Sources

Many growers believe more blue gives shorter, tighter plants, "denser" buds and higher potency. So some choose a cooler colour temperature or add blue diodes. These are reports from practice, not measurements. The app's tutorial does not set a blue fraction: it relies on a white LED and on intensity (PPFD 750–900 in flower).

Westmoreland et al. (2021) ran three crops in 1 m² sections at a fixed PPFD. As the blue fraction rose from 4% (HPS) to 20% (5000 K LED), dry flower yield fell linearly by about 12%. CBD and THC concentrations did not change, and the difference in plant height was not significant. Fixture efficacy mattered more: a white+red LED (10% blue, 2.51 µmol/J) gave 27% more flower per unit of electricity cost than HPS. The authors cite an earlier study (Magagnini) where, at roughly half the intensity, LEDs with 14–24% blue raised cannabinoid concentration but not total cannabinoid yield. A review by Desaulniers Brousseau et al. (2021) describes the blue-light findings as conflicting. In lettuce leaves, an absorbed blue photon was used least efficiently (Liu & van Iersel 2021). In cucumber, leaves grown under red alone worked abnormally; 7% blue was enough to prevent this (Hogewoning et al. 2010).

No comparisons exist on high-THC cultivars at home intensities without CO₂. Terpenes were not measured. The authors note that other cultivars may respond differently, and at lower intensity, for example in veg, colour may matter more.

Growers choose seeds by label — THC-rich, CBD-rich or 1:1. Some believe that stress, light or a late harvest can push a plant towards CBD or change its character. These are beliefs, not tested results.

The clearest evidence is a classic crossing study: de Meijer et al. (2003) crossed pure-CBD and pure-THC inbred plants (HortaPharm/ISCI, Italy); every F1 plant had a mixed CBD-THC chemotype, and the F2 offspring split into pure CBD, mixed and pure THC in a 1:2:1 ratio — one locus, B, with two codominant alleles, B_D and B_T. CBGA is turned into THCA or CBDA by two competing enzymes, THCA synthase and CBDA synthase. Whether a plant carries working or broken versions of these genes — linked at the so-called B locus — largely sets whether it is THC-, CBD- or CBG-dominant (Ahsan et al. 2026, review). Henry et al. (2020) found one marker in this gene cluster fixed for one form in every CBD-expressing variety and for the other form, or both, in THC-expressing ones. Environment changes amounts, not the type. Rodriguez-Morrison et al. (2021) raised light from 120 to 1,800 µmol/m²/s: flower yield and cannabinoids per m² rose 4.5-fold, but cannabinoid concentration did not change. In Alden's dissertation THC and CBD percentages rose and fell over weeks 4–10, yet CBD:THC stayed between about 21:1 and 28:1. Henry also found identical genotypes sold under different names (e.g. "Gorilla Glue" and "Holy Grail") and high variability within seed lines.

Henry tested North American samples, not UK or European seed. Whether stress or UV shifts the ratio in THC cultivars is untested here; epigenetic effects remain hypotheses.

The tutorial sets light by PPFD: 150–250 for seedlings, 400–600 in veg (usually 18/6), 750–900 in flower (12/12). Growers rarely discuss the daily total the plant actually gets.

DLI is the number of photosynthetic photons a square metre receives in one day (Faust & Logan 2018). For a steady lamp: DLI (mol·m⁻²·d⁻¹) = PPFD × 3600 × hours ÷ 1,000,000, i.e. PPFD × hours × 0.0036. Gavhane et al. (2023) used this: PPFD 200 for 12, 16 and 20 h gave 8.64, 11.5 and 14.4. The energy-to-photon factors in these papers (Faust: 0.0072664 mol per Wh of total sunlight; Gavhane: 1 W·m⁻² ≈ 2.0657 µmol·m⁻²·s⁻¹) are for sunlight only, not for LED lamps. Rodriguez-Morrison et al. (2021) note that DLI can track yield better than instantaneous PPFD, and their cannabis yield kept rising up to a flowering DLI of about 78. Lettuce, by contrast, did best at 11.5 and lost weight at 14.4, reached with 20 h days (Gavhane). Faust & Logan add that LEDs alone can now deliver 40–50, and that small plants with little leaf area may not use a high DLI.

