1. Wild Rose Blooms Signal When to Harvest Soapberries
Wild roses and soapberries are separate plants. Yet, the blooming of rose flowers serves as a signal to harvest soapberries. Records indicate that Indigenous people in Canada use this method [1]. One possible reason is that both plants grow based on the warmth of the same year. In warm springs, roses bloom early; in cold springs, they bloom late. The fruit also progresses according to that year’s weather. The flower acts like a clock hand showing how far the season has progressed. We can call this concept an "indicator plant."
Does this mean standard calendars are useless? To check this, let us look at examples of flower markers used in gardens.
2. Lilacs and Chicory Bloom as Signals for Garden Planting Tasks
Horticultural information from the University of Wisconsin-Madison lists tasks based on flowers. When lilacs are in full bloom, it is time to plant beans, cucumbers, and pumpkins [2]. The opening of the first chicory flowers marks the right time to prevent squash vine borers from damaging pumpkin vines [2].
Ordinary lilacs are considered a central plant for yearly comparison in observations. Because you can look at the same type of flower every year, you can say, "This year is X days earlier than last year" [2]. Can flowers replace calendar dates?
3. Why Calendar Dates Stay Fixed but Flower Bloom Times Shift Each Year
March 10 arrives on the same day every year, whether spring is warm or cold. However, the days when flowers bloom or insects appear change each year. Iowa State University Extension explains that plant and insect events tend to align with the heat accumulated up to that point [3]. The overlap between lilac blooming and the appearance of certain pests is one example of this trend [3].
Accumulated heat refers to adding up daily temperatures. These materials use a count called "growing degree days." Flowering often corresponds to a specific range of accumulated temperature [3]. In this way, flowers act like thermometers reading an addition scale.
Location also affects flowering times. The University of Wisconsin-Madison materials introduce "Hopkins’ Bioclimatic Law," which states that spring events happen later as you go north and east. As a rule of thumb, it is delayed by about 4 days for every degree of latitude north, and about 1.25 days for every degree of longitude east [2]. A difference of 5 degrees in latitude means about a 20-day difference. This is an empirical rule from North America and does not necessarily apply exactly to Japan.
4. Why Temperature Alone Cannot Explain Flower Timing
Iowa State University Extension notes that the growing degree day method relies only on temperature. It does not include rain, soil moisture, or daylight length [3]. Also, a garden’s microclimate may differ from the temperature at a nearby weather station. Therefore, these markers should be used as guides and combined with direct observation [3].
Furthermore, flower markers have strengths and weaknesses. The standards for color and hardness when identifying ripe fruit vary by plant species. There is no common standard for deciding which wild plant is the most reliable indicator. This is likely why local markers born from long-term observation, like the wild roses used by Indigenous people in Canada, are strong in their specific regions.
5. How a Flower Becomes Fruit After the Petals Fall
Fruit is what the ovary becomes after a flower is fertilized; it swells and forms the part that contains and protects seeds [4]. After petals close or fall, if the ovary begins to swell, it is a sign that fruit development has started [4].
In observations by the USA National Phenology Network, stages like flowers, swelling fruit, and ripe fruit are judged separately as "present" or "absent." This is because stages can overlap [5]. It is possible to record that on the same plant, flowers are still blooming but there are no ripe fruits yet.
There are rules for what counts as a "ripe fruit." For example, fruit from a capsule that has split and dropped all its seeds is not counted as ripe [5]. In University of Maine observation rules, wild strawberries are considered to have "ripe fruit present" if at least one ripe fruit is visible on the plant [6]. Fruit that has fallen or been taken away is recorded in a separate category [6].
These records accumulate. In Mount Rainier National Park in Washington State, citizens have participated in observing 17 species of wildflowers. They recorded four stages—buds, blooming, fruiting, and seed dispersal—at 28 sites. From 2013 to 2019, over 42,000 observations were collected [7].
6. How to Track Your Local Flowers' Bloom Day and Fruit Swelling
Pick one flower you see every year in your neighborhood. Write down the day it starts to bloom and the day petals fall and the swelling ovary is visible. This alone creates a one-year "flower calendar" for your town.
If you record the same flower the next year, you can see yearly shifts. If you compare this with spring temperature records, you can check yourself if flowers come earlier in warm years. This is the same thinking as using lilacs as a comparison standard in University of Wisconsin-Madison materials [2].
Observe wild fruits only; do not eat them. Observe in places where you are allowed to enter, and go out with an adult.