The small green, leaf-like flaps at the base of a flower are sepals, the flower’s outermost layer.1Encyclopedia Britannica. sepal Their main job is to wrap and protect the bud before it opens.2PubMed. Sepal They are far from leftover packaging, though. In some plants they feed the flower and the fruit, and in others they are the showy part people mistake for petals.3Photosynthetica. Photosynthetic performance of vegetative and reproductive structures of green hellebore (Helleborus viridis L. agg.)4PubMed Central. How to lose a whorl: the evolutionary and developmental biology of apetaly
Where Sepals Sit on a Flower
Flower parts are usually arranged in rings, one inside the other.5Encyclopaedia Britannica. calyx From the outside in, these are the sepals, the petals, the pollen-making stamens, and the seed-making center.6PubMed Central. Evolving Ideas on the Origin and Evolution of Flowers: New Perspectives in the Genomic Era Together the sepals make up the calyx, the usually green outer ring of the flower.7Encyclopaedia Britannica. calyx Not every flower has every ring.8Encyclopaedia Britannica. calyx
In some flowers each sepal is a separate piece.9Encyclopaedia Britannica. calyx In others the sepals are fused into a tube or cup with small teeth or lobes at the rim.10Encyclopaedia Britannica. calyx If the green parts under a bloom are fused, count the teeth at the rim to see how many sepals made it.11PubMed Central. Evolving Ideas on the Origin and Evolution of Flowers: New Perspectives in the Genomic Era
On a rose, the five green flaps that peel back from the bud as it opens are sepals. On a tomato or strawberry, the green star on top of the fruit is the calyx that stayed on. On a carnation, the sepals form a long green tube holding the petals.
Sepals are easy to confuse with leaves, but they are attached to the flower itself, not along the stem. They come in a ring of usually three, four, or five, while leaves usually grow singly or in pairs along the stem.
A quick check is to turn a bloom over and look at the outermost ring beneath the petals, whatever its color. That is the sepals. On a many-petaled flower like a rose, they sit folded back under the petals at full bloom, so a bud is the best time to see them. Some flowers, strawberries among them, have a second, smaller ring of green bits just outside the sepals. It looks like extra sepals but is a different structure.
If a flower seems to have no sepals, the plant may have dropped them very early, sometimes as the bud opened. In others the sepals are so small, thin, or hairy that they look like fuzz. A missing ring is normal variation for that species, not a sign of disease or poor care.
Protecting the Flower Bud
Inside the bud are the parts that will make pollen and seeds.12Encyclopedia Britannica. sepal13PubMed. Arabidopsis floral buds are locked through stress-induced sepal tip curving Once the flower opens, sepals often support the petals from below.14PubMed. Sepal
Some sepals lock the bud shut. In a common lab plant related to cabbage, the tip of each sepal curves inward early into a tiny stiff hook, and the hooks help keep the bud closed until it is ready to open.15PubMed. Arabidopsis floral buds are locked through stress-induced sepal tip curving As the flower opens, the edges of sepals and petals grow faster than their centers, which pushes them outward.16AskNature. How Uneven Growth Makes Blooms Burst Open — Biological Strategy — AskNature
Chewing insects and egg-laying pests go for buds because the tender parts inside are soft, and the sepals are the first barrier they meet. Sun and wind can dry out a young bud, and the sepals’ outer surface slows that water loss.
Some sepals defend with water. In some tropical plants the fused sepals form a cup filled with liquid that submerges the bud.17PubMed Central. The benefits of bathing buds: water calyces protect flowers from a microlepidopteran herbivore In one such plant, moth larvae destroyed almost a quarter of the flowers by eating buds, and keeping the bud under water cut moth egg-laying and damage by about half.18PubMed Central. The benefits of bathing buds: water calyces protect flowers from a microlepidopteran herbivore
As a bud begins to open, the sepals split apart at the tip first, showing a stripe of the petal color before anything else. On a rose, they keep pulling back until they fold down under the petals. Seeing them loosen is a useful sign that the bud will open soon.
Sepals That Make Food
Sepals are the flower part most often able to photosynthesize.19PubMed Central. Pathways of Photosynthesis in Non-Leaf Tissues Building a flower costs the plant a lot of energy, so sepals help recapture carbon the bud burns while growing inside.20PubMed Central. Pathways of Photosynthesis in Non-Leaf Tissues In some daisy-family plants, this activity was highest at the bud stage.21PubMed Central. Pathways of Photosynthesis in Non-Leaf Tissues
Sepals make sugar the same way leaves do, using chlorophyll to turn light and carbon dioxide into food. That is why sepals are usually green even though they are not leaves: the color is the working machinery.
