What Makes Flowers Bloom?

Most flowering plants bloom in order to perpetuate themselves. It is their way of attracting pollinators so that they can set seeds to produce a new generation. Some plants flower with such abandon that it seems as if their only purpose is to fill the world with color. Gardeners sometimes say that such a plant “flowers its head off.” Azaleas, roses, and French marigolds all demonstrate this ability. Yet even the most prolific plants require certain conditions in order to flower freely, or indeed at all. The following factors can seriously affect the performance of flowering plants.

Sunlight

The most important influence on flowering is sunlight. Most flowering plants require at least six hours of sunlight a day to flower. Even the small group of plants that will tolerate light shade—such as impatiens, begonias, and coleus—will not perform well in a heavily shaded garden. Sunlight is so critical that in some cases a 1 percent increase in light levels produces a 100 percent improvement in flowering. Removing a tree limb or painting a fence white may be all it takes to enable sun-loving plants to flower in your garden.

Temperature

Another influence on flowering is temperature, particularly nighttime temperature. Many flowering plants will survive high noontime heat provided that they have a cool respite during the night. Snapdragons and nasturtiums are examples. Yet other flowering plants are inhibited by low temperature; zinnias, hardy hibiscus, and other plants from southern climates need warm, sunny weather to flower spectacularly.

When certain flowering shrubs fail to bloom, the problem can sometimes be traced to a severe late frost that entered the bud sheaths and destroyed the flowers, yet spared the leaves. Called bud blast, this is a particular problem in early spring, when premature warming can cause buds to break dormancy, only to be hit by a cold blast before they open.

When hardy perennials and hardy bulbs fail to come back the following year, the cause is generally a period of alternating freezing and thawing. During a thaw, dormant plants may start to grow, making them vulnerable to damage by an unexpected freeze.

Day Length and Time of Day

The blooming of many flowers is influenced by day length and in some cases by time of day. For example, chrysanthemums start blooming as days shorten in autumn. Morning glories close in the afternoon and stop blooming entirely as day length dwindles. The moonflower, a type of morning glory, opens only during the evening and part of the morning.

Watering

All plants need water. Although many flowering plants tolerate drought, a regular supply of water will usually prolong their flowering, especially the flowering of plants such as impatiens and wax begonias, which enjoy cool soil. Since roots can absorb nutrients only in soluble form, they cannot take up these nutrients without water.

In small gardens, people are often quite content to water on an as-needed basis: When a plant shows signs of wilting, they douse it with a garden hose or sprinkler. However, a plant that shows thirst by wilting is already under stress and may never completely recover from emergency treatment. It is far better to make a habit of watering regularly so that plants never come under moisture stress.

A rule of thumb is to give established plants a good overnight soaking at least once a week in the absence of rainfall and preferably twice a week during prolonged drought.

Soil Fertility

A fertile soil has the proper balance of three essential plant nutrients—nitrogen, phosphorus, and potassium. Flowering plants are generally less greedy in their fertilizer needs than are vegetables, but the application of a granular fertilizer, raked or watered into the soil surface at the start of the season is good insurance. In general, the faster a plant grows, the more fertilizer it needs. If you’re not sure of the fertility of your soil, use moderate amounts of a fertilizer formulated for flowers.

Soil pH

The pH of a soil is a measure of its acidity or alkalinity. Wooded areas often have acid (low pH) soil; deserts tend to be alkaline (with high pH). Although some flowering plants will fare better in one type of soil than the other, most common flowers will perform well in a wide range of soils.

You can determine the pH of your soil by asking a neighbor who gardens, by purchasing an inexpensive soil test kit, or by submitting a soil sample to a laboratory recommended by your county agricultural agent. Some garden centers will test your soil for free. Minor adjustments to soil pH can be made by adding lime (to acid soil) or soil sulfur (to alkaline soil). However, if your soil pH is extreme, adjusting it may be difficult; instead you may choose to plant your garden in raised beds filled with topsoil from a nursery.