Methods for Evaluating Seed Ripeness

Use the following guides to determine seed ripeness.

Crown Position

When assessing ripeness, sample cones from all aspects and crown positions where cones will be collected. Cones on sunny and lower portions of the crown tend to ripen sooner than cones in other portions. Avoid sampling cones in the lower third of the live crown to exclude seed that is self-pollinated. The appearance of mature cones and ripe seed varies by species and geographic locale. Use guides created for your local area if they are available. Otherwise, rely on the general indicators below.

Cut Face Test

Cut the cone lengthwise with a cone cutter <show picture> or clean sharp knife to display the cross section or cut face of each side of the cone. Cut through the center of the cone to expose the seeds, except for true firs, which must be cut about ¼ to ½ inch from the center. From the cut face you can count the number of seeds and examine the effects of insects or disease, and the stage of ripeness.<diagram of cut face, also one of cone cutter> Specially designed cone cutting tools are available commercially or can be custom made. Refer to < MTDC illustration> as a guide for making your own cone cutter. You may need to cut individual seeds with a single-edge razor blade and use a 10-power hand lens to observe the cross section of the seed more clearly.

Indicators of Cone and Seed Maturity and Quality

Cone Color: As cones develop and the seeds ripen, the cone’s color generally changes from green to various shades of brown, olive-green, purple, or other colors. This color varies by species, within species, and by variety (see Seeds of Woody Plants in the United States). Cone color is only an indicator of ripeness, because each species has many color variations. Color changes are often subtle and difficult to distinguish.

In some species, the cones may not change color until after the cones have begun to shed their seeds. Some insect activity may cause cones to appear to be ripe. Used alone, color is the most unreliable test for cone maturity. Color should be used only used with other measures, such as the cut face test, to evaluate cone maturity.

The cut face of an immature cone will be light colored and will turn brown when it is exposed to air, just as an apple turns brown when it is exposed to air.

Seed Development: Ripeness can best be determined by exposing the seed’s cut face and inspecting the color and development of the seed embryo, seed coat, seed wing, and endosperm (nutrient tissue of the seed).

The cross section of high-quality mature seed should have a high percentage of healthy filled seed and have at least the minimum number of seed, based on regional guidelines. On one cut face, count the number of visible, cut seeds that are filled (the mature embryo and megametophyte are fully developed, undamaged, colored normally, and appear live and healthy). <diagram > When most seeds are mature and the cut face has at least the minimum quantity of healthy, filled seeds, cone collection should begin. The minimum number or percent of filled seed is determined by each region; refer to Regional Handbooks or direction for details.

If some seed has been damaged or some seed cavities are empty (no embryo or they are shriveled) it may not be feasible to collect cones. If there is a high percentage of immature seed (the embryo fills less than 75 percent of the seed cavity), you should delay collections.

The endosperm tissue should be white or creamy-colored. It should be developed past the "doughy stage." It should be firm enough to break into distinct granules when it is pressed with a sharp-pointed object such as a pen, pencil point, or sliver of wood. The texture of the mature endosperm is similar to that of coconut meat or a cut wheat kernel. Heating or collecting seed that is too moist will allow the endosperm to pull away from the seed coat after a period of drying. If the endosperm is milky, the seed is not mature.

The embryo cavity should be nearly filled with a white, light green, or yellow embryo. The embryo must have elongated over 90 percent of its potential length to ensure high viability. At high elevations, the embryo may only need to elongate to 75 to 80 percent of its potential length to ensure high viability, depending on the species. The embryo's potential length is the length of the cavity developed for it in the endosperm.

Seed cut tests conducted early in the season provide little information on ripening. There may be very little embryo development and the seed may just be filled endosperm tissue. Cut tests should increase in frequency when the embryo has elongated to 75 percent of the cavity length. Early tests can help evaluate whether the developing cone crop is likely to be worth collecting.

The final part of the seed cut test is leaving the seed in a warm room overnight to see if the endosperm shrinks away from the seed walls. Considerable shrinkage indicates that the seed has too much moisture and collection should be delayed.

The seed coat should be smooth, hardened, and be light tan (similar to a coconut shell) to black.

The seed wing should be fully developed and should not adhere to the cone scale.

As seeds and cones mature, the specific gravity of the cones decreases because water has been lost. Refer to the guides on determining cone ripeness based on the specific gravity of freshly picked, sound cones in Seeds of Woody Plants in North America. Do not test cones that are on the ground, in squirrel caches, picked from trees a day after the tree was felled, or that have insect damage. The specific gravity of cones may vary by geographic location, so do not rely on specific gravity alone to determine cone ripeness. A good guide is to test the specific gravity of five cones from different trees. Seeds are probably ripe if the cone’s specific gravity is lower than that listed in the guides.

If you are not experienced in recognizing cone and seed maturity, you should refer to publications with descriptions and photos of mature cones. One reference you may want to use is Characteristics of Mature Cones by Species from A Field Guide to Collecting Cones of British Columbia Conifers.