Hemp grows well only when the field, the nitrogen supply, the sowing date and the THC testing plan all match the crop type. This part covers soil and nutrition, crop rotation, pests and diseases, harvesting and drying CBD hemp, hemp in Thailand, and how THC sampling works in different countries.
Hemp falls into three crops: fibre hemp (grown for the stem), grain hemp (grown for seed) and CBD hemp (grown for cannabinoids in the female flowers). Each needs different spacing, varieties and harvest timing, as explained in Part 1. Industrial hemp is Cannabis sativa L. with THC, the intoxicating cannabinoid, below the legal limit.
Thai legal reminder: in Thailand, hemp means less than 1.0% THC in leaves and flowering tops, and hemp extracts may contain no more than 0.2% THC. Hemp flower at any THC level is a controlled herb under the Notification of the Ministry of Public Health Re: Controlled Herbs (Cannabis), B.E. 2568, in force since 26 June 2025. It may be sold only against a PT 33 (ภ.ท.33) prescription, for at most 30 days of use; advertising and online sale of flower are prohibited. The draft Cannabis Control Act is not law yet. → Cannabis as a controlled herb in Thailand.
This is Part 2. Part 1: Hemp Cultivation: Fiber, Grain, and CBD Varieties — What Determines Agronomic Outcome
3. Field, Soil and Nutrition
3.1 Soil and drainage
Hemp tolerates average soil fertility, but three things decide the yield.
- pH: aim for about 6.0–7.5 for all three crop types (University of Wisconsin Extension, cited by the Rodale Institute, a US organic farming research organisation). Below about pH 6, check for aluminium and manganese toxicity. At high pH, leaves can show iron and zinc deficiency even when the soil has enough; the pH is blocking uptake.
- Drainage: this matters most. HRDI, the Highland Research and Development Institute in Thailand, names root and stem rot from wet or waterlogged soil as the main problem in Thai hemp. The first 30 days after sowing are the riskiest. Choose well-drained loam or sandy loam, and dig drainage furrows in the rainy season. → Root rot in cannabis.
- Weeds: once a dense fibre crop closes its canopy, it shades weeds out. A widely spaced CBD crop never does, so it needs weeding by hand or machine, or a mulch, in the first weeks.
What to do: test soil pH before sowing and walk the field after the first heavy rain. Water standing for more than a day marks a spot to drain or avoid, because seedlings there are the ones most likely to rot.
3.2 Climate
Hemp seed needs soil warmer than 8 °C to germinate. That is never a problem in lowland Southeast Asia. Seedlings are killed by frost, which matters in temperate countries and on the highest Thai mountains. Long periods of high humidity during flowering raise the risk of grey mould (botrytis) in CBD hemp.
3.3 Nitrogen and other nutrients by crop type
The table gives one consistent set of figures. Adjust them to your own soil test.
| Crop type | Target pH | Total nitrogen (N) | Source |
|---|---|---|---|
| Fibre | 6.0–7.5 | up to about 150 kg N/ha (best rate without a cover crop in trials at Kutztown, Pennsylvania, USA) | Rodale Institute |
| Grain | 6.0–7.5 | about 100 kg N/ha (same trials) | Rodale Institute |
| CBD | 6.0–7.5 | No agreed best rate. General hemp guidance is 50–100 lb/ac (about 56–112 kg N/ha), split into several doses, with 20–25% at planting | University of Delaware Extension (USA) |
- Nitrogen during flowering (CBD hemp): the research disagrees. One study in a controlled growing environment found the highest cannabinoid levels with low nitrogen. A field trial in a high, dry desert area found that cannabinoid levels depended on variety and harvest date, not on the nitrogen rate. A practical approach is to give most of the nitrogen during vegetative growth and avoid heavy doses late in the season.
- Legume cover crops: in the Kutztown trials, hairy vetch and winter pea grown before hemp supplied enough nitrogen, with no extra fertiliser. A cover crop is a crop grown to feed and protect the soil rather than to sell; legumes add nitrogen from the air.
- Phosphorus, potassium and sulphur: apply according to the soil test. Heavily leached tropical soils may also need a fertiliser containing sulphate.
