Biostimulants are one of those words that sound a bit science-lab-ish at first. You might see them on bottles of seaweed feed, compost extracts, microbial inoculants, amino acid sprays, humic products, root drenches or “plant health tonics” and wonder: is this fertiliser, plant medicine, snake oil, or something genuinely useful?

The answer is: sometimes genuinely useful, sometimes over-marketed, and often misunderstood.

For gardeners, biostimulants can be thought of as products or natural preparations that help plants and soil life work better. They are not mainly about feeding plants in the same way as a nitrogen-rich fertiliser. Instead, they aim to stimulate natural plant and soil processes: better rooting, better nutrient uptake, improved tolerance to stress, stronger growth, better quality crops, or more active biology around the roots.

For regenerative farmers, market gardeners and growers, biostimulants sit in a bigger conversation: how do we reduce reliance on soluble inputs, improve soil function, build crop resilience, and support nutrient cycling without pretending there is a magic bottle that replaces good soil management?

That last bit matters. A biostimulant is not a substitute for compost, soil cover, diverse rotations, living roots, good drainage, appropriate grazing, careful cultivation or balanced nutrition. But used wisely, it can be a helpful tool.

What Is a Biostimulant?

A plant biostimulant is generally understood as a substance, microorganism or product that stimulates natural plant processes, independently of its nutrient content, with the aim of improving things like nutrient efficiency, tolerance to abiotic stress, crop quality or nutrient availability around the roots. The EU Fertilising Products Regulation defines plant biostimulants around this idea: they stimulate plant nutrition processes independently of the product’s nutrient content, aiming to improve nutrient use efficiency, abiotic stress tolerance, crop quality traits, or nutrient availability in the soil and rhizosphere.

That phrase “independently of nutrient content” is the key. A fertiliser feeds. A biostimulant stimulates.

The Royal Horticultural Society gives a very gardener-friendly explanation: biostimulants are not fertilisers, but products that may improve plant growth and crop yield by activating or boosting natural processes. They compare them loosely to vitamin tablets: not food in themselves, but something that may support normal functioning alongside food.

So, in plain English:

Fertiliser = nutrients. Biostimulant = support for plant and soil processes.

Of course, in real life, the line can get blurry. Seaweed products, for example, may contain small amounts of nutrients, plant compounds, trace elements and other substances. Some commercial products are sold as fertiliser “with added seaweed,” while others are sold mainly as seaweed extract or biostimulant. The RHS notes that fortified seaweed extracts usually contain added nitrogen, phosphorus and potassium and quote an NPK ratio on the label.

That means reading the label is important. A product can be a fertiliser, a biostimulant, or a blend of both.

Why Do Gardeners Need to Understand Biostimulants?

Because the gardening world is full of bottles with big promises.

“Stronger roots.” “Healthier plants.” “More flowers.” “Improved resilience.” “Bigger harvests.” “Natural growth enhancer.” “Soil activator.”

Some of these claims may be reasonable. Some may be vague. Some may depend hugely on context.

Understanding biostimulants helps you avoid two common mistakes. The first is dismissing them all as nonsense. The second is expecting them to fix poor soil, hungry plants, drought, compaction, disease pressure or bad growing conditions all by themselves.

A biostimulant is usually most helpful when it is part of a wider growing system. In a garden, that could mean compost, mulch, crop rotation, good watering, healthy potting compost, appropriate feeding and wildlife-friendly practices. In a regenerative farm or market garden, it could mean reduced soil disturbance, cover crops, compost extracts, diverse rotations, agroforestry, livestock integration, foliar monitoring and careful nutrient planning.

Think of biostimulants as a nudge, not a miracle.

They may help plants cope better, root better, use nutrients more efficiently or recover from stress. But they cannot overcome everything. A tomato plant in exhausted compost with no water and no light is not going to be saved by a splash of seaweed. A wheat crop in compacted anaerobic soil will not become regenerative because someone sprayed a biostimulant at flag leaf. Soil is cheeky like that. It insists on fundamentals.

What Are Biostimulants Used For?

Biostimulants are used in several main ways: to support plant growth, improve rooting, help crops tolerate stress, improve nutrient uptake, support soil biology, enhance crop quality, or improve establishment after planting.

Supporting Root Growth

Many gardeners use biostimulants when sowing, potting on, transplanting or planting out. The logic is simple: if a plant establishes a strong root system early, it has a better chance of finding water and nutrients later.

