This section deals with a crop that contains something it should not. The four main problems are pesticide residues, heavy metals, microbes such as moulds and bacteria, and mycotoxins (poisons that some moulds leave in the crop). Use it to discuss how each problem is found, the few ways to remove it, and when a batch can no longer be sold.
Prevent first, test second, clean up last
Work in this order: prevent contamination, test for it, and only then try to remove it. Removing it after the fact is called remediation, and it is the weakest of the three steps. Most contaminants can only be reduced, and some cannot be removed at all.
That means most of your effort belongs before harvest. Use clean soil, water and fertiliser, spray only what is allowed, and dry the crop properly. Testing then shows whether those steps worked.
Translator’s note: In Thailand you may only use pesticides registered with the Department of Agriculture (กรมวิชาการเกษตร), and only as the label directs. A product used off-label is a legal problem, and it is also likely to fail a residue test. The topic What You May Legally Spray Under Thai GACP explains how this works on certified farms. To keep pests down without heavy spraying, start with integrated pest management. This approach combines monitoring, prevention and biological control.
The main contaminant tests and what each one tells you
A contaminant test does more than give a pass or fail. Each group of tests points to a different stage of your work where something went wrong.
| Test group | What the lab measures | Where it usually comes from | What a failed result tells you |
|---|---|---|---|
| Pesticide residues | Traces of insecticides, fungicides and miticides | Your own sprays, drift from neighbouring fields, or mother plants and clones sprayed on another farm | A product reached the plant that your spray records do not explain, or one was used too close to harvest |
| Heavy metals | Usually lead, arsenic, cadmium and mercury. The exact list depends on the lab and the buyer’s specification | Soil, irrigation water, fertilisers and other inputs | One of your inputs is contaminated, and you need to find out which one |
| Residual solvents | Solvent left in an extract after processing | The extraction process. This test is mainly for extracts, not raw flower | The solvent was not fully removed from the extract |
| Microbes | Total yeast and mould, total bacteria, and sometimes named harmful bacteria. Which bacteria are named depends on the lab and the buyer | Slow or humid drying, poor storage, dirty handling | Your drying, storage or hygiene needs fixing |
| Mycotoxins | Toxins made by moulds, such as aflatoxins and ochratoxin A | Mould growing on a wet crop, before or after harvest | Mould grew at some stage. Killing the mould later may not remove toxins it has already made |
All of these results are reported on a certificate of analysis. This is the lab report that lists each test, the result, the limit it was compared against, and whether the sample passed. Always read the limit column as well as the pass or fail. A pass under one set of limits can be a fail under another.
Translator’s note: Use a laboratory accredited to ISO/IEC 17025, the international standard for testing laboratories. Accreditation only covers the specific tests listed in the lab’s scope. Ask for that scope, and check that it includes the test you are paying for. To hear how other growers found a lab, ask in the B2B Services section of this forum.
Heavy metals: where they come from and how to find the source
Cannabis roots absorb metals from the root zone along with nutrients. The metals then spread into the leaves and flowers. Cannabis and hemp take up metals easily, which is why they have been studied for cleaning contaminated land. That makes the crop sensitive to whatever is in your soil and water.
The usual sources are:
- Soil. Land that was used for industry or mining, or that sits beside a busy road, can carry metals from earlier use.
- Irrigation water. Wells, canals and rivers downstream of industry or mining can carry dissolved metals.
- Fertilisers and other inputs. Some phosphate fertilisers and mined minerals can contain cadmium or other metals.
- Growing media of unknown origin. Unlabelled soil mixes or bulk material without a known source.
When a flower sample fails for metals, use these steps to trace the source:
- Note which metal failed. Each input carries a different mix of metals, so this tells you which inputs to suspect first.
- Test each input separately. Send the soil or substrate, the irrigation water and each fertiliser or amendment as separate samples. The input that is high in the same metal is the likely source.
- Keep a sealed sample of every input lot and write down its lot number. If a later crop fails, you can test the exact material that crop received. You will not have to guess.
- Test new land, a new water source or a new supplier before you plant a full crop. One test costs much less than a whole crop that cannot be sold.
Translator’s note: Before you buy a fertiliser in Thailand, check that it is registered. The topic Verifying a Fertiliser Is Legally in Thailand explains how to read the registration and the label. Before renting or buying farmland, ask the owner or village head how the land was used before. Then test the soil and water before your first crop.
What remediation can and cannot do
Ozone and irradiation. Ozone is a reactive gas used to kill microbes in a room or on a product. Irradiation treats the product with radiation to kill microbes. Both lower microbial counts. Neither does anything about a systemic pesticide, meaning one the plant absorbs and carries inside its tissue rather than leaving on the surface.
