Algae vs. Lichens in the Garden: A Gentle Guide to Slippery Paths and Living Patina

Some mornings, I step into the garden before the city is fully awake and notice the way the stones keep secrets. A green sheen on the path, a faint dusting of pale coins on the wall—algae and lichens writing their quiet stories across surfaces we rush past. I’ve slipped a few times, I’ve smiled many times, and I’ve learned to ask a simple question: when should we tidy, and when should we let time do its art?

This is a love letter to balance. To safety and softness. To the kind of caretaking that refuses to panic, checks the facts, and works with life’s small ecosystems. If you’ve wondered whether to discourage algae or invite lichens to stay, I’ll walk beside you. We’ll keep the steps safe, the panes clear, and the old stones honest with their age.

What They Are, and Why They Show Up

Algae are simple photosynthetic organisms that bloom wherever moisture lingers and light is sufficient. On glass, stone, plastic, tarmac, or timber, they form a thin film or powdery scum—sometimes emerald, sometimes drab—especially after wet seasons. Their gift is efficiency; their flaw, on paths and steps, is slipperiness.

Lichens are something else entirely: a long marriage between a fungus and an alga (or cyanobacterium). The fungus makes the home; the alga makes the food; together they colonize places most plants cannot—dry walls, old bark, statues, headstones. They are slow. They are resilient. And they are often signs of cleaner air and stable microclimates. On trees, they are not parasites; they simply use the bark as a surface, like a painting resting on a wall.

Both appear where conditions suit them: damp shade and still air summon algae; steady light and time invite lichens. So our first choices are microclimate choices: light, airflow, drainage, patience.

Greenhouse Film: Light, Air, and a Clean Pane

In glasshouses or polytunnels, green film can creep across panes and staging, reducing light just when winter gives the least. I start with prevention: prune anything that casts heavy shade, check gutters, and keep floors free of pooling water. A window propped open on mild days or a fan to stir the air helps surfaces dry and keeps humidity honest.

For cleaning, I go gentle. Warm water, a soft cloth or sponge for glazing, and a mild neutral detergent for grubbier spots—especially on plastic coverings that scratch easily. I schedule one thorough clean each year, ideally before sowing starts, and small wipe-downs whenever I notice the view dimming.

Inside, I corral excess water with saucers or trays, empty them before they sour, and avoid watering late in the day. The aim is not sterile perfection; it’s light and dryness where needed, so algae don’t feel invited.

Slippery Steps, Safe Steps

On paths and steps, algae (and sometimes jelly-like cyanobacteria such as Nostoc) thrive where water sits and sun can’t reach. Safety comes first. I brush weekly with a stiff broom, lift moss from joints with a narrow weeding tool, and, if needed, use a pressure washer carefully—avoiding delicate mortar and directing water away from areas that already drain poorly.

Then I change the conditions: prune overhangs to let wind and light do their drying work; clear drains; lift the grade so water runs off; rake loose gravels to break crusts where growth begins. Chicken wire tacked to wooden treads adds immediate grip. Most of the time, culture beats chemistry: fewer puddles, fewer slips.

If after that the green film keeps returning, I consider a labeled “patio/path” cleaner appropriate for hard surfaces and local regulations, and I follow the label like scripture—no improvisation, no double-dosing “just in case.” Pets and children wait indoors until surfaces are rinsed and dry.

I sweep the slick path as soft morning light warms the lichens.

When (and If) You Reach for Chemicals

There are days when elbow grease and pruning don’t quite win. If you choose a chemical route, pick a product explicitly labeled for algae/moss on hard landscaping, heed local rules, and protect waterways. Many patio cleaners use quaternary ammonium compounds (like benzalkonium chloride); they work but may require repeats and must be kept out of drains and planted beds. Copper-based products can also kill algae but are toxic to aquatic life and carry strict environmental cautions. None of these are “set and forget.”

My safety ritual is simple: read the label in full; wear impervious gloves and eye protection; mix only in plastic containers meant for that task; keep the solution and rinse water away from ponds, storm drains, and vegetable beds; and store leftovers safely. If drift or splash seems likely, I don’t apply. I’d rather accept a little green and stay gentle with the life downstream.

And still, I circle back: the longest-lasting “treatment” is sunlight, airflow, good drainage, and regular brushing. Chemicals are a last resort, not a lifestyle.

Lichens: The Quiet Art on Stone and Bark

I have loved lichens since the first time I noticed them brightening a gravestone: tiny suns and moons—saffron, silver, smoke—shingled across old marble. They do not harm the stone in any ordinary sense, nor do they suck life from trees. They simply live where others would falter. On trunks and fences they add texture and remind us that our air has grown kinder in places where they flourish.

If you grow fruit trees, you might notice lichens where light reaches bark. Their presence usually says more about air, age, and canopy openness than about health; if a tree is failing, the cause almost always lies elsewhere—roots, water, pests, soil. Prune well, feed the soil, and the tree will answer. The lichen will come or go at its own speed.