Cannabis DLI data come mostly from one CBD-dominant cultivar in flower. No source here measured the best DLI for cannabis seedlings or veg; lettuce is a low-light crop.

Lamps with extra 730 nm diodes are marketed for faster finishing and bigger yields. Some growers run FR alone briefly after lights-off ("end of day") so plants "sleep" sooner. These are reports and marketing, not measurements; the app's tutorial doesn't mention FR.

Zhen & Bugbee (2020) added 701–750 nm photons to white light. Up to ~40% of the background, they raised canopy photosynthesis as much as the same number of white photons. FR alone worked poorly, gains shrank above 40%, and ~746 nm diodes were weakest. This is the Emerson effect: FR mainly drives photosystem I and rebalances the two photosystems. The authors propose counting PAR up to 750 nm. A lettuce leaf absorbed only 21% of FR versus 85% of white light. Plants lack the FR-using chlorophylls d and f of some cyanobacteria; a soybean canopy model suggests adding them could raise photosynthesis up to 26% (Wang et al. 2025). In lettuce, 50 µmol/m²/s FR doubled fresh weight and leaf area, mainly via leaf expansion, but lowered anthocyanins (Van Brenk et al. 2024). Bilodeau et al. (2019) review stem stretch under FR; FR late in the photoperiod aided flowering in long-day plants, while one cannabis cultivar (G-170) showed no flowering response to R:FR changes.

No source tests FR on cannabis to harvest: yield, bud density, cannabinoids. Zhen's measurements lasted hours, not a crop cycle. Whether end-of-day FR shortens cannabis flowering is untested here.

Many small growers finish on plain water. The app's tutorial teaches 7–10 days at pH 6.0, with runoff EC dropping from about 2.0 to below 0.6. Growers report a smoother taste and a "cleaner burn": reports, not controlled tasting. The tutorial says flushing "washes out stored mineral salts"; water rinses salts from the pot, not from leaves and buds.

Alden & Faust (2025) leached pots to runoff EC ≤ 0.2 mS/cm, then gave plain water for 0–4 weeks. Losses grew with length: 'THM Jack' lost about 0.01 kg/m² of flower per week (from 0.17); in 'Southern OG' total reproductive mass after one week matched the control, two weeks cost 10 % of flower mass and weeks 2–4 a further 22 %. THC % did not change; CBD % rose 0.5 points a week in one cultivar, yet CBD per m² fell in both. Flower N, P and K fell; older leaves yellowed and dropped. The authors suspect flushing began as a fix for salt build-up from overfeeding. Alden's dissertation reports the same experiment. Hershkowitz (2024) found plain water lowered yield, a nitrogen-free feed for up to 21 days did not (a separate trial), and cannabinoid yield per plant held. Leaf N fell while flower N stayed steady, suggesting the plant moved reserves from leaves to buds.

No study has measured taste, smoke or ash after flushing. The two studies disagree on flower nutrients. THC cultivars under small LEDs in coco or soil are untested.

Many small growers start from clones swapped or bought from others, or keep one mother plant for months. Viroids rarely appear in grower guides; plants that stay short and brittle and make loose, weak-smelling buds are usually blamed on genetics or feeding.