Translator’s note: hairy vetch and winter pea are temperate crops, and how they perform in Thai heat is not covered by these trials. A legume adapted to your area may play the same role, but this has not been tested before hemp in Thailand. Try it on a small plot first and compare it with a fertilised plot, so you learn whether the cover crop really replaces part of your fertiliser.
What to do: get a soil test before the season and set your nitrogen rate from the table for your crop type. For CBD hemp, put most of the nitrogen in before flowering.
4. Crop Rotation and Soil Health
Hemp works best as one crop in a rotation.
- Fewer weeds: a dense fibre canopy leaves fewer weeds for the next crop.
- Disease break, with limits: hemp does not host clubroot (Plasmodiophora brassicae), a disease of cabbage-family crops. But it is susceptible to Sclerotinia sclerotiorum (white mould), which also attacks sunflower, soybean and canola. In a rotation that already includes these crops, hemp gives no disease break.
- Heavy metals: hemp takes up heavy metals mainly when grown on contaminated land. The research on cleaning soil with plants (phytoremediation) used heavily contaminated soils (Linger et al., 2002; Rheay et al., 2021). A crop from such land is unsuitable for food or extraction.
- Seed hulls: ploughing hulls back into the soil adds little nitrogen. At 3% protein they contain about 0.48% N, roughly 5 kg of nitrogen per tonne of hulls.
Temperate rotations such as cereal → hemp → cereal or oilseed are well documented in Europe (Struik et al., 2000).
In Thailand, photoperiod hemp takes the rainy-season upland slot (Section 7). It therefore competes for land with other rainy-season upland crops. Avoid poorly drained rice paddies. A dry-season crop is possible only with autoflowering varieties and irrigation.
What to do: list the crops grown on your field in the last three years. If sunflower, soybean or canola is on the list, do not count on hemp to break white mould; and if the land has an industrial or mining history, test it for heavy metals before growing for food or extraction.
5. Pests and Diseases
Hemp borer (Grapholita delineana) and hemp flea beetle (Psylliodes attenuatus) are recorded mainly in Europe and Central Asia.
Two-spotted spider mite (Tetranychus urticae) is the main pest of CBD hemp grown under cover in hot, dry conditions. Predatory mites that eat spider mites (Phytoseiulus persimilis, Neoseiulus californicus) work best when released before the pest builds up, not after. → Spider mites and biological control.
Grey mould (Botrytis cinerea) does the most damage in dense CBD flowers. Prevent it by spacing plants more widely, removing lower leaves so air can move, and not watering from above once flowering starts. → Botrytis prevention.
Root and stem rot is the main field problem in Thailand. HRDI coated seed with Streptomyces (a soil bacterium) or Trichoderma harzianum (a beneficial fungus). Compared with uncoated seed, this cut seedling deaths at 15 days by up to 77% and 49% respectively. Avoid sowing just before a spell of heavy rain. → Trichoderma and other root microbes.
Translator’s note: in Thailand, use only pesticides and biological products registered with the Department of Agriculture (กรมวิชาการเกษตร), and follow the label. Check the registration of any Trichoderma, Streptomyces or predatory-mite product with the Department of Agriculture before you buy. Registration is your evidence for a GACP audit and protects you from fake or contaminated products.
What to do: in CBD hemp, check the undersides of leaves for mites once a week from the vegetative stage, so predators go in early. In the field, sow after a heavy-rain spell rather than just before one.
6. Harvesting and Drying CBD Hemp
6.1 Finding the harvest window
CBD and THC rise together as the flowers ripen (Part 1, Section 2.2). So the harvest window is set by laboratory testing, not by how the flowers look.
Where a 0.3% limit applies, NC State (North Carolina State University, USA) advises taking the compliance sample no later than about two weeks after floral initiation, the first sign of flowers. Under the rules of the US Department of Agriculture (USDA), harvest must then take place within 30 days of sampling.
A test by an approved laboratory documents that the crop is within the limit. It does not give permission to sell flower where flower is a controlled product. In Thailand, hemp flower remains a controlled herb (see the box at the top). Ask your licensing authority which laboratories it accepts for THC results. → How to read a certificate of analysis.
6.2 Drying
Flowers must be dried quickly to below about 12% moisture to prevent botrytis and loss of quality. Drying in open air is unreliable in humid Southeast Asian weather, so rooms with forced air or dedicated dryers are needed. Slow drying gives a hay-like smell and taste and more losses to mould. In Thailand, handling, storing and selling hemp flower fall under the controlled-herb rules. → Drying at 85–95% outdoor humidity.