So in the garden, I treat lichens as I treat freckles: not a flaw, a feature. I keep them off only where they create hazards—like steps—or obscure inscriptions meant to be read.

Where Lichens Thrive: Substrate, Light, and Time

Lichens like stable, relatively clean air and a surface that doesn’t move much: old terracotta, lime-rich mortar, porous stone, weathered timber. They drink from rain and dew, feed on light, and take spare nutrients from dust and bird droppings. They notice pH—some preferring the alkalinity of limestone or mortar, others the faint acidity of bark or sandstone.

They are slow because their partnership is slow: a careful exchange between fungus and alga. Months can pass without visible change, then one winter you realize a pale galaxy has spread across the pot you thought too plain. That is their pace. It cannot be forced, only invited.

If air quality is poor or surfaces are frequently scrubbed or sealed, lichens hesitate. If the site is calm, lightly shaded, and left undisturbed, they begin to write their soft script.

If You Want More Lichens: Patience-Based Methods

I know the internet abounds with recipes—yogurt or buttermilk “slurries,” secret starters, blended green potions. You may see results on porous stone, but evidence is mixed; often what grows first is a general biofilm that merely darkens the surface. I prefer a quieter approach that respects how lichens truly establish.

Choose the right canvas: rough, mineral-rich, unsealed stone; old terracotta; lime mortar. Place it where morning light kisses and afternoons cool—a stable place that gathers dew. Avoid frequent scrubbing, and never harvest rare lichens from the wild. If you have ethically sourced fragments from common species—say, chips shed naturally from stones in your own garden—press a few crumbs onto damp stone and mist lightly. Then wait. Weeks yield texture; months yield speckles; years yield tapestries.

If you do try a weak dairy wash for patina, keep it minimal, outdoors, and expect smell and inconsistency. Better, in my heart: set the stage and let the wind deliver what belongs.

Moss, Nostoc, and Other Lookalikes

Not everything green is algae. After rain, Nostoc can appear on compacted gravel as dark, gelatinous pads that dry to a crust. It is slippery, but the cure is cultural: rake to break compaction, improve drainage, and reduce routine wetness. Mosses, meanwhile, are miniature forests that take root in joints and shady corners; they can be lifted with a narrow tool or kept as velvet in places you don’t walk.

Distinguishing who’s who helps you choose the least invasive response—brushing where brushing suffices, redesigning where water lingers, and letting velvet be velvet where it harms nothing.

Gravestones and Old Walls: Care, Ethics, and Restraint

In burial grounds and on heritage stone, removal is never just removal. Lichens bind tender surfaces; harsh cleaning can pull off more than growth—it can strip history. Where names are obscured or safety requires action, use the gentlest means: soft brushes, plenty of clean water, and neutral non-ionic cleansers. Avoid wire brushes, grinders, strong acids, hypochlorite, and high-pressure jets. If in doubt, wait and seek conservation guidance; sometimes leaving lichens protects the stone best.

Old walls live longer with like for like. Lime mortar suits lime mortar; piecemeal repairs let colonizers wander back. If a memorial must be moved, keep the original orientation so the lichen community can adapt rather than die. Conservation is patience wearing work gloves.

In gardens, I keep the romance where it belongs: on statues and boundary walls, not on the steps that carry us. That line—between charm and hazard—is where care becomes an art.

A Simple Two-Week Plan for a Safer, Quieter Garden

Week One: Walk your routes after rain. Note where feet slip, where water sits, and where shade persists. Prune just enough to let light and wind reach the ground. Brush steps and paths thoroughly; clear drains; lift moss from joints. In greenhouses, wipe panes with warm water and a soft cloth; set a fan or crack windows on mild days.

Week Two: Re-walk the routes. If slime persists in critical areas, consider a labeled patio cleaner and follow the instructions exactly, keeping runoff out of beds and drains; otherwise, repeat brushing. Meanwhile, “plant” some character: place porous pots or a piece of unsealed stone in a calm, lightly shaded corner. Resist the urge to fuss. Give lichens the stage and time.

Choose Your Patina

I want your garden to keep you safe and also make you ache a little with beauty. Keep the algae off your steps and greenhouse panes. Let the lichens ghost your walls and pots if you love that sense of age. Every choice is a mood, a boundary, a story the house tells about what matters here.

When the light returns, follow it a little. It will show you where to sweep, and where to pause and whisper thank you.

References

Royal Horticultural Society (RHS) guidance on algae, lichens, liverworts, and mosses (greenhouses; hard surfaces; trees and shrubs).

Natural History Museum and JNCC resources on lichens as air-quality indicators.

British Lichen Society notes on lichen biology and conservation practice.

U.S. EPA and other authorities on environmental cautions for copper-based products.

Historic England, National Park Service (U.S.), and War Memorials Trust advice on cleaning historic stone and biological growth.

Disclaimer

This article offers general gardening information. Always follow local regulations and product labels. Keep pets and children away from treated areas until dry. Avoid allowing any cleaner or rinse water to reach waterways, ponds, or storm drains. For historic stones or memorials, consult professional conservators.