Oaks are notorious for being tricky to identify to species. Some Oaks can be confounding even to someone well versed in their identification and not all trees can be identified to species from photos or even in the field, especially early Spring growth and saplings. My hope with this short document is to provide a method for people who are learning about Oaks and want a set of tools to help get better identifications here on iNat as well as to learn more about how to identity oaks for themselves.
This is not a comprehensive guide to identifying Oaks. I have a goal of one day writing that book, but this is not it!
Before we get into the details, there are few things to discuss. First is terminology. Botany is jam-packed full of lingo. I am trying to avoid as much of it as I can, but there are certain words that can not be avoided. Here they are with some quick definitions:
Not everyone is going to have camera equipment that can capture the details (sometimes small or hard to photograph) discussed below. This is OK. Even if your camera can capture all of the details there is one thing that you can do that will not only improve the chances of getting an ID but also help you learn, take notes! A lot of features such as the feel of a leaf can not be captured in any photograph.
If you do these things it is highly likely you will be able to get an identification. If you want to make sure that your Oak is impossible to ID, then read the later section about what not to do.
Habitat info can be a huge help in figuring out what kind of Oak (or any other plant) you are looking at. Before you even start thinking about photos do a quick assessment of the habitat. Is wet or dry? What is the soil like (sandy, clay, alkaline/limestone, etc.)? On top of a hill or mountain? Or in a floodplain or river bank? Or on a prairie, barren, or glade? Getting into all of the details of the various habitats is beyond the scope of this short article, but at least think about it and add it any habitat notes to your observation. It can be tremendously useful.
If the tree is mature the overall growth form can be a critical clue. Step back and get a photo showing as much of the tree as you can. This can be difficult/impossible for large trees, just try and get as much as you can. If the tree is mature go ahead a grab and photo of the bark at about chest height.
Approach the tree and take a photo that shows a selection of leaves. Oak leaves are highly variable, even on a single tree, and quite often a photo of a single leaf is not enough evidence.
Find a leaf, preferably alive or recently fallen, and get a photograph of the underside of the leaf. If you camera will allow it, try and get any hairs that may be present in focus. If your camera can not close focus to show any hairs, then take notes of what you see. Understand that without this information some Oaks can not be identified to species. Even in the Winter the leaf texture can be useful, just make sure that the leaf you are looking at came from the same tree that you are looking at. Leaves blow around and can be displaced well away from the tree that they came from.
If you can reach a live limb, find and photograph a terminal bud cluster. In Winter this is pretty much mandatory. In early Spring, right before bud burst the shape changes and they can be very misleading. In later Spring and early Summer as the buds develop, they are not as useful. But from later Summer, through Fall and Winter (when the buds are mature) they alone can make the ID. As you document buds, you can try and document the twig vestiture and/or branch texture as well.
Look for acorns. Depending on the season and whether the tree is mature or not you may not find any. If you do, get a photo of an acorn and cap. The cap is actually more important for ID than the acorn itself. If the cap is not too old and worn the tree can often be IDed from this alone. Both the outside and the inside of the cap can be useful evidence for supporting an ID. It is fairly easy to photograph the outside of the cap showing the shape of the scales and the overall shape of the cap. The inside it tricker to photograph and unless you have a camera that can close focus it will be difficult to get a useful photo. Like with the leaf vestiture, take notes if your camera can not resolve the details.
Oaks have a very diverse and interesting gall fauna associated with them. Most of these galls are formed by tiny wasps. If you see any abnormal looking growths on the tree take photos. These can be uploaded as separate observations and sometimes the gall alone can ID the tree. If you get interested in galls (and why wouldn't you!) you can learn more at Gallformers. Host identification is critical for gall identification so your Oak identification skills will serve you well if you go on a gall journey.
I have the most experience with the Oaks of Eastern North America so this section is biased towards those, but the general process outlined earlier works for any Oak (or tree for that matter).
In Eastern North America there are 4 Sections of Oaks that you run across:
These are the Red Oaks. In most cases it is easy to tell that you are looking at a Red Oak as the leaves will have distinct bristle-tips, the acorn cap scales will be flat, and the interior surface of the acorn cap will have at least some pubescence.
A great many, maybe even a majority, of the Lobatae Oak observations on iNat are not identifiable to species. This in spite of the fact that there are a huge number of them that are IDed to species, often as Research Grade. The primary reason for this is that many Eastern Lobatae Oaks can not be IDed from the leaves, which is most often the evidence present in an iNat Oak observation.
There is a particular group that creates the most challenge. It consists of the following species:
With sufficient evidence, all can be IDed. Some are made easier due to limited ranges, but even those overlap with at least one of the others on the list. Leaf shapes with these can be very misleading, especially with saplings. In general small trees of any of these, other than sometimes Q. velutina, can not be IDed to species.
The other group that is tricky are the Lobatae oaks with (mostly) un-lobed leaves:
These are made somewhat easier due to range limitations. Q imbricaria is a tree mostly of the Midwest US with some populations present in the East. Q. laurifolia and Q. hemisphaerica are strictly southern oaks (NC to TX) with Q. phellos occurring across a wide range from NY to TX. Q. phellos is used heavily in cultivation and can show up outside of its natural range. Another gotcha is that to the untrained eye the Virentes Section Oaks (see later section) can look a lot like these.
