Biostimulants 101: What they are and why your crops need them

If you’ve been following the biologicals conversation for any amount of time, you’ve probably come across the word “biostimulant.” It’s everywhere right now — on product labels, at trade shows, in research papers, in marketing brochures. But what does it actually mean?

Here’s the honest truth: even within the industry, the definition isn’t always consistent. Different countries regulate biostimulants differently. Some companies use the term loosely. And for farmers trying to make practical decisions, the confusion can be a real barrier.

So let’s clear it up.

What is a biostimulant?

A biostimulant is any substance or microorganism that, when applied to a plant or its environment, enhances nutrient uptake, improves stress tolerance, or stimulates natural growth processes — without being a nutrient itself and without directly killing pests or pathogens.

That last part is key. A biostimulant doesn’t feed your plant the way a fertilizer does. And it doesn’t fight disease the way a fungicide does. Instead, it makes your plant better at doing what it already does naturally.

Think of it like this: if your crop were an athlete, a fertilizer would be food, a pesticide would be medicine, and a biostimulant would be the coach — helping the athlete perform at its best, recover faster from stress, and use its energy more efficiently.

What biostimulants are NOT

This is where a lot of confusion lives, so it’s worth being explicit:

Biostimulants ≠ Biofertilizers. Biofertilizers are microorganisms that directly make nutrients available to plants (like nitrogen-fixing bacteria or phosphorus-solubilizing fungi). Biostimulants improve how the plant accesses and uses those nutrients, but they don’t supply them directly.

Biostimulants ≠ Biopesticides. Biopesticides (including biocontrol agents) are designed to suppress or kill pests and pathogens. Biostimulants don’t have a direct biocidal action. However, a healthier, less-stressed plant is often more resistant to disease — so there’s an indirect connection.

Biostimulants ≠ Growth regulators. Plant growth regulators (PGRs) are synthetic hormones that force specific developmental responses. Biostimulants work with the plant’s own hormonal and metabolic pathways rather than overriding them.

In reality, the lines can blur. Some products have multiple functions. But understanding these distinctions helps you choose the right product for the right job.

The main types of biostimulants

Biostimulants come in many forms, but most fall into a few main categories. Here are the ones you’re most likely to encounter:

1. Humic and fulvic acids

These are organic compounds derived from decomposed plant material — essentially, the building blocks of rich, healthy soil. Humic acids improve soil structure and increase the soil’s capacity to hold nutrients. Fulvic acids are smaller molecules that act as natural chelators, helping transport micronutrients into the plant more efficiently.

If your soil has been depleted by years of chemical-heavy farming, humic and fulvic acids are often one of the first things to reintroduce. They don’t replace what’s been lost, but they help the soil start functioning again.

2. Seaweed (algae) extracts

Seaweed-based biostimulants — most commonly derived from brown algae like Ascophyllum nodosum — are among the most widely used in the world. They contain a complex mix of natural growth-promoting compounds: cytokinins, auxins, betaines, and polysaccharides like alginates and laminarin.

What does this mean in practice? Plants treated with seaweed extracts tend to show improved root development, better tolerance to drought and salinity stress, and more uniform fruit set. They’re particularly popular in high-value crops like grapes, berries, and vegetables.

3. Amino acids and protein hydrolysates

Amino acids are the building blocks of proteins — and plants need them for virtually everything: enzyme production, chlorophyll synthesis, stress defense. When plants are under stress (heat, drought, transplanting), their ability to produce amino acids drops. Applying amino acid-based biostimulants gives them a shortcut.

Protein hydrolysates are essentially pre-digested proteins (from plant or animal sources) broken down into free amino acids and peptides. They’re fast-acting and particularly useful during critical growth stages or recovery periods.

4. Microbial biostimulants

This is where microbiology meets agriculture in a powerful way. Certain microorganisms, when applied to seeds, roots, or soil, can stimulate plant growth through mechanisms that go beyond simple nutrient supply. The two most prominent examples:

Trichoderma species — these fungi colonize the root zone and promote root growth, improve nutrient uptake, and can even trigger the plant’s own defense mechanisms (a phenomenon called “induced systemic resistance”). Trichoderma is incredibly versatile and is used across a wide range of crops.

Mycorrhizal fungi — these form a symbiotic relationship with plant roots, extending the root’s effective reach by creating a network of fungal filaments (hyphae) that access water and nutrients the roots can’t reach on their own. In phosphorus-poor soils, mycorrhizae can be transformative.

How do biostimulants actually work?

Without getting too deep into the biochemistry, biostimulants influence plants at multiple levels:

At the root level: Many biostimulants promote root proliferation — more roots, more root hairs, deeper penetration. This means better water and nutrient uptake from the soil, especially under suboptimal conditions.

At the cellular level: Biostimulants can activate antioxidant defense systems, improve membrane stability, and enhance osmotic regulation. In plain language: the plant handles heat, cold, drought, and salinity better.

At the metabolic level: Some biostimulants improve photosynthetic efficiency or enhance nitrogen assimilation, meaning the plant converts sunlight and nutrients into biomass more effectively.

At the microbial level: Certain biostimulants — especially microbial ones — modify the rhizosphere (the soil zone immediately around the roots), encouraging beneficial microbial communities and suppressing pathogenic ones.

The important takeaway is that biostimulants don’t force the plant to do something unnatural. They optimize what the plant is already designed to do. That’s why their effects are often most visible when the plant is under stress — because that’s when the gap between “surviving” and “thriving” is widest.

When should you apply biostimulants?

Timing matters. Biostimulants aren’t a “spray and forget” product. For the best results, consider applying them at key moments in the crop cycle:

Seed treatment or transplanting: Microbial biostimulants and amino acids applied early help establish strong root systems from the start.

Vegetative growth stage: Seaweed extracts and humic acids during active growth support nutrient uptake and canopy development.

Pre-stress periods: If you know a heat wave, frost event, or dry spell is coming, applying a biostimulant beforehand can prime the plant’s defense mechanisms. 

Flowering and fruit set: This is a critical energy-demanding phase. Amino acids and seaweed extracts can improve pollination success and fruit uniformity.

Post-stress recovery: After a hailstorm, flood, or chemical burn, biostimulants help the plant bounce back faster by supporting repair processes.

Common myths vs. reality

Myth: “Biostimulants can replace fertilizers.”

Reality: They can’t. Plants still need macro and micronutrients. What biostimulants can do is make your fertilizer program more efficient, meaning you might get the same or better results with less fertilizer input. 

Myth: “If I can’t see immediate results, it’s not working.”

Reality: Biostimulant effects are often subtle and cumulative. You might not see a dramatic change after one application, but over a season — especially under stress conditions — the treated plots consistently outperform. Measure root mass, stress recovery time, and yield uniformity, not just above-ground visual impact.

Myth: “All biostimulants are the same.”

Reality: This is like saying all medicines are the same because they come in a pill. The source material, extraction method, concentration, and microbial strain all matter enormously. A cheap seaweed extract is not the same as a well-characterized Ascophyllum nodosum product with proven bioactivity. Quality and consistency are everything.

The bottom line

Biostimulants aren’t magic. They won’t rescue a fundamentally broken management program. But when used correctly — as part of an integrated approach alongside good nutrition, proper irrigation, and sound crop management — they can be the difference between a crop that merely survives and one that thrives.

For farmers transitioning from a purely chemical approach, biostimulants are often the easiest entry point into the biologicals world. They’re compatible with most conventional programs, relatively low-risk to trial, and their benefits compound over time as soil health improves.

In the next edition, we’ll go deeper into biofertilizers and the soil microbiome — the living foundation that makes everything else work.