post will go over some of the ways that plants keep track of the time of year as well as some examples of local invasives’ flowering times. This blog post is part of a series of posts called How Do Plants Work?
Photoperiodism
One of the most important mechanisms in the plant for determining their flowering time is photoperiodism (wow, try saying that three times fast!), or reacting to the length of night. Plants sense the lengthening or shortening of days in the same way that they sense the light’s direction, quality, and intensity, and can tell whether summer is beginning or ending.
Some species, like Yellow Lamium, use lengthening days, as well as the light quality and ambient temperature to tell when summer is beginning; that’s their cue to begin flowering. Other plants, like Chicory, use the shortening days of late summer as their cue to start producing flowers. Not all plants depend on day length to begin flowering, either: some continuously flower all summer, depending mostly on temperature.
Determining Flowering Time
The timing of plants’ flowering is based on a species’ individual needs. In some cases, a species tries to have its individual flowers come out simultaneously, like with Cherry Blossoms in Japan. In this case, the flowering time is so precise that tourists can plan their visits around it. Cherry Blossoms have evolved to flower simultaneously to maximize cross-pollination (i.e., pollination between plant individuals), rather than resorting to self-pollination (which would be more likely if plant individuals flowered at scattered times).
In other cases, the time of year a plant flowers depends on some environmental cue. This may be waiting for a certain temperature, moisture level, or day length to flower, as certain environmental cues may result in the best conditions for their future seeds.
The longer a plant can hold off before beginning to flower, the more starch resources it can accumulate from photosynthesis to build its seeds with. The more starch is accumulated, the better the quality of the seeds, and the more likely they are to germinate. However, holding off on flowering runs the risk of the plant dying before it is able to reproduce.
Since they don’t put a lot of energy into creating their seeds, annual plants tend to flower only a few weeks after popping out of the soil. Some perennials, on the other hand, may wait years before flowering. All plants go through three phases of growth:
- the juvenile phase, in which they aren’t able to make reproductive structures yet;
- the adult vegetative phase, in which they are able to make reproductive structures but haven’t started yet; and
- the adult reproductive phase in which they make reproductive structures.
In perennial plants, the juvenile and adult vegetative phases can be quite drawn out.
Examples of growth phases in invasive plants
English Ivy
English Ivy remains in its juvenile phase for around ten years before going through what is essentially plant puberty. In its juvenile phase, it acts as a trailing groundcover that crawls over and chokes other plants with 3-5 lobed leaves. As it enters its adult reproductive phase, English Ivy leaves begin to grow unlobed, the climbing vines begin to get thicker and woodier, and they develop small greenish-yellow flowers that mature into round fruits that are toxic to humans and animals.
Himalayan Blackberry
Himalayan Blackberry, as a biennial, remains in its juvenile phase for its first year of growth, transitioning to its adult reproductive phase for its second year. In its first year, its leaves are made up of 5 leaflets. Himalayan Blackberry spends this first year building up its starch reserves and spreading rapidly. In its second year, it transitions to leaves made of 3 leaflets and begins to form large clusters of white to pink flowers that mature into the large, juicy berries that this species is known for.
Vernalization
In order to flower, some plants require vernalization, or being exposed to the cold as a seed or juvenile plant. For instance, Hound’s-tongue seeds require a period of cold before they begin to germinate in March and April.
This requirement often occurs in perennial and biennial plants, especially if they produce large fruits like apples. Vernalization causes plants to enter a dormant phase in which they can funnel all of their energy toward resource collection. This promotes flowering once it warms back up to a certain temperature.
Unfortunately, climate change is heavily impacting vernalization in some species. Without a cold spell, the plants are unable to begin flowering, and can’t reproduce as a result. Flowering times that have traditionally been like clockwork are becoming less reliable as plants’ normal cues begin to occur at more and more abnormal times. This, of course, is true for invasive as well as native plants!
References
- Taiz, L., Zeiger, E., Møller, I. M., & Murphy, A. S. (2015). Plant Physiology and Development (6th ed.). Sinauer Associates Inc.
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Species mentioned in this post
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