Most people think about cleaning their solar panels in spring. The weather turns, the yard gets attention, and the panels look dusty after winter. It feels like the right moment.
On the Front Range, it usually is not. Spring is when the dirtiest part of the year is just getting started. This article explains how the seasons land on panels here, which months make a wash count, and why the hour of the day matters as much as the month.
What lands on panels, and when?
The Front Range year delivers a different film every few months. Roughly:
- April to June: pollen. A fine yellow-green layer from trees and grasses. See pollen on solar panels.
- Late May to June: cottonwood fluff. Heavy along creek corridors, matting onto the pollen.
- May to August: hail. Not soiling, but a reason to look at the glass.
- July to October: dust, grime and smoke. Dry months, road dust, field dust, and in some years ash. See dust and road grime.
- November to April: snow. It covers panels for a day or two after storms, then slides off.
Bird droppings, sap and leaves turn up any time. Our Front Range solar year lays out the full calendar month by month.
Why not wash in spring?
Because the pollen has not finished. A wash in April or early May clears the winter dust, and then the trees and grasses coat the glass again within weeks. Cottonwood fluff follows, and it sticks best to glass that already has pollen on it.
You end up paying for a clean that lasts a few weeks. Waiting until the spring shed is over gets you clean glass for the months that matter most.
So when is the best time?
For most homes, late June to early July. The pollen and cottonwood are done, and the longest, strongest days of the year are just ahead. Clean glass in July is worth more than clean glass in any other month, simply because there is more sun to catch.
If you wash twice a year, the second wash fits best in September. The dry months, road dust and any wildfire ash have built up, and the autumn sun is still strong enough for clean glass to matter.
That is the reasoning behind our two-season cleaning plan: one wash after the spring shed, one after the dry summer.
What if you only wash once?
Pick late June or early July for most roofs. It catches the heaviest spring film right before peak production.
There are exceptions:
- Near dirt roads, fields or construction, late summer dust may be the bigger load. September may be the better single wash.
- After a heavy smoke season, an early autumn wash can clear ash that rain has not moved. See wildfire ash on solar panels.
- Steep roofs with good rain exposure sometimes hold their output well enough that one wash every year or two is plenty.
Your own data is the tiebreaker. If clear-day output has slid well below the same month last year, that is a better signal than any calendar.
Does elevation change the timing?
It does, a little. The calendar above fits the plains and the lower suburbs best: Lakewood, Arvada, Littleton, Westminster and most of Denver.
Up in the foothills, spring runs later. Trees leaf out and shed later, snow can linger into May, and the dusty season starts later too. In Evergreen and the higher neighborhoods around it, a mid-July wash often lands better than a late June one. Pine needles and pitch also stay in the picture all year. Our page on Evergreen and the foothills covers what changes up there.
Closer to the creeks, cottonwood fluff sets the date more than the calendar does. Wait until the fluff has stopped falling on your street, then book.
Doesn’t rain take care of it?
Heavy rain helps. Light rain often does not, and it can make things worse.
A 2025 study in Solar Energy, by researchers from Taif University, King Abdulaziz University and the University of Southampton, found that light rain under 1 mm a day increased dust buildup and adhesion, causing cementation on flat modules. The test site was a desert in Jeddah, so it is not the Front Range. But the mechanism is easy to picture on our spring and late summer drizzles.
A sprinkle wets the dust or pollen without moving it. Then the sun bakes it into a crust that the next real storm struggles to lift. That is part of why a dry, dusty month followed by a few light showers can leave panels dirtier than before. We cover this in more detail in does rain clean solar panels?.
What about winter?
Usually not worth it. Production is low anyway, so a cleaner panel adds fewer kilowatt-hours. Freezing temperatures also rule out many days, since water should never freeze on glass or on the roof.
The exceptions are heavy droppings, which block light hard in any season, and a badly filmed array after a dry, smoky autumn. On a dry day above freezing, a winter wash can still make sense.
What time of day is best?
Early morning. This matters as much as the month.
- Cool glass. In full midday sun, panel glass gets much hotter than the air. Cool water on hot glass is a shock you do not want.
- Manufacturers agree. REC and Panasonic both ask for water at ambient temperature, which is easiest to respect on cool morning glass.
- Dew helps. Overnight moisture softens pollen, dust and droppings, so they release with less brushing.
- Less spotting. Water runs off cool glass before it dries in place.
Late afternoon can work too, once the glass has cooled. Midday in summer is the one window we avoid, and we book most washes from first light.
If you wash a ground mount yourself, the same rule applies. Start before the sun clears the trees, and stop when the glass turns warm to the back of your hand.
How do you know it is time?
Three quick checks:
- Look from the ground at a low angle. Film shows up best when light skims across the glass.
- Compare clear days in your app. This week against the same week last year, or against the days right after your last good rain. Our guide to reading your solar app shows how.
- Put a number on it. The output check shows what a small soiling loss is worth for your system, so you can see whether a wash pays.
If the glass looks fine and the numbers look normal, wait. We would rather tell you a wash can wait than sell you one that does not show up in your numbers.
What happens on a well-timed wash?
On a late June morning, our four-pass visit goes like this:
- Read. We pull your last 30 days from the app, including the spring months.
- Look. A walk-around and a glass-level look at every module, with photos.
- Rinse. Pure water at air temperature, soft brushes, top row down.
- Prove. Before and after photos, notes, and a production comparison from the next clear days.
You can read the whole method on our solar panel cleaning page.
The short version
- Spring is usually too early: pollen and cottonwood are still coming.
- Late June to early July is the best single wash for most roofs.
- September is the best second wash, after dust and smoke.
- Light rain does not count, and it can cement dust in place.
- Wash in the early morning, never on hot midday glass.
If you are planning next year, a late-June slot is the one to hold. You can book a wash now for next summer, or start with the output check to see whether your panels need one at all.