Flower Development and Time

Why does a person who suffers from ragweed pollen allergies only suffer during a specific time in the summer? The answer is that the ragweed plant produces pollen only during flowering and flowering occurs during a limited and specific time of the ragweed plant’s life. While this is a good thing for the ragweed allergy sufferer, the timing of flower development can have an impact on the appeal of a landscape plant or the success of seed production in a crop plant.

We can think of the time-limited aspect of flowering in two ways. First, a plant may have a very specific time period in which it produces flowers or the plant may flower over weeks or months. Second, the development of any individual flower on a plant involves many steps and stages but the period of time in which the anthers are shedding pollen or the stigma and ovaries are receptive to pollen is limited to several hours.

The time period at which a plant will produce flowers is often tied to the plant’s growth pattern and life cycle. For example, many plants produce flowers over a time period that lasts only several days. With perennial plants this is often during the early part of the growing season so the plant has time to produce seeds, the seed can disperse and the seedlings that grow from these seeds can become established. In contrast to this, other plants such as impatiens and beans will produce flowers and bloom over a longer time period. Plants that continue to produce flowers during a growing season are called indeterminant while determinant plants have a more specific period of flowering.

Perennials bloom during a specific time period. For instance, these Black-eyed Susans flower in a display of beautiful yellow and black flowers in mid summer.... Image by Kim Todd

...whereas mums create a display of many small white and yellow petals on small flowers in the fall. Image by Kim Todd

Annuals like these pink impatiens can bloom all season long. Image by Don Lee

Grain crops that are grasses tend to flower during a short time period. Some grain crops planted in the spring (corn, wheat, barely and oats) will flower by midsummer. In contrast, sorghum and pearl millet flower in late summer. Some soybean varieties will flower continuously for 7 to 11 weeks after emergence.

Oats in a South Dakota field produce mature grain, which makes the field a light brown... Image by Don Lee

...and are harvested mid summer while other crops are still flowering, leaving short stubble after harvest. Image by Don Lee

The development of any given flower on a plant always follows a specific sequence of stages. Early stages allow the stamen and pistil to develop from a series of regulated cell division and cell differentiation steps. These steps occur inside the plant or in an inconspicuous bud. The actual blooming of the flower is the dramatic final chapter in the flower’s life. In a few hours or a couple of days, the sepals and petals will enlarge and open and this is timed with the maturity of the stamen and pistil. Timing and opportunity are critical for reproductive success. When pollen is shed at the same time that the stigmas are ready to receive it, fertilization can occur. Successful sexual reproduction will result in seed development. If the pollen fails to land on a receptive stigma during this critical window of receptivity, no seed will develop.

A milkweed plant with the most developed flowers at the bottom. More developed flowers are typically more colored, are more open, and are larger than less developed flowers. Less developed flowers start as tight, small, green bundles of cells until they more developed and begin to open. Image by Don Lee