How to Get Rid of Nostoc Algae and Prevent Its Return

Nostoc is one of the most stubborn organisms you can find growing on lawns, patios, and gravel paths, and most of the products people reach for first do nothing against it. Getting rid of it requires understanding that Nostoc is not ordinary algae, that common herbicides and algaecides are largely useless here, and that the only lasting fix involves changing the conditions that invited it in the first place. The good news is that once you address the root causes, Nostoc rarely returns in force.

Nostoc Is Not Actually Algae

The dark, jelly-like blobs that appear on wet ground after rain are almost always colonies of Nostoc commune, a cyanobacterium sometimes called “star jelly” or “witch’s butter.” Although people call it algae, cyanobacteria are a distinct group of organisms. This matters for practical reasons: products designed to kill green algae often fail against Nostoc because its biology is fundamentally different. Nostoc forms thick colonies held together by a rubbery sheath of extracellular polysaccharides that protect the cells inside from drying out, freezing, and chemical exposure.¹ That gelatinous sheath is the reason Nostoc feels like wet rubber when you pick it up, and it is also the reason the organism is so hard to kill.

Nostoc has a cosmopolitan distribution, meaning it grows on every continent, including Antarctica. It thrives in disturbed, poorly drained areas where other plants struggle. It can fix its own nitrogen from the atmosphere, which means it does not need fertile soil to grow.² In fact, it actually prefers poor, compacted, nutrient-depleted soil where grass and other ground covers have thinned out. If your yard has bare patches with standing water after rain, that is Nostoc’s ideal habitat.

Why Common Products Fail

The first instinct most people have is to spray the Nostoc with an algaecide or herbicide they already own. Trials conducted at Oregon State University tested the most commonly recommended products and found that most of them are ineffective or counterproductive. Algaecides based on sodium carbonate peroxyhydrate and copper sulfate provided little to no control.³ These products work against thin-walled green algae but cannot penetrate the thick polysaccharide sheath that surrounds Nostoc colonies.

Glyphosate-based herbicides like Roundup are an even worse choice. Not only does glyphosate fail to control Nostoc, but killing the surrounding grass and weeds with glyphosate releases phosphorus from the decaying plant matter, and that phosphorus actually feeds the Nostoc.³ Spraying Roundup on or near Nostoc can make the problem worse than doing nothing at all. This is one of the most common and costly mistakes homeowners make with Nostoc.

Herbicidal soaps based on ammoniated soap of fatty acids or potassium salts of fatty acids can provide some limited effect, but they are inconsistent and rarely eliminate established colonies. If you have already tried one of these products and seen minimal results, you are not doing anything wrong; the products just are not well suited to this organism.

The One Chemical That Works

The same Oregon State University trials found that the most effective chemical product against Nostoc is pelargonic acid, sold under the brand name Scythe. Pelargonic acid is a naturally occurring fatty acid that strips the protective sheath from Nostoc cells and kills them on contact. In those trials, Scythe not only killed active Nostoc colonies but also prevented regrowth for several weeks.³ It is labeled for treatment of algae, moss, and liverworts in container nurseries, making it one of the few products with a label-appropriate use against organisms in this category.

There is an important timing detail: you must apply pelargonic acid when the Nostoc is hydrated and swollen after rain or watering. Dried Nostoc mats are not susceptible to chemical treatment.³ If you spray dried, dormant Nostoc, you are wasting product. Wait until after a rain event, when the colonies have puffed up into their characteristic dark green or olive-brown jelly blobs, and then apply. Pelargonic acid is non-selective, meaning it will damage or kill any plant it contacts, so keep it away from desirable grass and ornamentals.

Scythe is primarily marketed to commercial growers and nurseries, so you may not find it at a typical home-improvement store. Look for it through agricultural supply retailers or online. Some other pelargonic-acid-based products exist on the consumer market under different brand names, but check the active ingredient and concentration to make sure you are getting a comparable formulation.

