Plant Stress in Cannabis – Potency, Yield, Hermaphrodites

Many growers see plant stress shortly before harvest as a trick for more THC, the main intoxicating compound. Growers let plants wilt, puncture leaves, hang UV lamps or keep the room dark for two days. None of the stresses tested so far raised yield and potency together. For ice water and darkness before harvest no trial turned up at all. Some stresses cost harvest, and cold at the wrong time can trigger hermaphrodites.

Does drought before harvest raise THC?

Not reliably. In one cultivar potency rose by about an eighth after wilting, in two others nothing changed. Severe drought over weeks cost up to 62 % of yield. THC never rose. Watering moderately short, on the other hand, saved 20 to 33 % of water with no loss.

  • Water on the lean side, but never so far that leaves droop.
  • To test wilting, take cuttings of one mother plant, that is rooted clones. Let one group wilt and water the other normally. Compare potency and dry weight at the end.
  • If only potency rises while weight falls, that can simply be a dilution effect.

Is it worth wounding leaves or stems before harvest?

No. Punctured fan leaves raised THC by 9 % in one cultivar. These are the large fingered leaves. That worked eleven days before harvest. Seven days before harvest it no longer did. Outdoors, damaged leaves brought nothing. Early topping, cutting off the shoot tip, did bring more flower. Cutting back did not trigger more hermaphrodites either.

  • Do not puncture leaves or cut into the stem before harvest.
  • Top large plants twice: at transplanting and at the switch to flowering.
  • Keep the main branches. Without them almost the whole harvest was lost.

Do UV light or darkness before harvest help?

No. UV-A and UV-B are the invisible short-wave parts of light. In six trials with current cultivars they did not raise THC. Stronger light gave more yield at the same potency. Longer light periods act differently depending on the cultivar. 14 hours of light in flower clearly lowered THC in two cultivars. 13 hours raised it in another. For 48 to 72 hours of darkness before harvest no trial turned up. More on this in the article on UV and IR light.

  • Do not buy a UV lamp for more THC.
  • Raise light intensity instead. If the flower tips turn white, turn it back down.
  • Stay at 12 hours of light in flower unless your own comparison shows otherwise.
  • Skip the dark period before harvest. It costs days, and no benefit is documented.

What do cold and heat do in flower?

Long cold spells and heat do harm. A slightly cooler second half of flowering can help. Warm first weeks of flowering gave over 50 % more flower. In one of two cultivars, potency was twice as high after a cool second half. Ten days at 10 °C halved potency instead. For ice water on the roots no trial turned up.

  • Run the first half of flowering warm, around 26 °C as a daily average.
  • Run the second half cooler, about 20 °C by day and 17 °C at night.
  • Stay well above 13 °C around the switch to flowering, nights included. A thermometer with a minimum reading helps.

Does starving or flushing at the end help?

No. When a nutrient was missing from the start of flowering, the harvest shrank sharply and THC content stayed the same. Without nitrogen only a good quarter of the flower was left. Rinsing with plain water removed nutrients from the flower in no trial. In two trials it cost yield.

Which stress causes hermaphrodites, and when is it too late?

Hermaphrodites are female plants with male flowers. Four triggers are documented. Cold of about 13 °C at the switch to flowering. A short day on young plants. An abrupt change of light schedule. And the spray silver thiosulfate. Light leaks, pruning, heat and UV are not documented as triggers. Hermaphrodites also appear without stress, on 5 to 10 % of plants in one operation. Pollination cost up to three quarters of the active compounds. Seeds need three to four weeks to ripen. By calculation, stress in the last week can therefore hardly produce ripe seed.

  • From week four of flowering, check every week for anthers, the small pollen sacs of the male flower.
  • Start low on the stem and at the lowest flowers.
  • Take plants with anthers out of the room before the anthers open.
  • How the tendency is inherited is covered in the article on hermaphrodites.

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