Adkar-Purushothama et al. (2023) review hop latent viroid: a tiny circular RNA without a protein coat, long known in hops and confirmed in cannabis "dudding" in 2019. Susceptible cultivars show shorter internodes, smaller leaves, stunting, brittle stems and, in flower, smaller, looser buds with fewer trichomes; many cultivars carry it without symptoms. In hops it spreads by propagation and contaminated tools; pollen transmission is low, seed transmission has not been found and no insect vector is known. Headline figures (about 90 % of Californian facilities infected, up to 50 % fewer cannabinoids) come from industry surveys and web reports. There is no chemical cure; only a lab test (RT-PCR, RT-qPCR) on several leaves along the stem shows infection. Chiginsky et al. (2021) also found HLVd in Colorado hemp. Torres et al. (2025) freed some plantlets by culturing meristem tips at 30 °C: clean samples ranged from 0 to 94 % depending on cultivar. Cleaned plants are viroid-free, not resistant.

How much yield HLVd costs in a small home grow, and how often traded clones carry it, has not been measured.

Lamp marketing leans on watts and "full spectrum". Many growers now compare efficacy in µmol/J (photons out per joule in) and the PPFD map, the maker's grid of readings across the footprint.

Kusuma et al. (2020) define efficacy as µmol of 400–700 nm photons per joule; lumens are weighted for human eyes. Heat (about 10 % loss from 25 to 85 °C at the LED), the driver (85–95 % efficient) and protective covers (about 10 %) pull a fixture below its LEDs' rating. In 2020 the best third-party-tested fixtures reached 2.5–2.8 µmol/J (white + red) and 3.0 (blue + red), against 1.72 for a 1000 W HPS. Typical values were 2–3; practical limits are about 3.4 and 4.1. Westmoreland et al. (2021) found that raising blue from 4 to 20 % cut hemp yield by 12 %, while spectrum did not change CBD or THC concentration. But going from 1.72 to 2.51 µmol/J gave 27 % more flower per dollar of electricity: efficacy matters more than spectrum for cost.

The yield data come from one hemp cultivar under 1,000 ppm CO₂, with no tent trials on THC cultivars. Makers' PPFD maps and lamp ageing in a warm tent were not tested (Kusuma note that heat speeds LED ageing).

The tutorial teaches PPFD 750–900 µmol·m⁻²·s⁻¹ at the canopy top in bulk flower, then dimming to 60–70 % in late flower to protect terpenes. Growers report that top leaves can pale, bleach or curl upwards when light and heat push too hard. These are observations, not measured thresholds.

Rodriguez-Morrison et al. (2021) grew a CBD-dominant cultivar at PPFD 120–1,800. Dry flower yield rose linearly from 116 to 519 g/m² (4.5×) without levelling off, and top buds got denser. Cannabinoid concentration did not change, and terpenes rose slightly. A leaf stress indicator (Fv/Fm) fell as leaf-level light rose. On a high-THC cultivar with no added CO₂, Llewellyn et al. (2022) found 1.6× more dry flower at 1,000 than at 600, with no change in cannabinoid or terpene concentration. Hahm et al. (2025) saw hemp flower mass and CBD content rise from 200 to 600, with only three plants per treatment. In a greenhouse, Collado & Hernández (2025) got 4.1× the yield for 4.7× the light, while cannabinoids in lower flowers fell about 13 %.

No study used a home tent, a small lamp or late-flower dimming. Rodriguez-Morrison found terpenes slightly higher, not lower, under more light, so the benefit of dimming is untested. Potency results differ between cultivars, and high light in a warm, poorly ventilated tent hasn't been studied.

Small growers fight disease mostly with climate: the app's tutorial teaches RH below 50 % (40–45 %) in bloom and fans running 24/7 against botrytis. Many also hang sachets of predatory mites (Amblyseius swirskii) against pests and stop oil or other sprays on flowers after about week 3 of bloom. This is practice, not a tested protocol.

Punja et al. (2019) sampled commercial indoor, greenhouse and field cannabis in Canada. Roots carried Pythium and Fusarium (browning, stunting, sometimes death); leaves mostly powdery mildew (Golovinomyces); buds Botrytis, Penicillium and Fusarium. Spores travelled by air, water, cuttings and unpasteurised coco and entered through wounds; applied biocontrol fungi turned up in the air too. McGehee and Raudales (2021) found Pythium myriotylum in coco from one facility; on hemp it cut plant size by a third or more. Mahmoud et al. (2023) review bud rot: it shows in weeks 6–8 of flower, favoured by RH above 70 %, 17–24 °C, dense buds, crowding and condensation; many thresholds come from other crops, and no genetic resistance is known. For mildew, Pepin et al. (2021) found no disabling mutations of the two candidate susceptibility genes in 32 cultivars, and Seifi et al. (2025) found a strong resistance locus (PM2) in only two of 510 genotypes. On cucumber, A. swirskii held spider mites back only when it was established before the mites arrived (Messelink et al. 2009).

No study has tested which RH, fan set-up or spray cut-off prevents bud rot in a small tent.

Growers set the pH of every feed and use EC as a gauge of feed strength. The app's tutorial teaches pH 5.8–6.0 for seedlings and 5.9–6.1 in veg, with EC rising from 0.5–0.6 for seedlings to 1.8–2.2 in bulk flower. Many also check the runoff from the pot. These are practice values, not trial results.

Extension guides for container crops (Camberato et al., Purdue; Whipker et al. 2018) state that all nutrients are readily available at substrate pH 5.4–6.2; iron chlorosis appears above about 6.3–6.5, and low EC shows as yellowing lower leaves. They compare PourThru (leachate pushed out with distilled water; non-destructive, works in coir, but variable), saturated extract and 1:2 slurry. EC bands depend on the method: in PourThru, 1.0–2.6 mS/cm counts as "low" (fits seedlings) and 2.7–4.6 as "normal" — for ornamentals. They advise calibrating with standards (pH 4, 7; EC 1.41) and plotting trends. Sambo et al. (2019) note that iron, manganese and phosphate precipitate above pH 7. In cannabis, Kpai et al. (2024) saw solution pH and EC rise in DWC as plants drew water. Velechovský et al. (2024) fed pH 5.9 at EC 0.97 rising to 1.74; a richer feed (up to 2.34) did not significantly change final flower mass or cannabinoid concentration.

No source gives validated root-zone EC ranges for cannabis in coco, or checks simple runoff against PourThru. The cannabis studies report feed EC in larger or hydroponic set-ups.

The app's tutorial: harvest in the morning before lights on, dry at 60/60 (≈16 °C, 60 % RH) in darkness for 10–14 days, then cure in jars at 62 % (Boveda), opening daily for two weeks, then weekly. Ripeness is read from trichomes: clear → milky (peak THC) → amber ("CBN, sedation"), aiming for 75 % milky and 25 % amber. This is grower practice.

Birenboim et al. (2024) used open-air drying at 15 °C and 50–55 % RH as their "traditional" reference. After 14 days the buds still had not reached a stable dry weight, and by day 12 about 80 % of the high-THC buds carried heavy Alternaria mould with some Botrytis. In that chemovar THCA fell from 9.5 to 7.6 % of dry weight; THC rose about 3.5-fold but far less than the THCA lost. In the hybrid, THCA and CBDA held. Myrcene was the most fragile terpene (losses of 3–30 %), limonene fell 15–58 % in the hybrid, while in the high-THC chemovar caryophyllene and other sesquiterpenes rose 20–50 %. Kim et al. (2024) found that even dark, room-temperature air-drying converts part of the acids: CBD:CBDA was 1:95 in fresh trichome contents and 1:20 in dried buds. Reviews note that heat and storage time drive decarboxylation and that THCA can turn into CBN — chemistry, with no link to amber colour or sedation.

No source tested 60/60, jar curing or burping, or measured taste. Samples were tiny buds, not hanging branches.

Home growers often assume that a prescription lets them grow their own, that a few plants in a tent are too small to matter, that legally sold seeds make growing legal, or that hemp or CBD rules cover flowers. The official guidance says otherwise.

