Plum seeds need a period of cold stratification to break their natural dormancy and allow germination. This cold treatment mimics the natural conditions of a plum pit spending the winter months buried in the soil. Without this necessary chilling period, the seeds contain natural inhibitors that prevent them from sprouting, often resulting in near-zero success rates for germination. This biological requirement ensures the seed does not germinate prematurely during a warm spell in the autumn, which would lead to the death of the tender seedling once true winter temperatures arrive.
Understanding Seed Dormancy and Stratification
Plum seeds, like those of other stone fruits in the Prunus genus, exhibit physiological dormancy. This dormancy is caused by factors within the embryo, primarily the presence of growth-inhibiting hormones, such as abscisic acid (ABA). The dormant state prevents the embryo from expanding and penetrating the seed coverings, even if external conditions like moisture and warmth are ideal.
Cold stratification is the horticultural process used to overcome this physiological block. It involves exposing the seed to a prolonged period of cold temperatures in a moist environment. This sustained chilling initiates chemical changes within the seed, leading to the breakdown of germination inhibitors. Simultaneously, the cold period promotes the production of growth-stimulating hormones, specifically gibberellin (GA), which triggers the actual growth of the embryo. This process resets the seed’s internal chemistry, allowing it to germinate once warm conditions return.
Preparing Plum Seeds for Successful Germination
Before beginning the chilling process, the plum pits must be prepared to maximize the chances of successful germination. First, thoroughly clean all fruit pulp residue from the hard outer pit. Sugars from any remaining flesh can attract mold and fungi, which will damage the developing embryo during the long, moist stratification period.
The next step involves scarification, which means lightly cracking the hard outer shell of the pit to expose the inner seed. While stratifying the whole pit is possible, gently cracking the shell significantly improves germination rates by allowing moisture to penetrate directly to the seed. Use a nutcracker or light tap from a hammer to avoid damaging the soft seed inside.
Once the inner seeds are extracted, they should be soaked in room-temperature water for 12 to 24 hours. This soaking step hydrates the seed, initiating the biological processes required to break dormancy. Any seeds that float during this time should be discarded, as they often lack a viable embryo.
Practical Steps for Cold Stratification
Temperature and Medium Selection
Cold stratification is best performed in a refrigerator to ensure consistent temperature and moisture levels. The ideal temperature range for this chilling period is between 33 and 41 degrees Fahrenheit (1 to 5 degrees Celsius). Temperatures below freezing should be avoided, as this can damage the embryo, while temperatures above this range will not effectively break dormancy.
The seeds should be placed in a slightly moist medium, such as peat moss, coarse sand, or vermiculite. Peat moss is an excellent choice because its naturally acidic pH helps suppress the growth of mold and fungus. A ratio of five parts medium to one part water is recommended to achieve the proper moisture level, which should feel damp like a wrung-out sponge, but never soggy.
Chilling Duration and Monitoring
The medium and seeds should be sealed inside a labeled plastic bag or a container with small air holes. The required duration for the cold treatment is typically 90 to 120 days (three to four months). Checking the bag monthly is necessary to ensure the medium remains moist and to remove any seeds that show signs of mold.
Planting Germinated Seeds
After the required chilling period, or once small roots begin to emerge, the seeds are ready to be planted. Sow the germinated seeds about one to two inches deep in a pot filled with well-draining potting mix. Placing the pot in a warm location, ideally between 65 and 75 degrees Fahrenheit (18 and 24 degrees Celsius), with bright, indirect light will encourage the seedling to sprout.