What to do: measure the moisture of a sample from each drying batch before storage, and keep the result with the batch record. This shows whether your drying set-up reaches 12% in the rainy season, not only in the dry season.
7. Hemp in Thailand: Rules and Practice
Licensing. Private licensing of hemp began under the Ministerial Regulation on licensing for the production, import, export, distribution or possession of Category 5 narcotics specifically for hemp, B.E. 2563 (2020), which took effect on 29 January 2021. Licences covered purposes such as study, research, breeding and certified-seed production. Seed was certified by registration with the Department of Agriculture, with THC test results submitted to the Thai Food and Drug Administration (Thai FDA).
Varieties. Thailand’s certified varieties are domestic lines bred by HRDI with the Royal Project Foundation: RPF1, RPF2, RPF3 and RPF4. They were bred for fibre, registered with the Department of Agriculture in 2011 (B.E. 2554), and are reported at about 0.2% THC. RPF1 suits highlands above 1,000 m. RPF3 suits 500–1,000 m.
Where hemp is grown. Press reports name Mae Sai (Chiang Rai province), Phop Phra (Tak province) and Song Khwae (Nan province) as hemp areas. Hmong communities, notably in Phop Phra, have grown hemp for textiles for a long time.
Planting calendar for photoperiod RPF lines in northern Thailand (HRDI):
| Sowing | What happens |
|---|---|
| March–May (only where water is available) | Long vegetative phase; plants can reach about 5 m, so wide spacing is needed |
| June–July (recommended; HRDI’s general window is May–July) | About 3 months of vegetative growth, flowering late September–October, about 1 × 1 m spacing for seed |
| August–February (short days) | Short vegetative phase and early flowering; dense spacing (75 × 50 cm or closer) is needed, and the plants produce little biomass |
This is why the dry season is a poor time for photoperiod fibre hemp. Short days make plants flower early, and the stalk yield stays low. A dry-season crop needs autoflowering varieties plus irrigation. Avoid sowing just before heavy rain.
What to do: choose your RPF line by the altitude of your field (RPF1 above 1,000 m, RPF3 at 500–1,000 m) and sow in June–July. That date gives about three months of growth before the plants flower.
8. THC Limits, Sampling and Records
8.1 THC limits by country
| Country or region | Plant THC limit (dry weight) | Notes |
|---|---|---|
| European Union | 0.3% (since 2023; 0.2% before) | Certified varieties only |
| United States | 0.3% total THC (federal) | State and Tribal plans differ |
| Canada | 0.3% | — |
| Thailand | 1.0% in leaves and flowering tops; extracts no more than 0.2% | Flower is a controlled herb; the draft Act may change the limits |
| Switzerland | 1.0% | — |
| United Kingdom | 0.2% | — |
What to do: if you plan to export, test against the limit of the destination country, not only the Thai 1.0%. A crop legal in Thailand can fail a 0.3% or 0.2% market. → What can leave Thailand as hemp.
8.2 How official samples are taken
- European Union (Delegated Regulation (EU) 2022/126, Annex I): under Procedure A, inspectors check at least 30% of declared hemp areas. They sample from 20 days after flowering starts until 10 days after it ends. Each sample is a 30 cm section with at least one female flower head, taken from each of 50 plants per field. Under Procedure B, used where a prior-approval system exists, at least 20% of areas are checked. Samples are taken in the 10 days after flowering ends and consist of the upper third of 200 plants per field.
- United States (7 CFR 990.24 and USDA sampling guidelines): an authorised sampling agent collects samples no more than 30 days before the expected harvest, by cutting the top 5–8 inches (about 13–20 cm) of the flowering top. A state or Tribe may instead use a performance-based method, which must give 95% confidence that plants will not test above 0.3%. The number of plants sampled per lot depends on the plan that applies.
- Thailand: follow the sampling instructions of your licensing authority.
Do not mix a whole field into one sample. Keep a chain of custody: a signed record of who took each sample, when, and who handled it until the laboratory. For a 0.3% limit, set an internal target of about 0.2% (a margin of about 33%), so normal test variation does not push the crop over the limit.