Physical Removal

For small infestations, physically scraping or raking up the Nostoc colonies is straightforward and often the fastest approach. Use a flat-edged shovel, a stiff garden rake, or even a putty knife on hard surfaces like patios and walkways. On lawns, a dethatching rake works well. Bag and dispose of the removed material rather than composting it, because Nostoc colonies can survive composting conditions and re-establish when spread back onto the landscape.

Physical removal alone will not solve the problem if you do not also address the underlying conditions. Nostoc’s ability to survive desiccation means that even tiny fragments left behind can rehydrate and regrow when conditions are right. Research on Nostoc commune has shown that the speed at which dried colonies absorb water and resume activity depends on how long they have been dry, but even colonies that have been desiccated for extended periods can come back to life.⁴ In practical terms, this means you cannot simply scrape it up and assume you are done.

Why Nostoc Is So Hard to Kill Permanently

Understanding why Nostoc keeps coming back helps explain why prevention matters more than treatment. The organism has evolved a suite of survival tools that make it one of the most resilient life forms in terrestrial environments. Its extracellular polysaccharide sheath retains water during dry spells and acts as a physical barrier against UV radiation and temperature extremes.⁵ Related Nostoc species in desert environments also produce UV-absorbing pigments and accumulate internal sugars that prevent cellular damage during drought.⁵

When a Nostoc colony dries out, it does not die. It enters a dormant state, turning into a dark, papery crust that can persist for months or years. The moment rain arrives, the colony absorbs water, swells to many times its dry size, and resumes photosynthesis and nitrogen fixation within hours. Studies on Nostoc commune have confirmed that the photosynthetic apparatus inside the cells remains essentially undamaged through repeated cycles of drying and rehydration, thanks to the protective polysaccharide matrix.¹ Freezing and thawing cycles do not kill it either. This remarkable tolerance is why a colony you thought you eliminated in summer can reappear the following spring as if nothing happened.

Nostoc’s ability to fix atmospheric nitrogen compounds the problem. Most weeds and algae depend on soil nutrients, so improving drainage and reducing fertilizer can starve them out. Nostoc produces its own nitrogen, so it can colonize bare, nutrient-poor soil that nothing else will grow in.² Reducing soil fertility does not deter it; in fact, areas that are too poor for grass are often the exact areas where Nostoc moves in.

Fixing the Conditions That Invite Nostoc

Because chemical and physical removal can only address existing colonies, the real prevention strategy is making your landscape inhospitable to Nostoc. The organism consistently colonizes areas with three overlapping conditions: poor drainage, compacted soil, and sparse plant cover. Addressing any one of these helps; addressing all three usually eliminates the problem.

Drainage is the single most important factor. Nostoc needs prolonged surface moisture to stay hydrated and metabolically active. If water pools on your lawn, patio edges, or gravel paths after rain, that standing water is the primary invitation. Grading the soil to direct water away from flat areas, installing French drains or channel drains along hardscapes, and breaking up compacted subsoil all reduce the moisture window Nostoc depends on. On patios and walkways, check that the surface slopes slightly away from the house and that joints between pavers are not holding water.

Soil compaction is often both a cause of poor drainage and a direct contributor to Nostoc growth. Compacted soil prevents water from percolating downward, keeping the surface wet longer. It also inhibits root growth of grass and ground covers, creating the bare patches Nostoc exploits. Core aeration of lawns, done once or twice a year, opens up the soil structure, improves drainage, and helps grass fill in bare spots. For areas that are heavily compacted from foot traffic or equipment, a deeper mechanical aeration or tilling may be necessary before reseeding.

Establishing dense, healthy ground cover is the final piece. Nostoc cannot compete with vigorous turf or ground-cover plants for light and physical space. Overseeding thin lawns, planting shade-tolerant ground covers under trees, and maintaining adequate but not excessive fertilization all help close the gaps that Nostoc colonizes. On gravel paths and patios, consider whether the surface design itself is trapping moisture. Landscape fabric beneath gravel, proper base material, and adequate slope can reduce the chronically wet conditions that Nostoc favors.