The Home Office factsheet states that cannabis is a Class B drug under the Misuse of Drugs Act 1971 and that cultivating any plant of the genus Cannabis is an offence without a Home Office licence. Seeds are excluded from the definition, but growing a plant from one is cultivation. Cannabis-based products for medicinal use (CBPMs) have been in Schedule 2 of the 2001 Regulations since 2018: specialist doctors may prescribe them and pharmacists supply them under an exemption. The factsheet gives no cultivation exception for patients. Industrial hemp licences cover approved seed with THC not above 0.2%, for fibre or seed only; leaves and flowers are controlled, and CBD-oil applications fall outside this policy. Pure CBD is not controlled, but CBD products usually contain traces of THC and are controlled unless they meet the "exempt product" conditions (e.g. at most 1 mg of each controlled cannabinoid per container). GACP (EMA 2006) is a quality standard for producers of medicinal plants (records, traceability), not a home-grow guide or a permission.

The factsheets do not cover penalties, enforcement practice, Northern Ireland, or medicines and food rules. Whether a CBD product is exempt is decided case by case, and the guidance can change (0.2% is the figure "at the time of drafting").

A popular tip says: add a UV-B lamp late in flower, because "stress boosts THC and resin". These are reports from practice, not measurements. The app's tutorial uses no UV lamps; for the last days it rather recommends dimming the light to 60–70%.

Llewellyn et al. (2022) found no effect of UV-A or UV-A+UV-B on yield, cannabinoids or terpenes in the flowers. Only in the "sugar leaves" did UV-A+UV-B give about 30% more THC in its neutral form, with no change in total THC. The authors also noticed, without counting, more trichomes on those leaves and less powdery mildew. By contrast, raising PPFD from 600 to 1,000 gave 1.6 times more dry flower at the same cannabinoid concentration. Reviews (Bilodeau et al. 2019; Desaulniers Brousseau et al. 2021) cite a 1987 study in which UV-B for 3 h a day raised THC in leaves and buds. Llewellyn points out that the older cultivars had much less cannabinoid content. In another study, short-wave UV-B (287 nm) caused only harm. Nawkar et al. (2013) describe how small UV-B doses switch on protective responses in plants, while larger ones damage DNA, proteins and photosynthesis.

There is no dose–response for modern cultivars. Studies differ in spectrum, dose and timing, so they are hard to compare. Whether CBD-dominant or balanced cultivars respond differently is unknown.

Growers steer humidity by VPD. The app's tutorial teaches 0.4–0.8 kPa for seedlings (RH 65–75 %), 0.8–1.2 kPa in veg and 1.2–1.6 kPa in flower (RH 40–45 %). Many believe LED leaves run cooler than the air. These bands are practice, not cannabis trial results.

Grossiord et al. (2020): VPD is the gap between the water vapour air could hold and what it holds; at the same RH it climbs steeply with temperature. As VPD rises, stomata close: conductance falls, transpiration rises up to a point, photosynthesis falls less steeply. Species differ widely; leaf-to-air VPD, from leaf temperature, is more accurate. Inoue et al. (2021) held lettuce at a mean VPD of about 1.0 kPa: swings between 0.63 and 1.63 kPa cut conductance and photosynthesis and, after three weeks, gave 15 % less dry weight and 29 % less leaf area than swings of 0.86–1.32 kPa. They cite 0.5–0.8 kPa as suiting most plants and reduced transpiration above 2 kPa. In Nelson and Bugbee's (2015) model, well-watered leaves stay within about 2 °C of air under any light, about 1.3 °C cooler under LED than HPS; water-stressed leaves in still air can reach 8 °C above air under LED. Below 0.4 kPa moisture stays on plants and favours fungi (Mahmoud et al. 2023). None tests the stage bands on cannabis; the flower band sits above that general range and rests mainly on mould control.

No source here compares VPD bands for cannabis yield or potency.