Sweet Acorns - Quercus rotundifolia Lam.
(syn. Quercus ilex, Quercus rotundifolia subsp. ilex,
Q. ilex subsp. ballota)
Holm oak, Ballota oak

Quercus rotundifolia is found primarily in Spain and Portugal, North Africa (mainly Morocco), and areas of Southern France (Languedoc and Roussillon) and the Atlantic coast. It has very low levels of bitter tannin in the acorns.Another, almost identical species is found mainly in the coastal parts of central and eastern part of the Mediterranean. This is Q. ilex, and it has quite high levels of bitter tannins in the acorns. Unfortunately, the two names have been used fairly interchangeably by nurseries and others, creating confusion.

Quercus rotundifolia, Morroco, Fouad MsandaQuercus rotundifolia, Morocco. Fouad Msanda.
Photo credits
Fouad Msanda. Note the variation in shape and spininess between the two trees. Photo on right cropped to show leaf form. Q. rotundifolia from the westernmost part of the Anti-Atlas mountains, Morocco. See more photos at 'Plant Biodiversity of South-Western Morocco'.
Reproduced under a Creative Commons licence (see bottom of page).

Quercus rotundifolia has relatively 'spiny', smaller, and more rounded young leaves than Quercus ilex. Both new soft growth and the current season growth of older trees is also relatively 'spiny', with the leaf margins becoming essentially without spines as the leaves mature. According to 'Oaks of the World' (see the link below), the leaf colour in rotundifolia is "ash-green" or "glaucous", which I take to mean greyish-blue due to a powdery or waxy bloom. The underside of the leaf is densely pubescent ('fuzzy') and more more or less white. The leaf blade length is cited as 1.5 cm to 4 cm, but their photo of a leaf has a blade of about 5.5+ cm long! There are 5 - 8 vein pairs.

Quercus ilex, broadly speaking, has larger, oblong, narrower leaves with only a few spines. According to 'Oaks of the World', the leaves vary in size, the leaf blades varying from 3 cm to 7cm, and the leaf shape is also variable - more or less ovate, or more narrow-lanceolate, or rounded. The leaf margins are equally variable - it can be entire or toothed. When held up to the light, the leaf margin is clearly transparent. The leaf itself is dark green and shiny. The underside of the leaf is densely pubescent ('fuzzy'), with a tawny cast. There are 7 - 12 vein pairs.

The French Riviera population of Q. ilex shows great variability in leaf form, with a mix of characters of both species. Perhaps there has been some introgression of Q. rotundifolia genes.

Leaves of 3 New Zealand seedling Quercus rotundifolia

The branches on the right are twigs taken in autumn from 3 different seedlings purchased as Quercus ilex rotundifolia. There is variation between trees, and also within the tree, with some leaves being almost without spines, and others distinctly spiny. Leaf blade length is ~ 4.5 to 5.5 cm, which is within the actual length of a leaf blade illustrated by 'Oaks of the World', but over the range listed  by the same site for Q. rotundifolia. The blade length of these seedlings is within the variable range they list for Q. ilex, the variation in margin spininess and in leaf shape certainly fits with Q. ilex.

Sometimes the two species are referred to as simply 2 subspecies of  Quercus rotundifolia, namely Quercus rotundifolia subsp.rotundifolia and Quercus rotundifolia subsp.ilex. Others have called the 'sweet' Q. rotundifolia a subspecies of the bitter Q. ilex, that is, Q. ilex subsp. ballota. Others reject the 'two species' concept, and thus reject the erection of a species called Quercus rotundifolia entirely, and refer both to Quercus ilex, with the oak with the bitter acorn being Q. ilex ssp. ilex, and the oak with the sweet acorn being Q. ilex ssp.ballota. The current botanically 'preferred' name for the sweet oak is Quercus rotundifolia, so I'll stick with that until the taxonomists come up with a new scheme.

Natural Environment

This evergreen oak grows naturally over thousands of hectares of agriculturally low value, shallow, poorer soils in the seasonally dry climates of upland Spain, Portugal, and the Atlas mountains of Morocco. It is adapted to a wide variety of soil types. It can stand frost and short periods of light snow. In the upland, marine influenced climates of southwest Spain and Portugal, the trees are managed as a mixed grassland-open woodland, with intermittent cropping between trees (an 'agrosilvopastoral' system). The grasses and herbs support low-density mixed sheep, beef, goat and pig grazing at the wetter time of year, and when the grasses die out in summer the acorns from the oak trees (at densities of 30 to 50 trees per hectare), plus oak foliage and some saved crops support the animals until the grasses return. (The rainfall levels are quite similar to Auckland, but the pattern is much more markedly summer dry, with Southwest Spain receiving a median of around 10mm or less per summer month where Auckland has double that, and Spanish summer temperatures are a little higher, with median temperatures of the summer months plus early autumn all in the 25C - 28oC range). There are some summers storms in the oak regions, unpredictable in occurrence, somewhat similar to tropical cyclones hitting Auckland in late summer.

Quercus rotundifolia is well adapted to summer drought. It has been shown to access water from a water table that has shrunken in summer to 5 meters below the soil surface. It inhabits drier environments than other Iberian oaks, such as the cork oak, Quercus suber. Q. rotundifolia regenerates well from resprouts after fire, presumably an adaptation to the summer dry environments it lives in.

Foreign Diseases and pests
Acorns are restricted entry to New Zealand - for very, very good reasons. Oak (Quercus) seed can carry two serious diseases not present in New Zealand that would damage the local oak tree populations - the fungi Ceratocystis fagacearum (Oak Wilt disease) and Cryphonectria parasitica (Chestnut Blight). Chestnut Blight in oaks is damaging but not usually lethal, but infected trees becomes sources of spread of the disease. Of course, in chestnut trees, it is usually of extreme importance, and in America it has destroyed millions of native American chestnut trees. Chestnut blight can also damage pecan trees. To top it off, there are a variety of acorn-destroying weevils that are not present in New Zealand. It would be very easy to introduce these important pests in a few acorns picked up from a park in Europe, Asia, or America. In Spain, in some wet summer seasons these weevils introduce a bacterial pathogen (Brenneria sp. syn. Erwinia sp.) that destroys between 4% and 16% of acorns in trees infested with the weevils. The weevil larvae typically destroy around 20% of the acorn, but the bacteria cause the acorn to stop growing, and then fall off the tree prematurely. Acorns from other countries pose a high risk to New Zealand's oaks, chestnuts, and pecans!

The tree

This relatively slow-growing evergreen oak has dark green, leathery leaves that superficially resemble those of the Holly tree, but without Holly's sharp spines. Holly is in the genus Ilex. The species name 'ilex', applied to this oak (genus Quercus), is meant to reflect the similar leaf shapes. The shape of the crown is broadly spreading, somewhat globular. Mature height is variable from roughly 15 to 25 metres, but is easily controlled with pruning.

The flowers

The tree produces separate male pollen producing catkins and female flowers. This particular species, in common with some other oak species, has some individuals in the population which produce almost exclusively male flowers, and others that produce almost exclusively female flowers. Flowering is in spring, and the newly set acorns easily visible by january. The acorns are ripe about 6 months after pollination.

The nuts
The mean nut weight in Spain is 3.5 grams, but the range in size is from around  1.75 grams to 5.25 grams. The mean acorn length is 3.5 cm, and the mean width is about 1.6 cm.

The acorns are harvested not just for animal food, but, in some parts of Spain, as human food, as well. Apparently, in the past, they have been roasted on charcoal braziers and sold on the streets (around Christmas time) in the same way as chestnuts.Up until about the 1980's they have been an important food "for poor people" in times of food scarcity. 

In New Zealand, seedling trees are sometimes available from nurseries. These are seedlings derived from trees grown from acorns sourced from  Europe (probably) in the historic past. No attention was paid to selecting seed from trees with the sweetest acorns or the heaviest yield. We know almost nothing of the genetic worth of the resource already present in New Zealand from historic seed and plant introductions. In 1979, George Halliwell, of the (former) Ministry of Agriculture and Fisheries Advisory Service (now disbanded) wrote "Ilex oaks bear heavy crops in Hawkes Bay, Manawatu and Taranaki and are of good flavour."Clearly, New Zealand climatic conditions suit these trees.

If they were ever available, it would be better to buy plants grown from cuttings taken from producing trees - trees that are proven by the taste test to be Quercus rotundifolia.

Seeds should be kept in the refrigerator in damp sand, perlite or similar for 12 weeks to break seed dormancy. The seed can be sown anytime after the cold treatment, and seed germinate in about 2 to 6 weeks.

Semi-hardwood cuttings taken in summer and hardwood cuttings taken in winter  and hardwood cuttings. Semi-hardwood cuttings should be taken in the summer while hardwood cuttings should be taken in the winter. Cuttings usually take between 30 and 100 days to set root.


The trees grow easily and well in Auckland, at least. The odd tree suffers from 'sudden death syndrome' probably due to a root rotting fungus, but apart from that rare occurrence, the only problem of note is the caterpillar of the puriri moth Aenetus virescens. This dastardly little insect can ring bark the tree. Its ring-bark chewing is covered in silk, with bits of bark etc stuck to it, so its very easy not to notice the damage. It is easy to kill the caterpillar by sticking a bit of flexible wire down its hole. Fly spray down the hole would probably work, too.

The leaves are also eaten by case-moth caterpillars, but they don't do a lot of damage, and so are not particularly important.

The trees can be affected by Phytopthora cinnamoni, and this organism seems implicated in 'sudden oak decline syndrome'. Seedlings show quite a lot of variability in their suceptibility to Phytopthora root rot. The vast cork oak (Quercus suber) forests of Portugal are under serious danger of population crash due to this disease. Whether or not Phytopthora becomes a problem for the more genetically diverse Quercus rotundifolia forests of Spain is yet to be seen.

New buds seem to 'break' fairly readily from cut branches, even quite large branches, so the trees can be kept compact. Outer branches can be trimmed back to force multiple new shoots, increasing the number of  finer branches that bear flowers.


Seedling  Quercus rotundifolia grow quite quickly in suitable conditions. Spanish literature suggests that while seedlings will start flowering at about 8 years old, they will not start producing acorns until 15 to 20 years old. These trees may come into bearing from seed slightly more quickly in Auckland's moister climate and better soil conditions. A few of our seedlings had the first few acorns about 10 years after planting.

The trees are extremely variable in production, as populations all over Spain grow in varying soil and climatic influences. Some trees with large acorns and heavy bearing characteristics have been selected for replanting (mainly in Andalusia), but no attention has been paid to choosing ecotypes adapted to the variabilities of specific environments. The mean production in the Spanish dehesa system (at 50 trees to the hectare) is in the range 250 -600 kg/hectare.

In Spain, production is extremely variable, both by tree and by year. There has been little effort to identify superior trees, whether in cropping ability, acorn size, or adaptability to disease or climate extreme.  The mean crop size recorded (for mature trees) in one part of the Spanish dehesas, measured over a 6 year period, was 87 acorns per square meter (with a year to year variation of plus or minus 50 acorns). Unmanaged wild trees in the same locale had a mean of about 35 acorns per square meter. Variation depends in part on management - the dehesas are cropped from time to time, with some part of the fertiliser application residually available to the oaks, and this has been shown to increase the number of female flowers, and the ultimate crop size. The oaks are also trimmed for forage, sometimes lightly, sometimes up to 30% of the crown is removed. Pruning has no effect in a poor nut production year, and reduces production when done in a good year. These cultural influences aside, it is weather that probably plays the biggest part in driving year-to-year variation in cropping.

An unusually dry winter usually results in poor set in the spring, followed, naturally, by a poor crop in the autumn of the following year. Following the poor autumn crop - if winter rains are normal - the trees then usually both set heavily the following spring, and bear a heavy following summer crop.

But winter weather is not the only factor affecting crop size. Even if the winter is moist, and the trees set well in spring, the immature fruit will drop if the summer turns out to be exceptionally dry.

Spring weather is also critical. Holm oak pollen has the ability to absorb atmospheric moisture, so in a wet spring, the pollen becomes heavy with moisture, and doesn't disperse as well as it should, resulting in low pollination and a poor subsequent crop. One Spanish study (Garcia-Mozo et al. 2007) found variation in effective pollen emission as the single most important factor affecting crop load.  The mature trees in a Spanish man-made savannah (dehesa) are relatively widely spaced, so pollen has to travel some distance to the female flower. In New Zealand, there is the opportunity to grow trees in close proximity, which might help offset the fact that we ordinarily have wet springs, and thus 'heavy pollen' which disperses poorly.

Quercus rotundifolia acorns put on a lot of their size in late summer. Adequate soil moisture at this time will result in well-sized acorns, and conversely, drought at this critical time will mean smaller acorns. A very good spring flowering and set also results in smaller acorns, as the absolute numbers are so much greater. The opposite also applies; a small spring flowering and set results in fewer, but larger, acorns (absent any summer moisture limitations).

Most acorns contain bitter tannins. The most common oak in New Zealand, the deciduous English oak (Quercus robur) is a good example. The American red oaks are another example. But the American Eastern white oak, and a few other species have low levels of tannin. Some people call the acorns from these trees 'sweet acorns'. 'Sweet' is a relative term for these species. Even 'sweet' acorns need to have the tannins leached out. Fortunately, the tannins are water soluble. The most practical means is to shell the acorns and heat or boil them briefly, ditch the water, refill the pot, boil again, and keep repeating until the tannin is gone. This may need to be done 5 times before they are tannin-free. The acorn meat can either be dry roasted in an oven so it keeps, or put in the freezer until ready for use.

But the Holm oak (Quercus rotundifolia),known in Spain as 'Bellota', is usually essentially tannin free and slightly sweet to the taste without any processing - at least in Spain! Other countries report that the local Holm Oak acorns still require leaching (and putting the acorn kernels in a porous bag in the toilet cistern seems a favored solution over more complex methods - when the flush water runs clear, the tannin is gone!). If the thin 'skin' around the acorn is still present, it apparently should be removed, as it is said to be bitter.

Use for Animal Feed
Acorns in general are a good energy source for livestock. Depending on the species, they contain containing 47% to 60% starch, and from 7% to around 14% fats. The predominant fatty acids in acorn fats are oleic( 48-63%), linoleic (16.5-17%), palmitic (12.1-13%), stearic (3-6%), and  linolenic (1-5%). Myristic, palmitoleic, arachidic, 11-eicosenoic and behenic acids are found in amounts below 0.5% down to traces. Levels of cholesterol are low, only 0.1%.

The oleic acid concentrations in Quercus rotundifolia are greater than 63% of the fatty acids present. According to Cantos et al. 2003, Quercus rotundiflia has 18mg of α-tocopherol and 113mg of γ-tocopherol per kilogram of dry matter, for a total of 131 mg/kg of vitamin E. These researchers then go on to report on the bitter Quercus ilex, (from the central and eastern Mediterranean, characterised by high tannin levels in the acorns). Quercus ilex has 31mg of α-tocopherol and 66mg of γ-tocopherol per kilogram of dry matter, for a total of 97 mg/kg of vitamin E.

Quercus rotundifolia (and Q. ilex) acorns also contain antioxidant phenolic compounds, derivatives of gallic acid in the main. These are mainly present in the endosperm of the seed, rather than in the seed coat (unlike other oak species). Skin extracts from Q. rotundifolia, and Q. ilex showed 1.4, and 1.0 antioxidant efficiencies, respectively.

Tannins are also antioxidant compounds, and can make up as much as 7% of the weight of the acorns of some oaks. Tannins are 'anti-feeding' compounds, in that they tend to inhibit the uptake of proteins from food. However, in broiler chickens at least, it seems livestock acclimate themselves to the tannin content, and they are no longer a factor in reducing protein assimilation.

When North African broiler chickens were reared on food which substituted ground air-dried acorn meal (Q. ilex) for a third of the normal corn (Zea mays) component (which made up 2/3 of the diet) the chickens acorn fed chickens reached similar final (56 day) weights as the normal diet (albeit they grew more slowly for the first 35 days). Interestingly, abdominal adipose (fatty) tissue was 78% lower in the acorn fed group compared to those fed the normal diet. The thigh muscles of the acorn amended group had 2.5 grams of fat  per 100 g of muscle, where the normal group had 3.1 grams of fat per 100 grams. The chickens fed the oak acorn amended diet had more linoleic (C18:2) polyunsaturated fatty acids in their fat profile, and a higher proportion of polyunsaturated fatty acid (PUFA) overall, compared to the normal corn diet. The acorn amended diet gave an increase in the linolenic acid (an 18:3 polyunsaturated fatty acid) of 0.2%, to a total of 0.8%, relative to 0.6% in the normal diet.

Acorns from holm oaks have long been the basis for the famous oak- fattened dried hams ('Jamon') of Spain. The special Iberian pigs forage on acorns extensively in the acorn drop season, and animals are pastured in the dehesa at this time and fed exclusively on acorns and grass. A 100 kg pig will add another 60 kgs to its weight at this time, eating from 7 kg to 13 kgs of acorns a day.

Use for truffle production
Quercus rotundifolia is used as a host plant for the production of both the black truffle, Tuber melanosporum and the summer truffle Tuber aestivum var aestivum in southern Spain. Tuber melanospora (Perigord Black Truffle), Tuber borchii and Tuber aestivum inoculated onto Q. ilex (which may or may not be Q. rotundifolia...!) are available from Southern Woods Nursery in Christchurch (as at 2014).

The most helpful link I found in trying to identify my trees (bought as Quercus ilex var ballota, 'edible oak') was Oaks of the World:

The site
'Plant Biodiversity of South-Western Morocco' was also very useful:

Herminia García-Mozo, Eugenio Dominguez-Vilches, Carmen Galán. 2012 'A model to account for variations in  holm-oak (Quercus ilex subsp. ballota) acorn production in southern Spain'
Annals of Agricultural and Environmental Medicine 2012, Vol 19, No 3, 403-408

Kaddour Bouderoua1, Jacques Mourot and Ghalem Selselet-Attou. 2009.  'The Effect of Green Oak Acorn (Quercus ilex) Based Diet on Growth Performance and Meat Fatty Acid Composition of Broilers'
Asian-australasian journal of animal sciences 2009, vol. 22, no 6, pp. 843-848

Manuel León-Camacho, Isabel Viera-Alcaide, Isabel M. Vicario. 2004. .Acorn (Quercus spp.) fruit lipids: Saponifiable and unsaponifiable fractions: A detailed study
Journal of the American Oil Chemists' Society Volume 81, Issue 5 , pp 447-453

Cantos et al. 2003 Phenolic Compounds and Fatty Acids from Acorns (Quercus spp.), the Main Dietary Constituent of Free-Ranged Iberian Pigs
Journal Agric. Food Chem., 2003, 51 (21), pp 6248–6255

García-Mozo H, Gómez-Casero MT, Domínguez E, Galán C. 2007. Influence of pollen emission and weather-related factors on variations in holm-oak (Quercus ilex subsp. ballota) acorn production.
Environ Exper Bot 2007, 61:35–40

Gea-Izquierdo G, Cañellas I, G. Montero G. 2006. Acorn production in Spanish holm oak woodlands
Invest Agrar: Sist Recur For (2006) 15(3), 339-354

Sghaier-Hammami B et al 2013. Physiological and proteomics analyses of Holm oak (Quercus ilex subsp. ballota [Desf.] Samp.) responses to Phytophthora cinnamomi.
Plant Physiol Biochem. 2013, 71 pp191-202

Photos from Wikipedia Commons and from 'Plant Biodiversity of South-Western Morocco' have been reproduced unchanged (except where noted) and are licenced under a Creative Commons Attribution-NonCommercial 3.0 Unported License.http://creativecommons.org/licenses/by-nc/3.0/.