Oak Tree Catkins

Fairchild Oak in Ormond Beach, Florida.  
Fairchild Oak is many centuries old but is not the largest oak in Florida.  That title is held by Cellon Oak north of Gainesville.

So we're getting a lot of questions about the stuff falling from oak trees this April that is staining sidewalks and cars and looks like worms.  I've had several people ask me what's going on with all the "oak wormies."  The short answer is that those are tannic-rich oak flowers that are falling everywhere and staining things brown.

I have more than 50 mature live oaks on my property so I have abundant catkins this month.  Their abundance is forcing me to climb the roof every couple of days to blow all of the discarded male blooms off, before their tannins stain all the shingles brown.

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In spring, a single oak tree produces both male flowers and small female flowers. When a plant bears both male and female flowers it's said to monoecious. Most flowering plants have both sexes in the same flower, but a fraction of flowering plants including oaks have separate males and females.  The yellow, wormlike items are catkins, more technically known as aments. Catkins are clusters, or inflorescences, of male flowers. Each of the "bumps" on the catkins is a male flower consisting of a bract (a highly modified leaf), a lobed calyx and some pollen-producing stamens.  Once the stamens have released their pollen into the air, the entire catkin will fall from the tree. 

You have likely seen thousands of such spent catkins littering a sidewalk beneath an oak tree early in the spring. Other trees producing catkins include willows and birch. On a flowering oak twig you have to look close to see the female flowers; the future acorns. 

Some live oak catkins with spanish moss and some old and new leaves.  In Central Florida most of the live oaks have finished the process of pollination by May 1.

More on Monoecious Oaks

There are some plants that have separate male and female plants, but oaks — and approximately 6% of all flowering plants — are hermaphroditic. As an evolutionary strategy, the upside is it’s easier for them to fertilize their flowers if they are the only tree of their species in an area, but the downside is they sacrifice some genetic diversity and are more at risk of inbreeding.

There’s a whole coda here on the astonishingly diverse sex-lives of plants — which are much more varied than that of mammals (for example have you heard of buzz pollination).

Oak catkins on my back patio.

A handful of oak catkins.

Oaks put out their flowers quite early in the year, before the leaves are out. They flower early because they don’t want the leaves to get in the way of wind. The purpose of the lazy dangly shape of the oak catkin is to maximize the ability of the wind to deliver pollen from the male flowers to the female ones.

Interestingly, while wind pollination was the earliest method of plant reproduction, plants as a whole abandoned the wind about 100 million years ago, because relying on insects is so much more efficient. However, wind pollination re-emerged in times of environmental change, when there were fewer insects around to do the job.

Gutters quickly clog with oak catkins.

My woodland boardwalk covered with catkins

What does that mean for this small percentage of plants that now rely on the wind? Unlike bees or hummingbirds, the wind blows pollen indiscriminately; inaccurately.

And the evolutionary solution to this problem of making sure the pollen grains reach the female flowers is to produce a very vast amount of pollen grains. So that’s why you have seen all these male flowers forming bundles on the ground. It just reflects the massive production of pollen grains during the reproductive season of oaks.

Catkins on my air conditioning unit.  Cover the unit with screen to keep them from fouling the unit and causing a costly repair.

Catkins cover everything all the way down to the lake.

An oak is a tree or shrub in the genus Quercus, of the beech family, Fagaceae. There are approximately 600 extant species of oaks. The genus Quercus is native to the Northern Hemisphere, and includes deciduous and evergreen species extending from cool temperate to tropical latitudes in the Americas, Asia, Europe, and North Africa. North America has the largest number of oak species, with approximately 90 occurring in the United States, and 160 species endemic to Mexico. The second greatest center of oak diversity is China, which has approximately 100 species. 

The fruit of the oak tree is an acorn; each acorn contains one seed (rarely two or three) and takes 6–18 months to mature, depending on their species. The acorns and leaves contain tannic acid, which helps to guard from fungi and insects. 

Oaks produce more than 2000 acorns every year, but only one in 10,000 acorns will develop into oak tree. A lot of animals (pig, deer, squirrels, mice, ducks…) feed on acorns. 

Above and Below:  Cellon Oak, the largest oak tree in Florida.  This massive oak begs the question:  Can a tree live forever?

A COMMON QUESTION

My live oak trees have been dropping tons of greenish brown things over the past week or so. Can I just leave this stuff where it is covering my lawn? Can I leave it in bare areas where there is no lawn? If I have to pick it up, what can I do with it?  Are the stains it leaves permanent?

The things falling from the live oaks recently as we've said above are male catkins. Their structures carry the male flowers of the trees.

As to the question about what to do with the fallen catkins, here’s what I recommend. In areas where there is no grass, leave them. If the layer of catkins is less than an inch thick, try mowing, but always wear a mask as the resultant cloud of pollen will cause an instant allergy attack.   The catkins will mostly  disappear with mowing, and there is no need to remove them.

If, however, the grass is covered with a layer over an inch thick and is completely buried, rake up what you practically can and put it into beds as mulch or put it in your compost pile.

You can do the same thing with catkins you sweep up from drives, patios and walkways and roofs. Don’t just throw this stuff away in the trash — put it to good use.

The stains from the catkins' tannins will take time to disappear from your car, driveway and patio furniture.  A little bleach and water washes it away pretty quickly.

While the catkins themselves are not particularly sticky they will stick to spider webs or anything else that is a little damp (the roof).

MORE COMMON QUESTIONS
There’s thousands of catkins on my car, what do I do? They cluster together, move like tumbleweeds or like a snake bundle. Does each tassel represent an acorn? How does that pollination thing work?

For starters, those tassels, again,  have a scientific name: “catkins.” The catkin is the male part.

To answer the second part of this question: each catkin does not result in an acorn, because the catkin is the male part. The seeds come from the female flower of the oak tree, which is even less obvious than the male part.

And it’s not just that they have to produce many flowers; for wind pollination to work plants have to produce a ton of pollen, too. All of the pollen that you see coating your car and yard furniture during the late spring and early summer is a result of this phenomenon.

(A fun fact: the pollen you most often see is pine pollen, which is visible because pollen grains are abnormally large. It gets all over everything in part because pine pollens have little air sacs or wing-like structures to help it be carried farther.)

Not only does this super-abundant production strategy result in pollen and catkins all over your stuff, it’s also what brings us seasonal allergies: you don’t get substantial amounts of pollen in your nose from plants that are waiting for bees to come along and do the job, the pollen that irritates our immune systems is from all of these wind pollinators.

Every couple of days into May I have to get on the roof to remove all the catkins sticking to the eaves and clogging the gutters.

So, that’s how this pollination thing works. Those little “acorn wormies” or "acorn tassels" represent a little insight into the fascinating diversity of tree sex, and all of the downstream impacts on the rest of the world!

You can often find warblers at this time of year by looking up into flowering oak trees, as the male flowers are an important source of protein for birds in the form of insects. 

One of Florida's most neglected and forgotten massive oaks is Mammoth Oak, found near a 6-lane highway on the southeast end of The Villages.

Oaks (all parts) are host to more than 550 species of butterfly and moth larvae as well as many other invertebrates, many of which are attracted to catkins.

It turns out that birds are not the only creatures that visit oak catkins in order to secure a meal. Although we think of Gray Squirrels as consumers of nuts, seeds, fruit and fungi (and bird eggs and fledglings), their preferred food in the early spring includes the nutritious buds and catkins of oaks, elms and maples.

Apparently squirrel and warbler taste buds are not the same as humans’, as having consumed catkins I can say that their taste leaves a lot to be desired.