Products used for this purpose might include seaweed extract, humic acids, mycorrhizal fungi, amino acids or microbial inoculants. In practical terms, these might be applied as a root dip, seed treatment, watering-in solution or compost amendment.

For market gardeners, root-focused biostimulants can be especially interesting during transplant production. Healthy module plants with strong roots often establish more evenly, suffer less transplant shock and make better use of short growing windows.

Helping Plants Cope with Stress

One of the biggest reasons growers use biostimulants is to help plants tolerate abiotic stress. That means non-living stresses such as drought, heat, cold, salinity, waterlogging, nutrient stress or transplant shock.

This does not mean a biostimulant makes a plant invincible. It means certain products may support the plant’s own stress responses. Seaweed extracts, for example, are widely researched and commonly used because seaweeds live in harsh tidal environments and contain a complex range of biologically active compounds.

The European Biostimulants Industry Council describes plant biostimulants as useful in areas including productivity, nutrient use efficiency, stress relief, crop quality, economic performance and sustainability.

For regenerative farmers, this stress-resilience angle is important because climate instability is now part of everyday growing. Longer dry spells, sudden downpours, late frosts, heat spikes and unpredictable seasons all put pressure on crops. Biostimulants may be one tool in a broader resilience strategy, alongside soil organic matter, cover cropping, water management, shelterbelts and crop diversity.

Improving Nutrient Use Efficiency

A good biostimulant is not necessarily adding lots of nutrients. Instead, it may help the plant access or use nutrients more efficiently.

For example, microbial products may support nutrient cycling around roots. Humic and fulvic substances may influence nutrient availability and root uptake. Seaweed extracts may affect root growth and physiological processes. Mycorrhizal fungi can form associations with plant roots and extend the effective root system, particularly for phosphorus uptake, although results depend strongly on soil conditions, crop species and existing soil biology.

For farmers, nutrient use efficiency matters because fertiliser is expensive, energy-intensive and environmentally sensitive. If crops can make better use of existing nutrients, that can support profitability and reduce losses. But this is also where expectations need to be grounded. AHDB has reviewed biostimulant evidence in cereals and oilseed rape and notes that products need careful selection and management.

Supporting Soil Biology

Some biostimulants are aimed less at the leaf and more at the rhizosphere - the lively zone around plant roots where soil, microbes, root exudates and nutrients interact.

This includes compost teas, compost extracts, microbial inoculants, humic substances, molasses-based feeds and other biological preparations. Their purpose is often to increase microbial activity, improve nutrient cycling or encourage beneficial root-microbe relationships.

In a regenerative system, this makes intuitive sense: living roots feed soil organisms, and soil organisms help plants access nutrients. However, adding biology to soil is not always straightforward. If the habitat is wrong - compacted, bare, dry, chemically disrupted or low in organic matter - introduced microbes may not establish well.

That means the best “biostimulant” for soil biology might sometimes be a cover crop, compost mulch or reduced tillage rather than a bought product. Tiny underground livestock are fussy tenants.

Improving Crop Quality

Biostimulants are also used to improve quality traits: colour, flavour, firmness, shelf life, sugar levels, uniformity, flowering or fruit set. In horticulture, this is a major reason for interest.

For gardeners, that might mean better tomatoes, stronger seedlings, healthier brassicas or more resilient ornamentals. For commercial growers, quality improvements can affect marketability and waste reduction.

However, quality claims vary by crop, product and conditions. A product that helps strawberries under stress may not do much for carrots in fertile soil. Trialling matters.

Which Products Are Biostimulants?

Biostimulants come in several broad categories. You may already be using some without calling them biostimulants.

Seaweed Extracts

Seaweed is probably the most familiar biostimulant for gardeners. Liquid seaweed feeds, kelp extracts and seaweed-based root drenches are widely used in gardens, allotments and horticulture. The RHS notes seaweed’s long-standing reputation as a soil improver and fertiliser source, especially in coastal regions.

Seaweed extracts are often used for transplanting, general plant health, stress tolerance and rooting. They may be applied as foliar sprays or watered into the soil. Some are pure seaweed extracts, while others include added NPK fertiliser.

Humic and Fulvic Acids

Humic substances are derived from decomposed organic matter, lignite, leonardite, composts or other organic materials. They are often used to support nutrient availability, root development and soil structure.

Humic acids tend to be larger molecules and are often associated with soil applications. Fulvic acids are smaller and more soluble, so they are often used in foliar feeds and liquid nutrient products.

