Navigation Menu+

Nut Crops Review



174
Nut Crops – Section Review
Like the small fruits, nut crops are a group of botanically unrelated crops. They
are grouped together because the fruit type is a nut (generally), and harvesting, postharvest processing, nutritional value and marketing characteristics are relatively
similar. We focused on four major temperate-zone nuts, pecan, walnut, almond, and
pistachio, and ended up with two tropicals – cashew and macadamia. The latter serve
as a transition to the next section on tropical fruits.
While it is difficult to generalize about such a diverse set of crops, pay attention
to the harvest and post-harvest processing (hulling, drying, roasting, etc.), and look at
the nutritional value in comparison with that for fresh fruits. Notice the lack of vitamins
C and A, but much higher values for minerals, B-vitamins, protein, and fat. Since nuts
contain the seeds of the plant, it is not surprising that they are packed with nutrition.
Some important points to remember about nut crops:
• Nuts are often wind pollinated, unlike many of the species we have considered
thus far. The trees generally produce a lot of pollen, often in specialized male
inflorescences, to ensure that the female flowers will be fertilized. While
possible, we would not want parthenocarpic fruit set in nut crops, since the
seeds are the edible products.
• The terms monoecious and dioecious are useful in this section, as pecans and
walnuts are all monoecious, and the pistachio is dioecious. Take a look at these
definitions in the glossary. Also look at “protandrous” and “protogynous” under
“heterodichogamy”, and think about the implications for orchard design and
pollinizer placement.
• Walnut and pecan are closely related, both members of the Juglandaceae.
While they are not cross- or graft-compatible, there are several aspects of their
botany that are similar. One of the main differences, responsible for much higher
yields in walnuts than pecans, is the propensity to bear nuts from lateral buds on
1-yr-old wood in walnut, not just terminal buds. This one feature makes a big
difference in the yields per acre and orchard design between pecan and walnut.
• Nut crops often undergo a process known as alternate bearing, or irregular
bearing. In other words, yield is high one year then low the next. Reasons for
this revolve around two main theories: a hormonal theory and a carbohydrate
depletion theory. The hormonal theory states that hormones in the developing
nuts diffuse back into the stems and inhibit flower bud formation for next year’s
crop. The carbohydrate theory states that carbohydrate reserves are exhausted
in high crop years, such that none are left to allow flower bud formation for the
next year. In “off” years, either the lack of hormone inhibitors or the surplus of175
carbohydrates available allow for great flower bud set and a big crop the
following year. In pistachio, it works a bit different, where flower buds are set for
next year’s crop in “on” years, but fall off during the maturation of the current
season’s crop. Regardless of the mechanism, this creates a hardship on the
grower, since yields, and therefore prices, fluctuate drastically from year to year.
Some progress has been made with “thinning” nutlets in an “on” year in pecan,
which has the effect of smoothing out the variation in yield over time.
• The pecan is the most important native orchard species in North America.
Other than some chestnuts, none of the other nut crops are native.
• Notice the lower yields per acre of nut crops compared to fresh fruits, due to
the fact that nuts are dried to about 5-10% water content before sale, while fresh
fruits are sold at about 90% water content. For example, 1-2 tons per acre would
be a good yield for nuts, but yields of 10-20 tons/acre are typical in tree fruits.
Despite lower yields, profits in nuts can be quite high since the price per lb is
often over $1, while the price per lb of fresh fruits is generally 10-30 cents/lb.
• Nuts are generally harvested by shake-and-catch methods, where the trunk is
gripped by a mechanical arm, and shaken for a few seconds to dislodge the
nuts. Nuts are then swept from the ground or caught in “catch frames” and
funneled into storage bins. Few fresh fruits could be harvested this way since
they would be bruised or cut as they fell, rendering them unmarketable.
Harvesting costs are substantially lower (per lb) in nuts, despite a high initial
investment in a mechanical harvester.
• Pecans and walnuts are the largest orchard species grown, and it is common to
find planting densities of only 10-40 trees/acre, as opposed to 100 to several
hundred plants/acre in tree and small fruits.
• In pistachio, almond, and macadamia, the fruit starts out as a drupe, but is
marketed as a nut; review the definitions and structures of drupes and nuts and
try to figure out what makes this possible.