Spirulina Not Growing? Causes and Solutions

What's really behind it, and what you do now.

Your farm is probably not broken, it's young

This is the first thing we tell everyone who writes to us: most of the farms people tell us about aren't broken at all. They're just young.

Spirulina doesn't grow at the push of a button. The first batch takes longer than any thereafter, because the culture needs to establish itself. The cell count increases slowly at first, the water looks thin and pale, and if you then compare it to the rich dark green in the shop photo, you'll prematurely draw the wrong conclusion. This is the most common mistake of all. From the second cycle onwards, it goes much faster, because the colony then already has a solid base.

You can find the complete step-by-step setup on the Set up your farm page and in the blog article Growing your own Spirulina: from unboxing to first harvest in 14 days. This is about what comes after..

Light green instead of dark green: normal or a problem?

Light green water in the first few days is not an alarm signal. It is the normal state of a young culture.

Spirulina grows best between 30 and 37 degrees Celsius, feeling most comfortable at around 35 degrees. Cooler doesn't mean dead, but slow: below 25 degrees, the culture grows noticeably sluggishly, but remains productive with light. Light is the second factor anyway. Too little of it, and the algae produces less chlorophyll, the water remains pale. A north-facing window in November is not a location, but a test of patience.

If you don't see any visible change by day five or six, there's no need to panic. Wait at least until day 10 to 14 before drawing any conclusions. In the meantime, check whether the grow light reliably switches on and whether the farm is in a really warm place. Everything about light, battery and aeration is explained in detail on the Light, Battery, Bubbling page.

Slow growth: the two unspectacular main reasons

The most common reason why spirulina doesn't grow is unspectacular: too little light and too little warmth.

No contamination, no defective device, no bad culture. Simply the wrong environment. Before you try anything else, check three things:

The Care subscription delivers nutrient salt and fresh culture every three months, so you don't run out of anything.

And here's an honest statement we'd rather make beforehand than afterwards: Anyone who permanently lacks the ability to provide warmth and light to the farm should know this before buying. A culture that is continuously kept cool and does not receive artificial light will never yield the expected results. This is not a device error, but a matter of conditions.

Brown spots on the glass: mostly harmless diatoms

Brown spots on the glass are usually harmless diatoms. No reason to panic, but an indication to change something for the next batch.

Diatoms, also known as Bacillariophyceae, occasionally settle in Spirulina cultures, especially if the water used contains a lot of silicate. They compete with the Spirulina for nutrients and can slow down growth. They become visible as a brownish coating, often first at the edge of the glass or on the bottom.

Rinse the harvest thoroughly before consumption, and the coating is not an issue. If you want to prevent it, use filtered water next time. If it gets out of hand, a water change with a new setup is a clean cut. How this works in practice is described step-by-step on the Resetting your Spirulina Farm page. Important: Brown spots alone are not a safety concern. But if an unpleasant smell is added, different rules apply. More on that shortly.

Spirulina floats to the top: almost always the pump

When spirulina floats on the surface instead of being evenly distributed in the water, the pump is usually not running.

Spirulina needs movement. Air circulation distributes light and nutrients evenly through the water. Without circulation, the algae accumulate at the top, shade each other, and hardly grow. First, check if the battery is charged and if the bubbling is audible. The most common causes and solutions for this can be found on the Light, Battery, Bubbling page.

Why a Spirulina culture doesn't just go bad

Almost no one explains this, yet it's the most important thing regarding safety: The farm is not a hygiene lottery.

The nutrient salt creates a strongly alkaline environment with a pH value between 9 and 11. Spirulina loves these conditions. For most other organisms, mold, putrefactive bacteria, the usual suspects, these conditions are simply uninhabitable. This is precisely why Spirulina is cultivated on a large scale in strongly alkaline water, as described in the FAO overview report on the cultivation and use of Spirulina (2008). A Spirulina culture therefore does not simply "go bad" like a forgotten glass of water on the windowsill. This alkaline environment is the actual reason why fresh Spirulina can be safely grown at home.

That doesn't mean nothing can go wrong. But it does mean that the hurdle for real problems is significantly higher than many assume.

The one rule we are uncompromising about: trust your nose

Three points really matter. Regarding smell: Fresh spirulina has a subtle green and slightly earthy scent, similar to damp forest soil or freshly washed spinach. More on how fresh spirulina smells and tastes can be found in What does spirulina really taste like. Musty, foul, or chemical means: throw it away. As for color: dark green to blue-green is normal. Bluish-violet or grayish? Do not harvest. And for consistency: The paste is soft and uniform. Slimy or stringy in an unusual way? Throw it away.

A new spirulina culture costs less than a spoiled day. This is not an exaggeration, but a trade-off that we want to make clear for everyone.

If none of these three warning signs apply, the harvest is fine. The FAQ on our website addresses further safety questions if you are unsure about something specific.

If the culture truly fails: our replacement promise

It rarely happens. But if a culture truly fails, we will replace it within the first two harvest cycles. This is a promise, not fine print.

Simply write to us, explain what happened, and we'll take care of it. Then you start over with a fresh spirulina culture, and the next attempt begins with the knowledge gained from the first.

A living organism, not a machine

This is a living organism, not a machine. Sometimes it grows faster, sometimes slower, depending on the sun, room temperature, and time of year. This fluctuation is not a defect. It is the difference between something living and a can from the shelf.

Those who anticipate this are better prepared than those who expect a consistent supply chain. And once you know the rhythm of your farm, you also know when something is truly wrong. If you don't have a farm yet, but already know that a warm, bright window sill is free: The MySpirulina Starter Kit contains everything you need for the first batch.

Quick Overview: Symptom, Cause, Action

Symptom	Probable Cause	What you do
Light green water in the first few days	young culture, often also too little light or warmth	wait until day 10 to 14, check location
Barely visible growth	too little light, too cold, too little nutrient salt during preparation	check grow light and temperature, use full 45 grams when restarting
Brown film on the glass	Diatoms, often with silicate-rich water	rinse harvest, next batch with filtered water
Spirulina floats to the top	pump or aeration not running	charge battery, check cable and bubbling
Musty smell, bluish color, slimy consistency	real problem	do not harvest, dispose of culture, restart with fresh culture

Frequently Asked Questions

How long does spirulina cultivation take?

The first harvest usually takes 10 to 14 days. The very first batch often takes longer because the culture needs to establish itself. From the second cycle onwards, it goes noticeably faster.

Is home-grown spirulina safe?

Yes, under two conditions. The alkaline environment in the glass keeps most foreign organisms away, and the culture and nutrient salt are laboratory-tested. And you check each harvest for smell, color, and consistency. Restarting with a fresh, lab-tested starter culture every three months further reduces the risk.

Does spirulina also grow in winter?

Yes, with the integrated grow light. It grows slower because there is less daylight and often lower temperatures, but it does grow.

Can I still save a light green culture?

Mostly yes. In most cases, it lacks light or warmth, not health. Change location, check grow light, be patient for a few more days.

How often should I restart?

After each harvest, you restart with 250 to 500 milliliters of your own culture. Approximately every three months, we also recommend a fresh, lab-tested starter culture.