In some plants sepals feed far more than the flower. In green hellebore, the sepals work at about 80% the rate of mature leaves.22Photosynthetica. Photosynthetic performance of vegetative and reproductive structures of green hellebore (Helleborus viridis L. agg.) So in early spring, before its leaves appear, its sepals supply over 60% of the plant’s food.23Photosynthetica. Photosynthetic performance of vegetative and reproductive structures of green hellebore (Helleborus viridis L. agg.) Hellebore’s “petals” are really sepals, which is why they stay on for weeks, gradually turning greener as seeds develop.
In another woodland plant, larger sepals meant bigger fruit and heavier seeds.24PubMed. Function of leafy sepals in Paris polyphylla: photosynthate allocation and partitioning to the fruit and rhizome Darkening or removing the sepals made fruit smaller.25PubMed. Function of leafy sepals in Paris polyphylla: photosynthate allocation and partitioning to the fruit and rhizome For growers, that means leaving green sepals on developing fruit such as strawberries and tomatoes, since removing them takes away a small food source.
When Sepals Look Like Petals
Many plants without true petals have colorful, petal-like sepals instead.26PubMed Central. How to lose a whorl: the evolutionary and developmental biology of apetaly Buckwheat’s showy flower parts are sepals that attract insects.27PubMed Central. How to lose a whorl: the evolutionary and developmental biology of apetaly Where pollinators are scarce, buckwheat puts more resources into them.28PubMed Central. How to lose a whorl: the evolutionary and developmental biology of apetaly Some sepals also carry scent glands or nectar spurs, as in jewelweed.29Master Gardeners of Northern Virginia. Perianth, Calyx, Corolla, Petal, Sepal, Tepal
Hydrangeas are a familiar case. The colored “petals” are sepals. Aluminum in acidic soil triggers pigments that turn them blue, which is why gardeners push hydrangeas toward blue by adding aluminum sulfate to acidic soil.30PubMed Central. Comparative Transcriptome Analysis Unveils the Molecular Mechanism Underlying Sepal Colour Changes under Acidic pH Substratum in Hydrangea macrophylla
To tell colorful sepals from petals, count the rings from the outside. Sepals are always the outermost ring, so colored parts outside everything else are sepals. If a flower shows only one ring of color, those parts may be sepals, though some flowers have petals only, so this is a clue and not a rule. Tulips and lilies have tepals, flower parts that look like both a sepal and a petal, so there is no clear dividing line to find. Clematis and anemone blooms are made of colored sepals, even though they look like ordinary petaled flowers.
Bracts are another source of confusion. These are modified leaves that sit below the flower, not part of it. The red of a poinsettia is bracts, and the actual flowers are the small yellow clusters in the middle. Sepals attach directly at the base of the flower, so a colored part further down the stem is more likely a bract.
What Happens to Sepals After Blooming
Plant hormones decide when parts drop. The hormone ethylene triggers flowers and their parts to fall.31Journal of Experimental Botany. Abscission of flowers and floral parts Another hormone makes them less sensitive to ethylene.32Journal of Experimental Botany. Abscission of flowers and floral parts In spinach, pollination triggers sepals to stay on and keep growing.33PubMed Central. Auxin regulated metabolic changes underlying sepal retention and development after pollination in spinach
In some plants the sepals become fruit covers. In tomatillos, ground cherries and Chinese lanterns, the calyx swells into a papery husk around the fruit, protecting it while it ripens.34PubMed Central. Multifunctional Contribution of the Inflated Fruiting Calyx: Implication for Cuticular Barrier Profiles of the Solanaceous Genera Physalis, Alkekengi, and Nicandra
Plenty of familiar fruits keep their sepals. The small crown at the end of a blueberry is the remains of the old sepals. The green cap on an eggplant is an enlarged calyx, and a pomegranate’s crown is also left from the calyx.
Sepals also tell you something on cut flowers. On a rose bud, sepals curling back mean the bloom is starting to open. Browning or shriveling sepals point to an aging bloom that is past its best. Ripening fruit gives off ethylene, which can make flowers age and drop faster, so keep bouquets away from the fruit bowl.
Why Every Flower Starts With Sepals
One set of genes alone tells a flower bud to make sepals.35PubMed Central. Evolving Ideas on the Origin and Evolution of Flowers: New Perspectives in the Genomic Era Adding other genes turns the rings into petals, stamens, and the seed center.36PubMed Central. Evolving Ideas on the Origin and Evolution of Flowers: New Perspectives in the Genomic Era When a key group of genes is switched off, every part of the flower grows as a sepal.37PubMed. B and C floral organ identity functions require SEPALLATA MADS-box genes So sepals are the flower’s starting template, close to a leaf.
This helps explain odd flowers in gardens. Double flowers can arise when genes shift so that parts such as stamens turn into extra petals. Green-flowered roses show petals that have turned into sepal-like parts.