As one of the lessons from the Floret Online Workshop, Erin Benzakein covered in 2022 Module 6, Video 1 how to harvest zinnias (minute 4:07 is the beginning). She, along with many others, use the “wiggle test” to identify stems that are ready to harvest to keep the flower-heads upright. This test evaluates the stiffening of the stem near the flower head. This is a great test to evaluate the readiness for harvest, but with many types of zinnia, it may be only one sign.
Let’s begin with a quick overview of the anatomy of a zinnia. Zinnia flowers are considered composite inflorescence, meaning that the flowers are made up of ray and disk flowers. Another composite inflorescence is the sunflower.
The petal structure is called the capitula and the sepal-like attachment to the stem is called the phyllary.
Favorite cultivars to grow in the cut flower industry are the cupcake types of zinnia such as the Queen Lime series, Oklahoma series and the Lilliput series. The cupcake (fully double) capitula is made up of many more ray flowers than disk flowers in a cascading ring of petals. The phyllary of cupcake types are imbricate – meaning that it has overlapping bud scales that partially overlap and conceal the inner scales.
For ripeness to occur, the ray florets need to fully unfurl. As shown in the examples below, the capitula is not yet fully open AND the ray flowers are curled. There also is a visible center.
To harvest, wait until nearly all of the visible ray florets are open and there no longer is a prominent center of disc florets. This is essential for cupcake-types since zinnia flowers not continue to open once cut from the plant. Blown zinnia heads will no longer have a center and will be shaggy in appearance.
The imbricate nature of the phyllary is in large part the cause for my picking zinnia too early. Because of the “overlapping tiles on a roof,” it is hard to determine if there are more ray flowers to emerge in the capitula. In my citizen scientist findings, the ripest zinnia that hit their prime flower head without losing their color or vase life were those with nearly flat phyllaries. Caution: harvesting cupcake-type zinnias with truly flat phyllaries (blown or over-ripe blooms) resulted in less vibrant colors and shorter vase lives.
Ripe flowers had nearly flat phyllaries; less than 0.25 inches in height.
Unripe zinnias with phyllaries more than 0.25 inches (more than half the bloom height) often failed the wiggle test.
Stem features & the wiggle test
Zinnia stems are hollow to allow for the passage of water and nutrients from the roots to the rest of the plant. However, hollow stems are not great at holding up heavy flower heads! Thus, the “wiggle test” is evaluating the plant’s maturity in reinforcing the stem nearest the flower. Specifically, the strength is gained by the formation of xylem in the stem or xylogenesis. Strangely, the greatest amount of research on xylogenesis is in zinnia elgans!
To do an effective wiggle test, hold the stem below the second set of leaves (8 inches) closest to the flower. Gently shake the flower back and forth. If the flower head bends each way, it is not ready yet to harvest. If the stem holds the flower head straight, then the stem is mature.
Also, gently squeeze the stem. Do not make the hollow stem collapse, but test to ensure that it is hardened. This hardening should go up the stem to approximately 2 inches below the phyllary.
Zinnia bud age
At this time, I have insufficient data to share the zinnia age from bud to mature flower for fully double, cupcake-type zinnias. Early trends show that it takes 14 days for cupcake-type zinnia flowers to mature.