Acorn Maturity Indices

Acorns should be collected when they are fully mature and not before. Unlike multi-seeded fruits, such as pine or yellow-poplar, single-seeded fruits generally will not complete maturation after separation from the tree. The best maturity indices for acorns are (Bonner and Vozzo 1987): (Bonner, 1993)

1. Color of the pericarp.
2. Ease of separation of acorns from cups.
3. Cup scar color.
4. Cotyledon color. (Bonner, 1993)

In red oaks the pericarps should have lost their green color and be primarily dark brown or black before collection. An occasional exception to this rule can be made for southern red (Q. falcata Michx.) and cherrybark (Q. falcata var. pagodifolia Ell.) oaks. Individual trees of these species may produce mature acorns with a greenish tint to their pericarps. In white oaks brown and black are also good mature pericarp indicator colors, but again there are exceptions. Acorns from certain trees of white (Q. alba L.) and swamp chestnut (Q. michauxii Nutt.) oaks may be fully mature when pericarps are still yellow or even a mottled yellow and green. (Bonner, 1993)

These color changes are related to moisture loss with maturation. In Mississippi, white oak acorn moisture peaks at about 65 percent in early September, then drops to between 50 and 55 percent at maturity (Bonner 1976). Water oak (Q. nigra L.) acorns in the same region exhibit their maximum moisture content in August (65 to 70 percent), which then decreases to 35 or 40 percent at maturity (Bonner 1974c). When acorns are mature, their cups come away cleanly with only slight pressure. If attempts to remove the cups cause them to break apart and leave pieces attached to the acorn, then the acorns are not yet mature. This is a simple test to carry out when collecting from branches. Overcup oak (Q. lyrata Walt.) is an exception to this rule, as these acorns are disseminated with their enclosing cups attached. The cup tissue is full of small air spaces, which apparently allow the acorns to float and be spread by moving water. (Bonner, 1993)

In red oaks, the cup scars on mature acorns are "bright" in color. On acorns of southern red and cherrybark oaks the scars may be bright pink or orange when first exposed. These colors fade within a few days of cup loss, however, so many good acorns collected from the ground may not show these bright colors. This index is most useful in checking maturity of acorns still attached to trees. (Bonner, 1993)

The last test for maturity is to examine a cross-section of acorn for cotyledon color. Species with naturally high fat contents, such as water oak, should have dark yellow to orange cotyledons. A pale yellow or white cotyledon indicates immaturity. Species with low fat content, such as Shumard oak (Q. shumardii Buckl.) and all white oaks, should have creamy white or light yellow cotyledons. Immature coloration in these acorns is almost the same as that of mature acorns. The higher the fat content, the deeper the orange color of the cotyledons. Mature overcup oak acorns are 50 percent carbohydrate and less than 1 percent fat (Bonner 1974a), and have almost white cotyledons. Cutting acorns in half also provides an opportunity to assess insect damage in the field. If insect larvae are found in more than 25 percent of the acorns, then collection crews should realize that additional acorns may be needed. (Bonner, 1993)