8.3 Records
Keep these records for at least as long as your licence requires:
- field maps with the variety and lot number of each block;
- seed invoices and seed certificates;
- logs of fertilisers and other inputs;
- sampling records with the laboratory certificates of analysis (CoA);
- yields by block.
Inspectors may compare the seed you bought with the area you planted. Matching records prove that no seed went elsewhere.
What to do: set up one notebook or spreadsheet per season with these five headings before sowing. Filling it in as you go is far easier than rebuilding it before an inspection. → GACP records for cannabis.
Summary
- Match the crop type (fibre, grain or CBD) to the processing and buyers available to you.
- Choose certified varieties suited to your latitude and altitude.
- Keep pH at 6.0–7.5 and nitrogen within the ranges in Section 3.3.
- Test CBD hemp early, because THC rises as the flowers mature.
- In northern Thailand, sow photoperiod RPF varieties at the start of the rainy season, in well-drained fields.
- In the dry season, grow only autoflowering varieties, and only with irrigation.
- Thai hemp flower remains a controlled herb, and the legal limits may change once the draft Cannabis Control Act is passed.
Have you grown RPF lines or tested a CBD crop in Thailand? Share your sowing dates, yields or test results below this topic so other growers can compare.
References
- Linger, P., Müssig, J., Fischer, H., & Kobert, J. (2002). Industrial hemp (Cannabis sativa L.) growing on heavy metal contaminated soil: fibre quality and phytoremediation potential. Industrial Crops and Products, 16(1), 33–42. Redirecting
- Struik, P. C., Amaducci, S., Bullard, M. J., Stutterheim, N. C., Venturi, G., & Cromack, H. T. H. (2000). Agronomy of fibre hemp (Cannabis sativa L.) in Europe. Industrial Crops and Products, 11(2–3), 107–118.
- Rheay, H. T., Omondi, E. C., & Brewer, C. E. (2021). Potential of hemp (Cannabis sativa L.) for paired phytoremediation and bioenergy production. GCB Bioenergy, 13(4), 525–536.
- Suchoff, D., Linder, E., & Henriquez Inoa, S. (2022). Avoiding a hot crop: minimizing the risk of non-compliant THC tests through proper harvest timing. NC State Extension AG-937. Avoiding a "Hot" Crop: Minimizing the Risk of Non-compliant THC Tests through Proper Harvest Timing | NC State Extension Publications
- Rodale Institute. Growing industrial hemp as an organic cash crop. https://rodaleinstitute.org/science/articles/growing-industrial-hemp-as-an-organic-cash-crop/
- University of Delaware Extension (2020). Recommended nutrient rates, miscellaneous crops. https://udel.edu/content/dam/udelImages/canr/pdfs/extension/environmental-stewardship/nutrient-recommendations/UD_Extension_Recommended_2020_Misc_Crops_All.pdf
- Floral hemp (Cannabis sativa L.) responses to nitrogen fertilization under field conditions in the high desert. Floral hemp (Cannabis sativa L.) responses to nitrogen fertilization under field conditions in the high desert - PMC
- Commission Delegated Regulation (EU) 2022/126, Annex I. EUR-Lex - 02022R0126-20260327 - EN - EUR-Lex
- 7 CFR § 990.24, Responsibility of a USDA licensee prior to harvest. 7 CFR § 990.24 - Responsibility of a USDA licensee prior to harvest. | Electronic Code of Federal Regulations (e-CFR) | US Law | LII / Legal Information Institute
- HRDI (2022). ปลูกกัญชงผลิตเมล็ดเพื่อการบริโภค [Growing hemp for food seed]. ปลูกกัญชงผลิตเมล็ดเพื่อการบริโภค | สวพส.
- HRDI. ปลูกกัญชงอย่างไร…ให้รอด? [How to grow hemp so it survives]. ปลูกกัญชงอย่างไร…ให้รอด? | สวพส.
- Thai Publica (2021). HRDI hemp seed sales (RPF varieties). สถาบันวิจัยและพัฒนาพื้นที่สูง เปิดขาย "เมล็ดกัญชง2 พันธุ์" สั่งซื้อ 26 เม.ย. - 5 พ.ค.64 - ThaiPublica
- Nishimura & Asahi (2021). Legalization of Hemp-Related Business in Thailand. https://www.nishimura.com/en/knowledge/publications/20210525-62786