A Step-by-Step Approach

Putting all of this together, here is a practical sequence for dealing with an active Nostoc problem:

Remove active colonies: After rain, when colonies are swollen and visible, scrape or rake them up and bag them for disposal. On hard surfaces, a pressure washer on a low setting can dislodge the jelly-like mass.
Apply pelargonic acid if needed: For heavy infestations or areas where physical removal is impractical, apply a pelargonic-acid product to hydrated colonies. Avoid contact with desirable plants.
Improve drainage: Grade soil, install drains, or add organic matter to break up compacted ground. This is the step that prevents recurrence.
Aerate and reseed: Core-aerate compacted lawns and overseed with grass varieties suited to your conditions. Shade-tolerant mixes are especially useful under trees, where Nostoc often thrives.
Monitor after rain: Check treated areas after the next few rain events. Small reappearances are normal, since dormant fragments can rehydrate. Spot-treat or remove them promptly before they spread.

Patience is part of the process. Because dormant Nostoc can survive in the soil for a long time, you may see small patches return even after you have corrected the drainage and filled in bare spots. Each time you remove or treat a returning patch, the overall colony mass decreases. Once the grass or ground cover is thick enough to shade the soil surface and drainage keeps the area from staying waterlogged, the regrowth usually stops on its own within a season or two.

Nostoc on Hard Surfaces

Nostoc on patios, sidewalks, driveways, and gravel is a slightly different problem than Nostoc on lawns. You do not have to worry about damaging turf when treating it, but you also cannot solve the problem by planting grass. On hard surfaces, the key variables are texture and slope. Rough-textured concrete, flagstone, and unsealed brick all hold moisture in their pores and joints, giving Nostoc a foothold. Smooth, sealed surfaces shed water faster and are much less prone to colonization.

If Nostoc keeps returning to a patio or walkway, check whether the grout lines or sand joints between pavers are sitting below the surface of the stones. Sunken joints trap water and organic debris, creating perfect microhabitats. Re-sanding or re-grouting to bring the joints flush with the stone surface can help. Polymeric sand, which hardens after wetting, resists the moisture retention that regular joint sand allows. For gravel paths, raking the gravel periodically to expose fresh surfaces and prevent a crust of organic matter from building up can reduce Nostoc establishment.

Pressure washing is effective for removing Nostoc from hard surfaces, but it is a treatment, not a cure. If the surface still holds water after you clean it, the colonies will come back. Combining pressure washing with a drainage correction, even something as simple as shimming a paver to redirect water flow or clearing a clogged edge drain, addresses the underlying problem.

When Nostoc Is Not the Enemy

It is worth noting that Nostoc is not inherently harmful. In many ecosystems, it plays a valuable role as a pioneer organism that colonizes bare, degraded ground and enriches the soil with nitrogen. Desert biological soil crusts, which help prevent erosion and kickstart ecological recovery in arid landscapes, often contain Nostoc species as a key component.⁵ In your own yard, the appearance of Nostoc is a signal that something about the site conditions is off: too wet, too compacted, too bare. Rather than viewing it purely as a pest, you can read it as a diagnostic indicator. The same fixes that eliminate Nostoc, improving drainage, loosening compacted soil, and establishing healthy plant cover, also make for a better-functioning landscape overall.

Some species of Nostoc are even eaten as food in parts of Asia and have a long history of use in traditional cuisine. The terrestrial species that colonizes lawns and sidewalks is not toxic in the way some other cyanobacteria are, though it is not considered edible in a Western context and nobody is going to mistake the slimy blobs on a patio for a delicacy. The point is simply that Nostoc’s presence says more about your site conditions than it does about any failure on your part. Fix the conditions, and the organism quietly bows out.

1
PubMed Central. Crucial role of extracellular polysaccharides in desiccation and freezing tolerance in the terrestrial cyanobacterium Nostoc commune
2
PubMed Central. Dried Nostoc commune exhibits nitrogen-fixing activity using glucose under dark conditions after rehydration
3
BYGL | Ohio State University Extension. The Inside Story on Obnoxious Nostoc
4
PubMed Central. Unusual water flux in the extracellular polysaccharide of the cyanobacterium Nostoc commune
5
PubMed Central. Molecular Mechanisms of Nostoc flagelliforme Environmental Adaptation: A Comprehensive Review