Common Name: Mexican Blue Oak

General Information

Quercus oblongifolia is a evergreen shrub or a small tree with a broadly spreading crown, it can grow from 5 - 10 metres tall. The bole can be 50cm in diameter[, ].
The tree is harvested from the wild for local use as a fod, medicine and source of materials. It is occasionally harvested commercially for its wood and is grown as an ornamental.
Quercus oblongifolia currently has a large range and is recorded as widespread within many of the areas of the USA where it occurs. Distribution and threats in Mexico are less known, but no serious and extensive threats seem to be present. The plant is classified as 'Least Concern' in the IUCN Red List of Threatened Species(2013)[].

Known Hazards

All parts of the plant contain tannins. Whilst tannins are found in many foods, and have a range of medicinal uses. They are usually only present in low concentrations. In some foods made from oaks (particularly the seeds), the tannin content can be quite high unless the food is treated to reduce tannin content.
Tannins are only of low toxicity and, because of their bitter taste and astringency, are unlikely to be eaten in large quantities. However, if they are taken in excess, they can cause stomach pains; constipation followed by bloody diarrhoea: excessive thirst; and excessive urination[].

Botanical References

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Range

South-western N. America - Arizona, New Mexico, Texas, northern Mexico.

Habitat

A common tree of open oak woodlands in foothills, mountain slopes and in canyons, at elevations from 1,500 - 2,000 metres[]. A dominant tree within mid-elevation oak woodlands, forming both pure and mixed stands[].

Properties

Conservation Status	Least Concern
Edibility Rating
Medicinal Rating
Other Uses Rating
Habit	Evergreen Shrub
Height	8.00 m
Pollinators	Wind
Cultivation Status	Ornamental, Wild

Cultivation Details

Quercus oblongifolia is a moderately cold-hardy tree, tolerating temperatures down to around -15°c when dormant. It is found in semiarid to arid climates with biseasonal rainfall[]. It grows best in areas with hot summers. In areas with cooler summers, such as the maritime regions of the temperate zone, it often grows poorly, failing to properly ripen its wood and suffering frost damage over the winter[, ].
Prefers a sunny position but can tolerate some shade[]. Prefers a good deep fertile loam which can be on the stiff side[, ]. It is found wild on soils that are often thin, sandy, rocky, and poorly developed, often derived from granitic parent materials or mixed alluvium-colluvium[]. Tolerates moderate exposure, surviving well but being somewhat stunted[].
The tree flowers on new growth produced in spring, the seed ripening in its first year[, ].
Trees are usually top-killed in fires but, especially in younger trees, will often resprout prolifically from the base[].
Seedlings soon develop a taproot and become intolerant of root disturbance, they should be planted into their permanent positions whilst young[].
Hybridizes freely with other members of the genus[].
Plants in this genus are notably resistant to honey fungus[].

Edible Uses

Seed - raw or cooked[, ]. A staple food for some native North American tribes[]. Very sweet[, ]. The light brown, ovoid or oblong seed is 12 - 19mm long and 7 - 12mm wide[, ].
The seed is usually cooked before eating, though it can also be eaten raw. It can be eaten whole, though it is more commonly dried, then ground into a powder and used as a thickening in stews etc or mixed with cereals for making bread.
In some species, especially many of those classified as 'white oaks', the seeds are low in tannins and have a more or less sweet and agreeable flavour. The seed of most species, however, have a very bitter flavour, due especially to the presence of tannins. In these species there are various processes that can remove or at least reduce the amount of these bitter substances (although other water-soluble substances, including some minerals, will also be removed).
Tannins are water-soluble and therefore the easiest way to remove or reduce tannin levels is by soaking in water. A few different methods are listed:-
A traditional method of preparing the seed was to bury it in boggy ground overwinter and allow the wet soil to gradually leach the tannins. The germinating seed was dug up in the spring when it would have lost most of its astringency and bitterness.
Another method was to wrap the seeds in a cloth bag and place them in a stream for several weeks.
Drying the seed and grinding it to a powder before soaking speeds up the process. The fastest method is to use hot water, by cooking the powder and changing the water several times until the cooking water is no longer bitter. Alternatively, you can use cold water (which is reported to produce the best quality flour). In this case, you soak the powdered seed in cold water for 12 - 24 hours then discard the water. Repeat this process for a number of times until the soak water is no longer bitter.

The roasted seed of many Quercus species has been used as a coffee substitute.

Medicinal

Quercus (oak) species are used in the traditional medicine of many cultures, being valued especially for their tannins. Various parts of the plant can be used, most frequently it is the leaves, bark, seeds, seed cups or the galls that are produced as a result of insect damage. A decoction or infusion is astringent, antibacterial, antifungal, antiseptic, styptic and haemostatic. It is taken internally to treat conditions such as acute diarrhea, dysentery and haemorrhages. Externally, it is used as a mouthwash to treat toothache or gum problems and is applied topically as a wash on cuts, burns, various skin problems, haemorrhoids and oral, genital and anal mucosa inflammation[]. Extracts of the plant can be added to ointments and used for the healing of cuts[, ].

Other Uses

The leaves of most species in this genus are more or less rich in tannins. A mulch of the partially decayed leaves can be placed around vulnerable plants in order to repel slugs, snails, grubs etc, and these will in time break down to add humus and nutrients to the soil. Fresh leaves should be used with caution, however, since as these decay they utilize some of the nitrogen in the soil and thus can inhibit plant growth[, ].

Oak galls are excrescences that are sometimes produced in great numbers on the tree and are caused by the activity of the larvae of different insects. The insects live inside these galls, obtaining their nutrient therein. When the insect pupates and leaves, the gall can be used as a rich source of tannin, that can also be used as a dyestuff and is also used by many cultures to make ink[, ].
The bark of oak trees is also usually rich in tannins and can be used as a dyestuff and for waterproofing rope[, ].

The wood is very heavy, hard, strong, but brittle. It checks severely when drying. It is used in small quantities for furniture production[, ]. It is sometimes used as a fuel[].
The wood is a favoured fuel - burning well and giving off a lot of heat[, , ].

Propagation

Seed - it quickly loses viability if it is allowed to dry out. It can be stored moist and cool overwinter but is best sown as soon as it is ripe in an outdoor seed bed, though it must be protected from mice, squirrels etc. Small quantities of seed can be sown in deep pots in a cold frame. Plants produce a deep taproot and need to be planted out into their permanent positions as soon as possible, in fact seed sown in situ will produce the best trees[]. Trees should not be left in a nursery bed for more than 2 growing seasons without being moved or they will transplant very badly.