Written by Hayley McKay
Illustrated by Sally Kyunga Kim
Spring is in the air—birds are chirping, the weather is warming, and as the proverb goes, ‘April showers bring May flowers.’ Although it can sometimes be dreary, rain in the spring is really important for plants as they start to grow, and without water, they wouldn’t survive!
Seeds of annual plants, like Helianthus annuus—also known as the common sunflower—lie dormant in the ground over the winter. The seeds were produced by the previous year’s sunflowers which die off at the end of the summer, leaving behind their seeds to sprout the following spring.
Sunflower seeds possess almost everything the future plant needs to start growing; the only things missing are warmer temperatures and water! During the winter, the seeds know they shouldn’t start germinating because they can sense the temperatures are too cold for developing seedlings to survive. As soon as the seeds start sensing warmer temperatures and the thawing ground starts to allow water to penetrate, the seeds will begin absorbing water in a dormancy-breaking process known as ‘imbibition.’
During imbibition, the seeds start to expand as water enters the cells of the developing plants. Water allows metabolic processes inside the seeds to increase as the seedlings prepare to germinate. Once the seedlings are ready, the seeds crack and a root (or ‘radicle’) emerges, followed by a stem (or ‘hypocotyl’) and two immature leaves (or ‘cotyledons’) which grow straight upwards towards the light from the sun. At this stage, the seedlings are able to grow with the help of the nutrients packed into the seeds from which they emerged, but these resources are limited. The young sunflowers will have to begin photosynthesizing to make their own food soon!
As the sunflower seedlings continue to grow, they develop more true leaves and become greener—a sign that their chloroplasts are starting to work! Chloroplasts are organelles in plant cells that catalyze the photosynthesis reaction where water, carbon dioxide and light are turned into sugar, oxygen and water. In addition to water and light, plants also need minerals and nutrients like nitrogen, potassium and phosphorus, which they absorb through their roots from the soil.
For a while, the sunflowers will continue growing in a vegetative state—a non-reproductive phase of their life cycle where they increase in size and accumulate resources before they begin blooming. When the time is right (usually when days begin to shorten) the sunflowers will send out a bud. Before the buds open, they engage in what’s known as ‘heliotropism.’ This is when the faces of the buds follow the sun across the sky throughout the day and reorient themselves at night to face east again for the sunrise. Interestingly, this phenomenon only happens while the flowers are immature; once the buds open (a process known as ‘anthesis’), they remain locked in an east-facing position.
Sunflower flowers, like many others in the aster family, are interesting because what we consider one ‘flower’ is actually a collection of hundreds of individual flowers. The flowers are situated in a complex spiral formation starting at the center of the head, following the Fibonacci sequence to allow for the most efficient packing of individual florets mathematically possible.
The flowers on the outside ring of the sunflower head (called ‘ray flowers’) develop one large petal each to form the recognizable ring of yellow petals, but they aren’t capable of producing seeds. Their sole purpose is to act as a bright and colourful beacon to attract bees, butterflies and other insects to come and drink the nectar of the interior ‘disk flowers’ and help pollinate them. These disk flowers are so tiny, it’s hard to even tell they’re flowers! They don’t have the same enlarged petal as the ray flowers, but they do possess functioning reproductive organs that allow each disk flower to produce a single seed when fertilized.
Like most flowering plants, sunflower disk flowers have both male and female reproductive organs, making them bisexual flowers. Fertilization of a disk flower happens when pollen from a male anther (a barrel-shaped organ covered in pollen grains, sitting in the center of the disk flower’s mini petals) is brought in contact with another disk flower’s female stigma (a wispy tentacle-like organ which develops on top of the anther). Once attached to a stigma, the pollen travels down through the floret to the ovule at the base of the flower and fertilizes it, eventually producing a single sunflower seed where the disk flower once was.
Once the majority of the disk flowers have been pollinated and begin to produce seeds, the flower head will start to senesce, an end-of-life process where the tissue starts to dry out and seeds mature. This will usually happen near the end of the summer growing season; the sunflower plant will die, and its mature seeds will be left to either harvest or remain where the plant grew, spending the winter in dormancy and beginning the process all over again the next spring. If left over the winter, decomposing flower heads with seeds also become great natural bird feeders for chickadees and other birds who brave the winter months!
Sunflowers rely on pollinators like bees and butterflies to fertilize their flowers so they can produce seeds for the next generation. However, pollinator populations are on the decline in many parts of the world, mostly as a result of habitat loss and pesticide use. Growing native North American varieties of sunflowers in your garden can help; while they need the pollinators, the pollinators also need them. Many ornamental garden plants are poor sources of food for pollinators because they’ve been bred to be sterile, meaning they don’t produce pollen to reproduce. But the native Helianthus annuus sunflower is rich in nectar and pollen and can help support and promote a healthy pollinator ecosystem.
Not only are sunflowers pretty and eco-friendly, they’re also easy to grow! All you need are a handful of seeds (not the roasted kind you eat) and a fertilized plot of soil: they like to be placed about an inch under the soil and spaced about six inches apart (since they grow to about three meters tall!). It’s best to start planting seeds once overnight temperatures reach a consistent ten degrees Celsius which usually happens in April or May. Make sure the soil stays moist while the seeds are germinating, but don’t overwater once the seedlings start to grow – April showers will probably be all you need to get your May (sun)flowers growing!