Common Name: Blue Oak

General Information

Quercus douglasii is a deciduous tree with a compact, round canopy; it can grow 6 - 20 metres tall. Usually with a single trunk, sometimes the tree develops a few to several main trunks[]. The bole can be 36 - 60cm in diameter. Plants can retain their leaves all year round when growing in moist sites[].
The tree was traditionally used as a food, medicine and source of materials, though it is little used at present. It is often planted in restoration projects and is grown as an ornamental.
Although Quercus douglasii faces low recruitment rates due to a variety of factors that are not well understood, this species currently occupies a sufficient area and is present in a variety of habitat types. It avoids intense pressure from development and agriculture, due to its growth on poor soils and slopes, and very hot, dry regions. More research is required to understand possible reduction in recruitment and if population size is being significantly reduced. 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

, ,

Range

South-western N. America - California

Habitat

Dry soils in valleys, rolling hills and lower mountain slopes; at elevations up to 1,350 metres[, ], often forming extensive stands[].

Properties

Conservation Status	Least Concern
Edibility Rating
Medicinal Rating
Other Uses Rating
Habit	Semi-deciduous Tree
Height	12.00 m
Growth Rate	Slow
Pollinators	Wind
Self-fertile	No
Cultivation Status	Wild

Cultivation Details

Quercus douglasii is a moderately cold-hardy tree, tolerating temperatures down to around -15°c when dormant. 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[, ]. This species is the only tree of California that occupies the lowest, sunniest slopes and plains, and can withstand temperatures around 37°c for weeks at a time, growing in areas with an annual rainfall ranging from 380 - 890mm.
Grows best in a sunny position[], but young plants tolerate reasonable levels of side shade[]. Prefers a good deep fertile loam which can be on the stiff side[, ]. Characteristic soils in the wild are shallow, skeletal, infertile and well drained[]. Tolerates moderate exposure, surviving well but being somewhat stunted[]. Established plants are very drought tolerant[].
The plant gains its common name from its moisture-conserving leaves, which are covered on the upper surface by waxy coating that gives a blue hue
Seedlings soon develop a taproot and become intolerant of root disturbance, they should be planted into their permanent positions whilst young[].
A slow-growing but long-lived tree. Seed production is cyclic, a year of high yields being followed by 2 - 4 years of low production[].
The tree flowers on new growth produced in spring, the seed ripening in its first year[, ].
The plant resprouts from the base following logging or beingr topkilled by fire[].
This species is notably resistant to honey fungus[, ].
Hybridizes freely with other members of the genus[].

Edible Uses

Seed - cooked[]. The seed has been used as a staple food by some native North American Indian tribes[]. Somewhat sweet[, , , ]. A good size, it can be 20 - 30mm long and 10 - 16mm 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[, ].
A poultice of the ground galls and salt has been used in the treatment of burns, sores and cuts[]. It has also been used as a wash for sore eyes[].

The leaves have been chewed as a treatment for sore throats[].

Agroforestry Uses:

Blue oak is planted for the restoration of wildlife habitats, riparian zones, and watersheds. The roots bind the soil of steep watersheds, reducing the incidence of mass soil movement downslope into permanent and ephemeral streams. Trees are established from acorn plantings or from transplanting nursery stock. Blue oak has been successfully used to revegetate upper streambanks in various sites[]

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 branches have been used to make rims for twined work baskets[].

The acorn meal has been used to mend cracks in clay pots[].

The seed cups are used as buttons[].

The wood is hard, heavy, strong, brittle. It has a strong cross-grain and is difficult to split[]. It was used traditionally for making bowls and other implements, and is still used for making fence posts, but is of little commercial value[, , ]
The wood is often used for fuel[, , ].

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.