THE TERPENES GUIDE: HEMP AND CANNABIS

A terpene is a hydrocarbon that plants assemble from a five-carbon building block, isoprene (C₅H₈). The units link head to tail and the resulting chain can then fold into one or more rings. This logic, known as the isoprene rule, is why terpene formulas come in multiples of five carbons: C₁₀ for monoterpenes, C₁₅ for sesquiterpenes, C₂₀ for diterpenes. The name goes back to August Kekulé, who coined it in 1866 for the C₁₀H₁₆ hydrocarbons found in turpentine.

Terpenes make up the largest family of natural products known to science, and Cannabis sativa L. is an unusually rich source. The literature lists 110 terpenes and 121 terpenoids described for the species. Any single flower expresses only part of that catalogue, and almost always with a small group of dominant molecules on top.

Terpenes vs terpenoids: it comes down to oxygen

Strictly speaking, a terpene contains nothing but carbon and hydrogen: myrcene, limonene and pinene are textbook cases. Once an oxygen-bearing group appears (alcohol, ether, ketone, aldehyde, ester) chemists call the molecule a terpenoid. Linalool and geraniol are alcohols, eucalyptol is a cyclic ether and caryophyllene oxide is an epoxide. Industry usage blurs the two words, so this guide says terpenes in the broad sense and names the functional group in each profile.

Flavonoids and flavorants are something else

Flavonoids belong to the polyphenol family. They act as plant pigments and are practically non-volatile, which means they contribute almost nothing to smell. Hemp produces several of them, including three found nowhere else: cannflavins A, B and C. Flavorants are another story again: non-terpene volatiles such as esters, thiols and indoles that recent research has detected in resins at trace levels, and which turn out to drive some fruity and gassy aromas.

Terpene classes by number of isoprene units

Most hemp terpenes are produced in the glandular trichomes, the tiny mushroom-shaped hairs that frost the bracts of female flowers. The largest type, the capitate-stalked trichome, carries a disc of secretory cells at its tip. Those cells pump resin into a pocket under the cuticle, forming the glossy head you can see under a loupe. That head, packed with terpenes and cannabinoids, is the raw material of every sieved resin.

Two pathways, one building block

Every terpene starts from two C₅ molecules: IPP (isopentenyl diphosphate) and its isomer DMAPP. The plant makes them along two separate routes. The MEP pathway (methylerythritol phosphate) runs inside plastids and needs seven steps to get from pyruvate and glyceraldehyde-3-phosphate to IPP and DMAPP. The mevalonate pathway (MVA) works in the cytosol, starting from acetyl-CoA. As a rule of thumb, MEP feeds the monoterpenes and mevalonate feeds the sesquiterpenes, with some cross-talk between compartments.

Terpene synthases decide the shape

Prenyltransferase enzymes then join two units into geranyl diphosphate (GPP, 10 carbons) or three units into farnesyl diphosphate (FPP, 15 carbons). These linear precursors are folded by terpene synthases (TPS), which set the final architecture: open chain, single ring or fused rings. Some synthases make a single product, others a blend. In the Finola hemp cultivar, CsTPS3 yields only myrcene, while CsTPS9 releases β-caryophyllene and α-humulene at roughly 2.5 to 1, almost exactly the ratio measured in the flowers. Work from the University of British Columbia counted 33 complete TPS gene models in the Purple Kush genome and about thirty characterised enzymes overall.

One detail ties everything together: GPP is also a building block of cannabinoids, which are terpenophenolic compounds. Combined with olivetolic acid it gives cannabigerolic acid (CBGA), the starting point of the acidic cannabinoids covered in our cannabinoid guide. Terpenes and cannabinoids share the same factory and the same raw materials, which helps explain the moderate correlation between total terpene and total cannabinoid content seen in large lab datasets.

If you remember one split, make it this one. Monoterpenes carry 10 carbons (about 136 g/mol for the hydrocarbons), sesquiterpenes 15 (about 204 g/mol). That extra third of mass changes how they behave. Monoterpenes are light and highly volatile: they are the bright top notes you notice the moment a jar opens, citrus, pine, fresh herbs. Sesquiterpenes are heavier, leave more slowly and hold the base notes: pepper, wood, hops.

Lab data shows the gap clearly. In a reference study published in the Journal of Natural Products in 1996, the volatile oil of fresh buds was 92.5% monoterpenes and 6.8% sesquiterpenes. After one week of air drying and three months of storage at room temperature, the monoterpene share had dropped to 62% while the sesquiterpene share had climbed to 35.6%. The sesquiterpenes did not multiply: the lighter monoterpenes simply escaped faster.

Size and functional groups govern how volatile a terpene is. Hydrocarbon monoterpenes boil between 155 and 186 °C at atmospheric pressure. Add an alcohol group and the molecules start hydrogen bonding with each other, pushing the figure up: 198 °C for linalool, 230 °C for geraniol. Sesquiterpenes sit well above 250 °C, with β-caryophyllene boiling at around 262 to 264 °C.

A boiling point is not the temperature at which aroma starts to disappear, though. Terpenes drift into the air at room temperature, slowly and continuously, which is exactly why a flower has a smell at all. For heavier molecules, reference tables often quote values measured under reduced pressure (106 to 107 °C at 5 mmHg for humulene, for example), and those are not comparable with figures at one atmosphere. Perceived intensity also depends on odour threshold: (R)-linalool can be detected at 0.8 parts per billion, so a trace compound can weigh more in the bouquet than a major one.

Values are atmospheric boiling points from reference data sheets (PubChem and compiled data). Camphene, borneol, fenchol and guaiol are solids at room temperature, which makes them a little less mobile than their liquid neighbours.

That last family produced the biggest surprise of recent years. In 2021 a team publishing in ACS Omega showed that the signature, pungent cannabis scent comes from a family of prenylated volatile sulfur compounds, led by 3-methyl-2-butene-1-thiol. Their levels rise sharply at the end of flowering, peak during curing and fall away after about ten days of storage. A second study in 2023, covering 31 resins, linked tropical notes to thiols bearing a 3-mercaptohexyl group, fruity notes to around thirty esters, and chemical or savoury notes to indole and skatole. Terpenes lay the foundation; these traces add the personality.

Even in a highly aromatic flower, all terpenes together amount to only a few percent of dry weight. An analysis of more than 41,000 commercial samples from US testing labs found an average close to 2% by weight, with most individual terpenes below 0.2%. Industrial hemp grown for fibre or seed often sits far lower. A terpene profile therefore reads like a ranking: one dominant terpene, two or three supporting ones, then a long tail of minor compounds.

Chemotype beats strain name

The same study sorted profiles into three groups: one rich in β-caryophyllene and limonene, one rich in myrcene and pinene, and a rarer group led by terpinolene and myrcene. The first two held 87% of samples. More importantly, the commercial labels indica, sativa and hybrid lined up poorly with the measured chemistry. For a professional buyer, the chemotype (the profile actually tested) is more reliable than any name or category on a bag. In CBD-rich hemp, myrcene is usually the leading monoterpene and β-caryophyllene the main sesquiterpene, as explained in our CBD guide.

Flower or resin: two readings of one bouquet

A sieved resin concentrates the trichome heads, so it holds the very reservoir of terpenes. Its profile mirrors the source flower with a few twists: the lightest monoterpenes are more exposed during separation and drying, while sesquiterpenes and sulfur compounds hold on better. That is why a pressed hash or a cold-sieved resin can come across deeper and spicier than the fresh flower. Temperature, mesh size and starting material matter as much as genetics.

Reading a name with care

Names such as Lemon Pop, Tangerine or Papaya naturally suggest limonene or fruity notes, and breeders' noses often inspired them. But a name is not a lab report: two batches of the same genetics grown in different conditions can show different terpene rankings. The product pages in our flower and resin catalogue describe the aroma observed batch by batch.

Genetics write the terpene potential; everything between seed and jar decides how much of it survives. The literature lists chemotype, location and growing method, season, microclimate, developmental stage, post-harvest handling, and how long and how a crop is stored. Three moments matter most.

During cultivation

Monoterpenes build up over the plant's life, from leaves to young flowers and then mature flowers, where trichomes are densest. Light, temperature, nutrition and harvest timing all shape the final intensity. How indoor, outdoor and greenhouse growing compare in look and aroma is covered in our guide to indoor, outdoor and greenhouse flowers.

During drying and curing

This is where most aroma is lost. In the 1996 study quoted above, a single week of air drying at room temperature removed 31% of the volatile oil, and after three months in a paper bag the loss reached 55%. Myrcene fell from 67% to 33% of the oil, while limonene and the pinenes stayed remarkably stable. Slow, cool drying away from light limits the damage, and curing in a closed container lets moisture even out without letting volatiles slip away. Curing is also when the sulfur compounds behind the loudest aromas reach their peak.

During storage

Three enemies work together: light (UV attacks unsaturated molecules), heat (it speeds up the loss of volatiles) and oxygen (it oxidises). Myrcene is unstable in air and tends to polymerise; limonene oxidises to carveol, carvone and limonene oxide; β-caryophyllene turns into caryophyllene oxide; linalool oxidises too. Repeated handling breaks trichome heads and exposes the resin. Our practical advice on containers, temperature, relative humidity and stock rotation is gathered in the guide on storing flowers and resins.

The reference technique is gas chromatography. The sample goes into a heated injector, a carrier gas (often helium) sweeps the molecules through a capillary column several tens of metres long, and each compound exits at a characteristic moment called its retention time. Light monoterpenes come out first, sesquiterpenes later.

GC-MS and GC-FID

Coupled with a mass spectrometer (GC-MS), the system identifies each peak from its fragmentation pattern, matched against reference libraries (NIST, Wiley, Adams) and retention indices. A flame ionisation detector (GC-FID) is often preferred for quantification because its response is very consistent. Many labs combine both: the mass spectrometer to name, the FID to count.

Sample prep: liquid injection or headspace

Two approaches dominate. Either a few tens of milligrams of plant material are extracted with a solvent such as hexane or ethanol before injection, or the sample is warmed in a sealed vial and only the volatile-laden air above it is drawn off: that is headspace sampling, sometimes with a solid-phase microextraction (SPME) fibre. Headspace is very sensitive to monoterpenes but can under-report the heaviest sesquiterpenes. For deeper work, two-dimensional chromatography (GC×GC) with time-of-flight mass spectrometry resolves hundreds of trace compounds, including the esters and thiols mentioned above.

How to read a terpene report

A terpene profile lists quantified molecules, usually as percent by weight or mg/g, plus a total. A few reading habits help: check how many terpenes the panel covers (a total built on 10 compounds is not comparable with one built on 40), note the limit of quantification, and remember that a standard column does not separate enantiomers such as (+)- and (−)-limonene. Green Connection batches are lab tested; for background on other measured compounds, see our CBG guide and CBN guide.

Terpenes come from two kinds of source. Cannabis-derived (or hemp-derived) terpenes are the ones the plant made itself, present in the flower or resin, or in an aromatic extract taken from the plant. Botanical terpenes come from other plants: limonene from citrus peel, pinene and carene from turpentine, caryophyllene from clove or pepper, linalool from lavender or coriander. They are then blended to imitate a given profile.

A molecule is a molecule: the myrcene in hops and the myrcene in a hemp flower share the same formula and structure. The differences lie elsewhere. First, stereochemistry: the limonene that dominates citrus peel is the (+) enantiomer with its orange scent, whereas the Finola enzyme CsTPS1 makes mainly (−)-limonene, closer to pine. Second, completeness: a blend rebuilt from fifteen terpenes lacks the dozens of minor compounds, esters and thiols that give a natural bouquet its character. Third, traceability: professional buyers need to know what they are getting, and transparency about aroma origin is part of trust.