In regenerative growing, humic substances are sometimes used alongside compost, fish hydrolysate, seaweed, molasses or microbial inoculants.

Amino Acids and Protein Hydrolysates

Amino acid products are made from broken-down proteins, often from plant or animal sources. They may be used to support plant metabolism, recovery from stress and nutrient uptake.

Gardeners may see these as “plant tonics” or ingredients in premium feeds. Farmers may use them as foliar sprays during stressful growth stages.

Microbial Inoculants

These include beneficial bacteria, fungi and other microorganisms. Examples include mycorrhizal fungi, nitrogen-fixing bacteria, phosphorus-solubilising bacteria and other plant growth-promoting microbes.

Mycorrhizal fungi are popular in gardening, especially at planting time for trees, shrubs and perennials. They are less relevant for brassicas and some other non-mycorrhizal plants. This is a classic example of why product choice matters: the best biostimulant depends on the plant, soil and purpose.

Compost Teas and Compost Extracts

Compost teas and extracts are made by steeping compost in water, sometimes with aeration and microbial foods. They are used to apply soluble compounds and microorganisms from compost to soil or leaves.

This area can be quite controversial because quality varies dramatically. A well-made extract from excellent compost is very different from a smelly bucket of anaerobic mystery soup. Glamorous? No. Important? Absolutely.

Chitosan and Other Natural Compounds

Chitosan, derived from chitin, is another biostimulant-type ingredient sometimes used to support plant defence responses and stress tolerance. Other plant extracts, botanical ferments, microbial metabolites and polysaccharides may also be used.

Silicon Products

Silicon is sometimes grouped near the biostimulant world because it can support plant strength and stress tolerance, especially in crops like cereals, cucumbers and grasses. Depending on the product and regulatory context, it may be marketed as a nutrient, beneficial element or biostimulant-like input.

What Is the Best Biostimulant?

The honest answer is: the best biostimulant is the one that solves a specific problem in your specific growing system.

That may sound annoyingly sensible, but it is true.

For a gardener planting out tomatoes, the best option might be a seaweed root drench. For a tree grower, it might be mycorrhizal fungi at planting. For a market gardener raising transplants, it might be seaweed plus good compost and careful watering. For an arable farmer, it might be a carefully trialled foliar product used at a stress-sensitive growth stage. For a regenerative grower, it might not be a product at all - it might be compost, cover crops and living roots.

In general:

For beginners: seaweed extract is the easiest, most accessible biostimulant to understand and use. For planting perennials and trees: mycorrhizal fungi can be worth considering. For soil-focused growers: compost extracts and humic substances may fit well. For commercial crops: product choice should be based on trials, crop stage, soil tests, tissue tests and cost-benefit.

The “best” product is rarely the one with the loudest label.

Are Biostimulants Worth It?

Sometimes, yes. Sometimes, no.

This is where we need to be refreshingly grown-up about it. There is evidence that some biostimulants can improve growth or yield, but results are variable. AHDB’s work on crop biostimulants has found some evidence of benefits for growth and/or yield in arable crops, but also notes limited evidence for some claimed effects and that results vary depending on crop type, conditions and product use.

Another AHDB review found evidence for yield increases in several product categories, but much of the evidence came from controlled conditions or non-UK field conditions rather than UK field trials.

For gardeners, “worth it” might not mean yield response measured to the decimal point. It might mean stronger seedlings, better transplant establishment or healthier-looking plants. If a £7 bottle of seaweed lasts all season and helps you feel more confident with your seedlings, that may be worth it.

For farmers, “worth it” is more demanding. A product needs to pay its way. That means asking:

Does it increase yield? Does it improve quality? Does it reduce losses? Does it help during stress? Does it allow lower input use without reducing output? Does it work consistently enough to justify cost and labour? Can I test it on-farm?

A regenerative farmer should be especially wary of replacing one input treadmill with another. The aim is not to swap synthetic dependency for biological dependency. The aim is to build a living system where inputs become more strategic, more targeted and, ideally, less necessary over time.

How Should Gardeners Use Biostimulants?

For gardeners, the simplest route is to use biostimulants during moments when plants need support.

Good times include:

When sowing seeds. When pricking out seedlings. When potting on. When transplanting outdoors. After weather stress. During flowering and fruiting. When plants are growing in containers. When establishing trees, shrubs and perennials.

A seaweed liquid feed or root drench is often the easiest place to begin. Use it according to the instructions, and do not assume stronger is better. Overdoing any input can cause problems, especially if the product also contains nutrients.

