September 12, 2015

1 - Temperate climate plants seeds and buds are dormant to survive the winter and need special treatment to grow in the spring.

Yard and Garden September 12, 2015

Q.

I read that I have to store seeds from my apples in the refrigerator if I want to plant the seeds. Is that true? If it is true, why is it necessary? I thought they just fell to the ground naturally and grew.

-Jason R.

A.

You can plant the seeds directly in the ground if you live in a location with sufficient cold weather to chill the ground during the winter. In southern New Mexico, a location on the north side of a structure where the sun does not warm the soil in the winter may be adequate. In northern and high elevation locations in New Mexico the soil even in sunny locations will provide sufficient "chilling".

Seeds of trees and shrubs native to temperate climates with a distinct winter are dormant when the fruit ripens. This means that they will not germinate if they are planted in warm soil. If they could germinate immediately, the tender new plants would freeze and die during the first winter. They are programmed to remain dormant through the winter, waiting to germinate in the spring as the temperatures warm. This process is regulated by metabolism in the seeds (and in the buds of these plants) that produces gibberellic acid, a plant hormone that regulates this type of dormancy. The production of this hormone is essentially a biochemical calendar that measures the length of the winter. Some plants must delay germination for a long winter in their native habitat; some must delay through warming and cooling cycles. The biochemical pathway manufacturing gibberellic acid accounts for these variations in climate. The process counts the "chill units" experienced by seeds and buds and varies for each type of plant based on the anticipated duration of the winter in its native environment. Once sufficient chill units have accumulated, the seeds and buds then begin to accumulate "heat units" to delay germination to avoid late freezing weather.

At temperatures below freezing, the chill unit accumulation process slows dramatically, so freezing time is not counted. At temperatures above 50 degrees the process reverses. A certain number of chill units must accumulate before sufficient gibberellic acid is formed to allow growth. This varies greatly between species. In a refrigerator you can maintain conditions near optimal temperatures for gibberellic acid synthesis (near 39 degrees) so that seeds are ready to germinate after 6 to 8 weeks for most species of temperate trees and shrubs. Seeds may begin sprouting in the refrigerator if they are left too long. Once seeds have received enough chilling in the refrigerator the seeds may be planted in pots indoors and allowed to grow, or planted outside. If it is still cold outside they will wait until enough heat units have accumulated before beginning growth. If grown in pots indoors, they should germinate fairly rapidly and be ready to plant outside after danger of freezing has past.

There are other forms of dormancy for other plants, but this biochemical dormancy is most common in plants from temperate climates that must delay germination until after winter has past.

Marisa Y. Thompson, PhD, is the Extension Horticulture Specialist, in the Department of Extension Plant Sciences at the New Mexico State University Los Lunas Agricultural Science Center, email: desertblooms@nmsu.edu, office: 505-865-7340, ext. 113.

Links:

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