Every year, somewhere between late May and the end of June, the creek corridors of the Front Range start to snow in summer. Cottonwoods along Clear Creek, Bear Creek, Ralston Creek and the South Platte let go of their seed, and white fluff drifts across yards, pools, windshields and roofs.
If your solar panels sit within a few blocks of a big cottonwood, you have probably watched it pile up along the bottom edge of the array. This article covers what that fluff does to your output, what it does not do, and when a wash actually makes sense.
What is the fluff, exactly?
It is seed. Female cottonwood trees release small seeds wrapped in fine cotton fibers, and the fibers let them ride the wind a long way. Male trees make no fluff at all.
That is why one street gets buried and the next one barely notices. It also explains why a neighbor two doors down can have a clean roof while yours looks like it caught a pillow fight.
Does dry fluff block much light?
Less than it looks. Dry fluff is mostly air, it is loose, and it moves. On a tilted panel in a breeze, a good share of it rolls off by itself within a day or two.
From the street, a white drift along the bottom row looks alarming. Up close, the cells above it are often getting nearly full sun. What matters is not the fluff alone but where it settles and what it settles on.
So when does it become a problem?
It becomes a problem when it gets wet. Cottonwood season overlaps the tail end of the spring pollen, so by June most Front Range glass already carries a thin yellow-green film. Our page on pollen on solar panels shows what that film looks like up close.
Then comes moisture. A heavy dew or a short shower wets the fibers, they mat flat against the glass, and they trap pollen and dust underneath. When the sun dries it all out, you are left with a felted strip that rain alone struggles to move.
A few things make it worse:
- Low-pitch roofs. Water moves slowly and drops its load before it reaches the edge.
- Frames with a tall bottom lip. The lip acts like a dam for fibers and grit.
- Sap from nearby trees. Sticky residue holds the fibers in place.
- Bird traffic. Fluff and droppings together form a crust that bakes on in June sun.
Why does the bottom edge matter?
Panels shed water downhill, so the lower edge of every module collects whatever rain and dew carry. Even in a normal year, that edge shows a faint line of grime. In cottonwood season, the line gets thicker and fuzzier.
A dirty strip along the bottom can cost more than its size suggests. Cells inside a module are wired in series, so a shaded strip across the bottom cells can drag down more of the panel than the dirty area alone. How much depends on the module and the inverter, which is why we look at your own numbers instead of guessing.
Should you wash during cottonwood season?
Usually not. If you wash in early June and the trees keep shedding for three more weeks, you pay for clean glass and then watch it collect again.
Our advice for roofs near cottonwoods is simple: wait until the fluff stops falling, then wash once. On most of the Front Range, that lands in the last week of June or the first week of July. Foothill neighborhoods often run a little later than the plains, and some years start early.
This timing is the reason the two-season cleaning plan books its spring wash for late June. One visit clears the pollen and the cotton together, right before the long July days when the sun is highest and clean glass is worth the most.
When is waiting the wrong call?
There are a few cases where we would not wait for the trees to finish:
- Droppings on the glass. A bird dropping casts a hard shadow on the cells below it. Our page on bird droppings on solar panels explains why those are worth lifting promptly.
- A clear fall in your numbers. If clear-day output is sliding well below last June, look before you wait.
- Sap under the fluff. Some trees drop sticky residue that rain will not touch. See leaves, seeds and sap for what that looks like.
- A nest starting underneath. Pigeons happily line a nest with cotton and twigs, and the gap under a panel is shelter they like.
Will a storm wash it off?
Sometimes. A proper soaking afternoon storm on a steep roof can move a lot of loose fluff and pollen. A light sprinkle is another story: it wets the film without moving it, which is exactly when matting starts.
We go into this in more detail in does rain clean solar panels?, including a study on why light rain can leave panels dirtier than before. The short version for June is that a quick shower followed by sun is the worst combination for cotton.
How can you check from the ground?
You do not need to climb anything to decide whether your panels need attention. Try this on a bright morning:
- Find a low angle. Stand where you can see the glass from the side, so light skims across it. Film shows up best this way.
- Look at the bottom edges. A thin white line is normal. A wide grey or tan band that has not moved after a windy day is not.
- Open your app. Compare a clear day this week with a clear day from late May. Our guide to reading your solar app walks through the daily and monthly views.
- Put a number on it. The output check shows what a small soiling loss is worth for your system before you spend anything.
If the band is thin and your clear-day numbers look normal for the season, leave it alone. The next good storm and the summer wind will take care of a lot.
What should you not do?
Most of the damage we see from cottonwood season comes from well-meant cleanups:
- No leaf blower on the roof. Climbing a pitched roof with a blower in June is a bad trade for a few fibers.
- No garden hose from the yard. Tap water carries dissolved minerals, and drops that dry on warm glass leave white rings. Our page on hard water spots shows the result.
- No scraping. Matted fibers lift with water and a soft brush. A blade or a rough pad scratches the glass and its coating.
- No soap. Residue holds the next round of dust and pollen.
If your array is a ground mount or a single-story roof you can reach from the ground, our clean it yourself guide covers a safe way to do it.
What do we do on a cottonwood roof?
On a roof under cottonwoods, the four-pass visit looks like this:
- Read. We pull the last 30 days from your app, so we know what your clear-day output looked like before and during the shed.
- Look. A walk-around and a glass-level look at every module, with photos of the bottom edges and the gap underneath, where cotton and twigs collect.
- Rinse. Pure water at air temperature and soft brushes, worked from the top row down, with extra passes along the frame lip.
- Prove. Before and after photos, notes, and a comparison of the next clear days against your baseline.
We book these washes early in the morning. The glass is cool, and the night’s dew has already softened the mat, so the fibers release with water instead of effort. You can read the whole method on our solar panel cleaning page.
We also note any cotton packed under the array edges. It is often the first sign that birds have found the gap, and it is easier to deal with in July than in October.
Where does it land hardest?
Anywhere with mature cottonwoods nearby. That means the creek corridors more than any single town: Clear Creek through Wheat Ridge and Golden, Bear Creek through Lakewood, Ralston Creek through Arvada, and the South Platte through Littleton.
Older streets with big shade trees also catch more, even far from water. If you can see a cottonwood’s crown from your roof, assume the fluff will find you.
The short version
- Dry fluff looks worse than it is, and much of it blows away.
- The real trouble is fluff, pollen and moisture matted along the bottom edge.
- Wait until the trees finish, then wash once, early in the morning.
- Trust your clear-day numbers more than the view from the street.
If you are not sure whether this year’s cotton is costing you anything, start with the output check and a look at your app. If the numbers say it is time, you can book a late-June morning and we will bring the water.