It is also worth separating fertiliser and biostimulant thinking. If your plants are pale because they lack nitrogen, they probably need nitrogen. If your potting compost is exhausted, they need feeding or repotting. If they are wilting because they are dry, they need water. Biostimulants support plant processes; they do not repeal biology.

How Can Regenerative Farmers Use Biostimulants?

For regenerative farmers, biostimulants are best used as part of a systems-based approach.

They may be useful for:

Seed treatment. Improving establishment of cover crops. Supporting cash crops during stressful periods. Helping transplants establish in market gardens. Stimulating biological activity alongside compost or amendments. Supporting nutrient efficiency where fertiliser use is being reduced. Improving resilience in drought-prone or low-organic-matter soils.

But the key is measurement. Try strips, controls and comparisons. Leave an untreated area. Compare yield, quality, rooting, disease pressure, crop colour, Brix, tissue tests or whatever metric matters for your enterprise.

A simple on-farm trial is worth more than a glossy brochure.

For example, a market gardener could trial seaweed extract on half of a brassica transplant batch and compare establishment. An arable farmer could compare treated and untreated tramlines. A pasture-based farm might test biological amendments after mob grazing or compost application. The point is not to become cynical; it is to become observant.

Regenerative agriculture is built on feedback. Biostimulants should be judged by feedback too.

Biostimulants and Soil Health

The most exciting thing about biostimulants is not the bottle. It is the shift in thinking.

Conventional growing often asks: “What nutrient is missing?” Biostimulant thinking asks: “How well is the plant-soil system functioning?”

That opens a different set of questions.

Are roots exploring the soil well? Is the rhizosphere biologically active? Are nutrients present but unavailable? Is the plant under stress? Is soil structure limiting growth? Is water infiltration poor? Is the crop lacking biology, minerals, energy or air?

For gardeners, this might lead to better compost use, mulching and plant diversity. For farmers, it might lead to soil testing, sap analysis, cover crops, composting, livestock integration and reduced disturbance.

Biostimulants are most powerful when they encourage us to see plants as part of a living web rather than as green machines waiting for pellets.

Common Mistakes with Biostimulants

The first mistake is expecting instant miracles. Some products may show visible effects, but many work subtly or only under certain conditions.

The second mistake is using them when plants actually need basic care. Water, light, air, drainage and nutrients still matter.

The third mistake is applying products without a purpose. “Because it sounds good” is not a strategy. Decide whether you are trying to improve rooting, reduce stress, support soil biology or improve quality.

The fourth mistake is ignoring labels. Some biostimulants are also fertilisers. Some are suitable for organic systems; others may not be. Some need foliar application. Others should go into the root zone.

The fifth mistake is not trialling. This is especially important for farms. If the product costs money and labour, test it properly.

So, Are Biostimulants Just a Trend?

No, but the hype around them can be trendy.

The basic idea is ancient. Coastal growers have used seaweed for centuries. Farmers have long used composts, manures, ferments, plant extracts and biological preparations to support growth. What is new is the language, the commercial product range, the regulation and the scientific interest.

The modern biostimulant industry is trying to understand, standardise and sell some of these effects. That can be useful. It can also lead to overclaiming. Both things can be true at once.

For gardeners, biostimulants offer a way to support plant health more gently and biologically. For regenerative farmers, they offer possible tools for resilience, nutrient efficiency and transition away from high-input systems. But they work best when paired with observation, soil health principles and common sense.

A Useful Tool, Not a Magic Potion

A biostimulant is not quite fertiliser, not quite medicine, and definitely not fairy dust. It is best understood as a support for natural plant and soil processes.

For a gardener, that might mean using seaweed extract when potting on seedlings, adding mycorrhizal fungi when planting fruit bushes, or experimenting with compost extracts to support soil life.

For a regenerative farmer, it might mean using targeted biological products during transition, improving crop resilience, reducing nutrient waste, or supporting plants through stress while the wider soil system improves.

The big takeaway is this:

Biostimulants work best when the basics are already in place.

Healthy soil, good compost, living roots, plant diversity, careful watering, balanced minerals and thoughtful management will always do the heavy lifting. Biostimulants can help - sometimes impressively - but they are supporting actors, not the whole show.

Use them with curiosity. Test them with honesty. Don’t be seduced by miracle claims. And remember: the most powerful biostimulant in any garden or farm is often a thriving, living soil.

Research

The Chemical Biology of Plant Biostimulants – Seaweed Carbohydrates: