Hemp trichomes: 3 types, and the gland where CBD is made

A trichome is an outgrowth of the epidermis, the cell layer that covers a plant: a hair, made of one or several cells, or a gland that makes and stores molecules. The word comes from the Greek trichos, "hair". According to a review published in 2012 in the International Journal of Molecular Sciences, it extends the epidermis and is in most cases not connected to the plant's vascular system.

Nearly all plants carry them: non-glandular hairs on most flowering plants, and even on some gymnosperms and bryophytes, and glandular trichomes on about 30% of vascular plants. Thale cress, the model plant of botanists, has only single-celled hairs, unbranched or with 2 to 5 branches.

On hemp, the glandular trichome is shaped like a tiny match, with a round head on a stalk. A disc of secretory cells at the base of the head makes the cannabinoids and terpenes. A review published in 2021 in Frontiers in Plant Science explains that these molecules are then stored in a cavity above the disc, beneath a thin cuticle. This pocket of resin is what glitters on a "frosty" flower.

The plant makes CBDA, the acidic form, and heat or time later converts it into CBD. Our article on CBDA explains the mechanism.

Hemp carries two families of trichomes: non-glandular hairs, stiff and pointed, which secrete nothing, and glandular trichomes, bulbous, capitate sessile and capitate stalked, which make the resin. The UNODC manual for drug analysis laboratories (2022) describes these five forms, and a Canadian team measured the glands on two THC-rich varieties, in a paper published in 2023 in the Journal of Cannabis Research.

French sources also call the capitate stalked trichome "capité tigé", "pédicellé" or "à capitule tigé"; the sessile one is "capité sessile", and the bulbous one "bulbeux".

Stalked trichomes gather on the bracts that wrap each female flower, mostly on their lower surface, and spread to the small leaves of the inflorescence. The large leaves mostly carry hairs and sessile glands, and the roots hold only traces of cannabinoids. A study published in 2020 in Scientific Reports analysed three varieties organ by organ, two THC-dominant and a third in which CBD dominated.

Flowers are therefore sold by the gram, and leaves mostly end up in extraction or infusions. Flavonoids are the exception, more abundant in the leaf: trichomes do not explain all of the plant's chemistry. On dried flowers of four other genotypes, the 2023 study counted 12 to 30 capitate trichomes per square millimetre of bract, and 2004 work it cites found up to 20 times more THC in a stalked head than in a sessile one.

A stalked trichome starts as a sessile-looking gland, then a stalk made of several cells lifts it up as flowering progresses. First put forward in 1977, the idea was supported in 2020 by counts of secretory cells and by the fluorescence specific to each head.

In 1977, a study in the American Journal of Botany saw bulbous and sessile glands appear on very young bracts and stalked ones later, with a stalk recruited from neighbouring cells of the epidermis and of the layer beneath; the authors suggested they derive from sessile glands.

In 2020, a study published in The Plant Journal counted 12 to 16 secretory cells in stalked trichomes, whose heads emit a blue fluorescence linked to high cannabinoid levels, against 8 cells and a red-shifted fluorescence in the true sessile glands of leaves and mature flowers. The terpene profile of stalked trichomes is also clearly dominated by monoterpenes, the light molecules responsible for much of the smell (see our article on terpenes), and these counts hold for hemp as well as for drug-type varieties, according to the 2021 review.

Under the electron microscope, the 2023 study shows a stalk built from cells of the epidermis and hypodermis, with 6 cells around a central core in cross-section. Sessile glands dominated at week 3 of flowering and stalked ones from week 6 to week 8, although not every sessile gland was transformed. Resin secretion and head ageing began around week 5, a timeline that varies with genetics and with the stresses the plant undergoes.

As for the molecule, a 2005 study showed that the enzyme that makes THCA is expressed in the secretory cells and passes into the storage cavity, where its authors place the formation of THCA. Since this compound can be toxic to plant cells, keeping it apart protects the plant, the 2021 review notes. The common precursor, CBGA, becomes CBDA or THCA depending on which enzyme is active, a chain traced in our article on where CBD comes from. The tools behind these studies: electron microscopy, fluorescence, cell counts and the active genes of isolated trichomes.

In many plants trichomes serve as a defence; in hemp, these roles remain mostly hypotheses. The strongest lead is chemical: in the laboratory, CBGA and THCA kill cultured insect cells. The idea of a UV shield, for its part, rests on a link between UV-B and THC that a 2021 trial did not find.

The first two rows come from the 2021 review: they are leads, and a test on cultured insect cells does not show what happens in the whole plant. The UV hypothesis goes back to 1983: the plant would make markedly more THC under stronger UV-B. In 2021, a team at the University of Guelph lit two indoor-grown varieties with UV-B LEDs, 3.5 hours a day for 9 weeks of flowering, at biologically effective doses of 0.16 to 13 kJ·m⁻²·d⁻¹: neither the flower yield nor the cannabinoids increased. In one of the varieties, total Δ⁹-THC and total CBD even fell as the dose rose, and terpenes declined in both.

Trichomes are not specific to hemp. The stinging hairs of nettle and the oil glands of tomato, mint or hop are trichomes too, with other shapes and other molecules, and none of these plants makes CBD.

Nettle

A 2018 study of 43 species from 5 families describes the stinging hair as "the plant equivalent of the hypodermic syringe": in the nettle family, Urticaceae, its tip is silicified while its base and shaft are mostly calcified. In Urtica thunbergiana, a 2006 analysis found mainly histamine, oxalic acid and tartaric acid in these hairs.

Tomato

According to the 2012 review, tomato carries 8 types of trichomes, 4 of them glandular (I, IV, VI and VII). Type VI, with 4 secretory cells on a stalk of 2 cells, contains terpenoids, and the sticky exudates of tomato glands can trap small arthropods.

Peppermint

Its peltate gland has 1 basal cell, 1 stalk cell and 8 secretory disc cells, under a cuticle where the oil builds up, according to a 2000 study: the same number as the sessile gland of hemp, a mere structural parallel. Basil, lavender, oregano and thyme, from the same family, are also grown for the oil of their trichomes.

Hop

A relative of hemp in the Cannabaceae family, hop has bract glands that resemble those of hemp (1976 study). The glands of its cones accumulate essential oils, bitter acids and prenylflavonoids, and two of its enzymes make myrcene, caryophyllene and humulene, according to a 2008 study. It does not make CBD for all that, as our article on CBD outside hemp explains.

The "pistils" of the trade, those white then orange filaments, are the stigmas of the female flower, part of its pistil. Trichomes are the resin glands on the bracts around them. Both change colour as they mature, and neither tells you the CBD content.

According to the UNODC, each female flower sits in a small green bract that encloses an ovary, from which two long stigmas emerge. Each unit of the inflorescence carries reduced leaves, bracts and one or two flowers, and trichomes form on the bract before the stigmas lengthen, according to a 2019 study of the female plant. In the 2023 study, the stigmas went from whitish-yellow to reddish-brown as flowering progressed, then to tan-orange after drying. This browning is not only a matter of age: the 2021 UV-B trial lists it among the damage caused by the radiation.

As flowering progresses, the trichome head turns from translucent to milky, then amber. The 2023 study measured this on photos converted into the CIELAB colour space: over the weeks, the translucency of the heads fell and their "red score" rose, until a high proportion were brown late in flowering. According to the authors, this browning probably comes from the oxidation of phenolic compounds.

Some growers read this shift to amber as a sign of advanced maturity, the 2023 study reports. The authors note, however, that the heads on a single bract are not all the same age, which makes any measure of maturity harder. Colour is still only a visual clue, and it says nothing about the CBD content.

Intact trichome heads point to a flower handled with care, without replacing an analysis. In 2023, researchers saw under the microscope that machine trimming detached or collapsed many heads, while drying alone did them little harm.

On dried flowers of a single genotype trimmed by hand, the heads stayed intact; trimmed by machine, many had come off or collapsed, especially at the edges of the bracts, leaving bare stalks. Drying, 5 days between 21 and 24 °C at 50 to 55% relative humidity, shrivelled the stalks but spared most heads. For the authors, handling therefore matters more than drying, which could affect the final cannabinoid content. On the plant late in flowering, their electron microscope also shows the signs of age: resin droplets of about 2 µm, heads stuck together in clumps of 5 or 6, then wrinkled.

When buying, a very frosty flower does not necessarily have a high content, and only the batch analysis gives the CBD and Δ⁹-THC content. Before choosing a flower, see which lines to check in our guide to reading a certificate of analysis. To go further: storing your flowers, telling mould from trichomes, trim and small buds, and the kief of a four-piece grinder.

All concentrates rest on one move: detaching the trichome heads, which a narrowed zone at the top of the stalk makes easy to dislodge, especially during drying, according to the 2023 study.

• Mechanical separation: the heads are collected dry on stacked sieves, where cold makes them brittle (dry sift), with static electricity (static sift), or in ice water filtered through fine-mesh bags (ice-o-lator).
• Extraction: a solvent or supercritical CO₂ dissolves the resin, a process described in our article on CO₂ extraction.

The vocabulary of hash follows from head size. Once detached, heads measure 40 to 110 µm: a 120 µm mesh lets them through and mostly retains heads still on their stalk, clumps of resin and debris. A narrow fraction of free heads can qualify for the full melt grade, and the micron tool turns each mesh into a size window. Our article static, frozen, 120u: what a hash name tells you gives the details with batch figures.

French law carries over the ceiling set for cultivated varieties: the Δ⁹-THC content of hemp extracts, and of the products that contain them, is 0.30% at most (order of 30 December 2021, Article 1, III; status checked on 20 October 2026). Yet isolating the heads concentrates THC along with CBD, so a resin made from compliant flowers can exceed that ceiling. Our article on homemade hash puts figures on this trap, the one on legal CBD hash sets out the framework, and the legal THC rate tool converts a rate into milligrams.