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Horticulture Principles and Practices

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<strong>and</strong> plants that flower under only certain day-length conditions are called photoperiodic.<br />

Four photoperiodic responses in plants are a basis for classifying horticultural plants.<br />

1. Short-day plants (or long-night plants). Short-day plants will not flower under<br />

continuous light. They require a photoperiod of less than a certain critical value<br />

within a 24-hour daily cycle. For example, strawberry (Fragaria x ananasia)<br />

requires 10 hours of light or less <strong>and</strong> violet (Viola papilionacea) requires<br />

11 hours. Poinsettia (Euphorbia pulcherrima) requires 12.5 hours of daylight <strong>and</strong><br />

cocklebur (Xanthium strumarium) requires about 16 hours or less of light. When<br />

planted in the field, short-day plants flower in early spring or fall (Table 4–3).<br />

2. Long-day plants (or short-night plants). Long-day plants are plants that flower<br />

only when light periods are longer than a certain critical length (Table 4–4).<br />

These plants flower mainly in summer <strong>and</strong> include annuals such as henbane<br />

(Hyoscyamus niger), which requires more than 10 hours of light, <strong>and</strong> spinach<br />

(Spinacia oleracea), which requires 13 hours of light. Baby’s breath (Gypsophila<br />

paniculata) requires 16 hours or more of daylight in order to flower.<br />

3. Day-neutral plants. Day-neutral plants are not responsive to photoperiod <strong>and</strong><br />

flower according to the developmental stage. Plants in this category include<br />

tomato, corn, <strong>and</strong> cucumber (Table 4–5).<br />

4. Intermediate-day plants. Certain grasses such as Indian grass do not flower if the<br />

days are too short or too long. These plants are said to have two critical photoperiods<br />

<strong>and</strong> are categorized as intermediate-day plants.<br />

The Role of Darkness in Photoperiodism Photoperiodic plants in actuality track<br />

or measure the duration of darkness or dark period rather than duration of light. Thus,<br />

short-day plants (or long-night plants) flower only if they receive continuous darkness<br />

for equal to or more than a critical value (Figure 4–7). If the dark period is interrupted<br />

by light of sufficient intensity for even a minute, flowering will not be induced. Similarly,<br />

a long-day plant (or short-night plant) will not flower if the critical duration of darkness is<br />

exceeded. However, if a long-night period is interrupted by light, flowering will be induced.<br />

TABLE 4–3<br />

Plant<br />

Selected Short-Day Plants<br />

Scientific Name<br />

Chrysanthemum<br />

Gardenia<br />

Poinsettia<br />

Kalanchoe<br />

Bryophyllum<br />

Orchid<br />

Strawberry<br />

Violet<br />

Chrysanthemum x morifolium<br />

Gardenia jasminoides<br />

Euphorbia pulcherrima<br />

Kalancho blossfeldiana<br />

Bryophyllum pinnatum<br />

Cattleya trianae<br />

Fragaria x ananasia<br />

Viola papilionaceae<br />

TABLE 4–4<br />

Plant<br />

Selected Long-Day Plants<br />

Scientific Name<br />

Baby’s breath<br />

Spider plant<br />

Sedum<br />

Evening primrose<br />

Bentgrass<br />

Fuchsia<br />

Rex begonia<br />

Gypsophila paniculata<br />

Chlorophytum comosum<br />

Sedum spectabile<br />

Oenothera spp.<br />

Agrostis palustris<br />

Fuchsia x hybrida<br />

Begonia rex<br />

4.2 Aboveground Environment 105

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