Translator’s note: Ozone irritates the lungs, so keep people out of a room while it is being treated. Before you treat any batch you plan to sell, ask your buyer in writing whether they accept flower treated with ozone or irradiation, and whether the treatment must be declared.
Extraction. Turning a failed batch into an extract does not reliably clean it. The extract weighs far less than the flower it came from. A pesticide that dissolves along with the cannabinoids therefore ends up more concentrated, not less. For how the extraction methods differ, see Cannabis Extraction Explained.
Heavy metals. No common process reliably removes heavy metals the plant has taken up through its roots. The metals are inside the plant tissue, so washing, drying, ozone and irradiation do not reach them. Your only real protection against metals is the input testing described above.
When a batch cannot be sold
A batch is not saleable when a contaminant is over the limit and no process reliably removes it. A systemic pesticide or a heavy metal above the limit usually puts a batch in this position. Before you spend money on treatment, check which kind of contaminant failed. That tells you whether treatment has any real chance of working.
Once a batch fails, separate it straight away and label it clearly. This step is called quarantine. Do not mix it into clean batches, because that spreads the contaminant into product that had already passed.
Translator’s note: Licensed Thai producers should follow the conditions of their licence when a controlled-herb batch fails and has to be destroyed. Ask the licensing office what it requires. Keep a written record of how, when and by whom the batch was destroyed.
The buyer’s market sets the limits
A regulated market is a country or system where cannabis products must meet official limits and be tested before sale. A product going to a regulated market is tested against that market’s limits, not the producing country’s. A batch can pass in one country and fail in another on the same certificate, because the limits are different.
Translator’s note: For a Thai exporter, the buyer’s country sets the limits, for example Germany or another EU country. Before you plant, ask the buyer for their full specification in writing, with every test and limit. Then check that your lab can measure down to those limits. The topic Testing Cannabis Flower Before Export covers the lab side. For product sold inside Thailand, ask the licensing office or your lab which limits apply to your licence.
Records: why an unrecorded clean-up counts against you
Good Agricultural and Collection Practice (GACP) is the quality standard for growing and harvesting medicinal plants. Good Manufacturing Practice (GMP) is the standard for processing and making medicines. Under both, a failed test is a deviation, meaning a departure from an approved procedure or specification. A deviation must be recorded and investigated.
If you try to clean a failed batch without recording it, the auditor will treat that as a finding. A finding is a problem the auditor writes down because it does not meet the standard. This applies even if the treatment worked. The paperwork around a deviation matters as much as the treatment itself.
For every failed batch, record:
- Batch number, date, test result and lab report number. These link the problem to one specific batch.
- Where the batch is held in quarantine, and who was told. This shows the batch was controlled and not sold by mistake.
- The likely cause. Note which input or step you suspect and how you checked it. This is how you stop the same failure next season.
- The decision and who approved it. The options are treat and retest, sell to a market whose limits it meets, or destroy.
- Treatment details, if any. Record the method, settings, date and the person who carried it out. Without these, a passing retest proves nothing to an auditor.
- The retest result. This shows whether the treatment worked.
- Corrective and preventive action (CAPA). Write down what you will change so the failure does not happen again, and when you will check that the change worked.
Translator’s note: For how GACP handles storage, testing, release and audits, and how the Thai rules fit in, see Good Agricultural and Collection Practice for Cannabis, Part 2. Keep the deviation records with your other batch records so an inspector can follow one batch from start to finish.
Translator’s note: the Thai rainy season
Heat of 30–35 °C and high humidity in the rainy season make mould on drying flower much more likely. More mould means a higher risk of failed microbial and mycotoxin tests. Ozone and irradiation cannot undo this reliably. Prevention has to happen in the drying room.
If you have no climate-controlled drying room, this rainy-season risk is the main reason to set one up. The topic Drying Cannabis at 85–95% Ambient Humidity compares setups that suit Thai conditions. A power cut during drying can let humidity rise for hours. Send any batch dried through a power cut for microbial and mycotoxin testing before you decide what to do with it.
What to do this season
- Get your buyer’s full test specification in writing before planting. That way you know which limits you are growing to.
- Test soil, water and each fertiliser once before a new crop or a new supplier. This rules out heavy metals while you can still change the input.
- Keep a sealed sample and the lot number of every input. If a batch fails, you can trace the cause.
- Spray only registered products, as the label directs, and record every spray. Your records will then explain any residue the lab finds.
- Set up a deviation form now, before anything fails. When a batch fails, you will record each step as it happens, not reconstruct it later.
Related threads
The subject is also covered in threads already linked above. They include Pesticide Residues on Cannabis, Mycotoxins on Cannabis Flower, How to Read a Cannabis Certificate of Analysis and Integrated Pest Management for Cannabis.
If you have a failed result or are worried about an input, post your question below this topic. Include the test that failed, the result and limit, your growing medium and inputs, and the crop stage. Leave out lab and supplier names.