Further complicating matters, there is a lot of hybridization amongst Lobatae oaks.
These are the White Oaks. Generally if it is not a Lobatae Oak and you are not in an area dominated by Live Oaks (see next section) then it is most likely a Quercus Oak. The leaves will not have bristle tips, the acorn cap scales will be warty, and the inner surface of the acorn cap will be glabrous (no hairs).
There are 4 species of "chestnut leaved" oaks that occur in North America (all in the East). There are 2 others that can at times appear "chestnut leaved" but upon closer examination are not.
These four share many similarities in leaf shape and dentation (marginal teeth). So much so, that the Flora of North America has this to say about them:
The four species of the chestnut oak group in eastern North America (Quercus montana, Q. michauxii, Q. muhlenbergii, and Q. prinoides) are somewhat difficult to distinguish unless careful attention is paid to features of leaf vestiture and fruit and cup morphology. Attempts to identify these species mostly or solely on basis of leaf shape and dentition (as in many other oak species complexes) have resulted in a plethora of misidentified material in herbaria and erroneous reports in the literature.
The other two that can sometimes be confused as being "chestnut leaved" are:
These are the Eastern Live Oaks. The two biggest clues that you are looking at a Virentes Oak are: the leaves are not deciduous (all falling off in the Fall) and you are in a Southeastern US State (SE VA to TX). There are four species in the Virentes Section that occur in the US (none in Canada) and three of them can be difficult to separate without sufficient evidence.
These are cultivated and sometimes invasive European Oaks.
Is this Q. montana (Chestnut Oak)? A lot of people might quickly agree that it is? | |
What about this one? | |
Or this one, which was taken in the Blue Ridge mountains in Virginia which are loaded with Q. montana? |
None of these are Q. montana. The first is Sassafras albidum (Sassafras), the second Oxydendrum arboreum (Sourwood), and the third Q. rubra var rubra (Northern Red Oak). These are all examples where insufficient evidence of a common species (and common observation type) can lead to erroneous identifications.
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Thank you for this useful document.
Thanks for this awesome resource!
This is fantastic Jeff!
Something I would add that can be helpful for ID: keep track of your surroundings... Where did you find your oak?
EX:
Riparian area
Dry ridgetop
Limestone/alkaline soil
Prairies/Barrens/Glades
Sandy soil
In non-cultivated situations, this information can help narrow down some of the more confusing groups.
@calconey Thank you! Will add a section on habitat. It is something that I have been working on adding to a larger document (maybe even one day a book) that I am slowly working on but is a definite miss from this one.
Very nice - thanks for putting this together.
Very informative! Thanks
Thanks, this will be very useful
Thanks so much for your coverage of the Oak. There are approximately 50 varieties east of the Mississippi alone, a daunting task to I.D. I for one hope you are able to publish your book. Thanks again!!
Mike J.
Thank you. Glad to have some organization of the families of oaks found in our mountainous area of Virginia. So sounds like they can be broadly grouped into the following; Red oaks, Lobed and unlobed, white oaks, chestnut leaved oaks, and non native oaks. This will help. Thank you.
Thank you for this guide!
This is a great guide! I've been looking for this information and this is formatted in a very helpful way. Thank you for letting me know about it.
Kudos for preparing this!
California oaks also hybridize frequently--fortunately they do this only within sections. But in some cases identifying native California oaks to section requires a sharp photo of the leaf underside. Two experts have given me a lot of help in identifying the host plant for oak galls in southern California. Both have both looked at reasonably sharp photos showing the entire top of an oak leaf with a gall perched on it and said they could not ID the oak to section, let alone species, without a photo of the leaf underside.
The trichomes (hairs) on the underside play an important role in identifying oak leaves--at least in California when you do not have photos of other parts of the tree. Photos of the inside of the acorn cap can also be helpful in IDing oaks to section in California.
Nice document! Thank you for sharing this.
Thanks for sharing!
Thank-you for. posting. For some reason, sometime in late summer, I got it into my head to learn my oaks better. I realized I had about 3 species in my head for my locality, when in fact there are 10 found commonly and another 7 found rarely. It's slow going, but I'm making headway.
I'm wondering if you could tell me what the best resource for bud identification is. I've been using the Princeton guide, and the descriptions and drawings are not helping me tease it out. Same with acorn caps. There are a few stand outs, long buds and slight fringe on the acorn cap of Q velustris. I'm hoping to find a good resource with close up photographs and explanations of the observation points.
The other question I have is when you've got all the photos needed, and you start taking a close look and realize this is likely a hybrid, what do you do -- leave it in the section, or make an educated guess about the cross. I'll tag you in my most recent conundrum.
Thanks for the guide,
Karen
Thank you so much. Will begin using this when I have a question about an oak identification.
Love this guide! I learn every time I read it.
I just found this, great tips, and I will certainly put them to use. For oaks as well as other plants.
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