Grow three indoor plants: Part 8: End of flowering and the last weeks before harvest

In the penultimate part of our indoor growing series, we will focus on the final weeks before harvest. We will explain why flushing is important and what kind of climate helps prevent mould, which could ruin your plans for a plentiful harvest. Finally, we will look at how to time the harvest for maximum levels of active compounds and terpenes.

In the previous part, we briefly discussed climate control in the flowering stage and the importance of VPD. We also covered how to fertilise plants at peak bloom and showed you defoliation and options for shaping plants during flowering. If you followed our guide, your plants are now in the peak flowering stage. The buds are slowly reaching their final size and beginning to ripen. You can tell this by the first browning pistils (the white hairs on the flowers) and the enlarging bracts (the small leaves around the pistils).

Fertilising in the last two weeks before harvest and flushing

Most indoor-grown varieties flower for 8 to 10 weeks, which means they are at peak bloom roughly in the 6th to 8th week after switching to a 12/12 regime. The exception is long-flowering sativas, which can flower for 12 to 16 weeks. About two weeks before harvest, plants begin to ripen and their nutrient requirements decrease. It is good practice not to fertilise the plants during this period and to water them only with clean water with reduced pH, or to use finishers or ripening accelerators.

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Flushing has several benefits. First of all, you save on fertilisers, but that is far from the most important point. Starving the plants speeds up ripening and, in some varieties, brings out their naturally purple colouring, which many growers value. Last but not least, flushing fertilisers out of the substrate results in a smoother taste of the dried material. If you have decided to use finishers or ripening support products in the last weeks before harvest (Final Part, EYVA Flush, Canna Flush, Final Solution, Flawless Finish), first water the plants once with clean water with reduced pH to flush part of the fertilisers out of the substrate.

Climate and plant care in the last weeks before harvest

The last weeks just before harvest are critical for plants, especially in terms of the risk of mould. The most dangerous is botrytis (Botrytis cinerea), grey mould, which causes flower rot. The easiest way to prevent this is to reduce the humidity in the growing area to 40–50%. If you cannot achieve this with ventilation, look for a suitable air dehumidifier. The temperature in the growing area should not exceed 28 °C during the day so that volatile aromatic compounds – terpenes – are not lost.

Once you reduce the nutrient supply and the plants begin to ripen, the first drying leaves will appear on them. Remove these every day to prevent the spread of pathogens. As harvest approaches, check the flowers every day for signs of mould, especially the largest buds. If you notice even the slightest sign of mould, harvest the plant immediately and discard the affected parts.

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How to recognise ripe buds and time the harvest

Medicinal varieties are best harvested at peak bloom. If you harvest the plants too early, they will contain fewer active compounds and aromatic terpenes. Harvesting too late, on the other hand, means that some of the active compounds and terpenes have already begun to degrade. But how can you time the harvest so that the buds are just ripe enough?

There are two main indicators of flower ripeness. The less precise one is to estimate ripeness by the colour of the pistils, which gradually dry out and turn brown as harvest approaches. Most varieties are best harvested when about half to two-thirds of the pistils have darkened. However, the truth is that this method is not 100% reliable for all varieties, and it is also a good idea to use other indicators, such as the size of the bracts and the overall appearance of the plant.

A more accurate method of determining ripeness is by the colour and transparency of the trichome heads. Trichomes are small glands scattered across the surface of the flowers and leaves of the plant, which at first glance look like a layer of shiny resin. Trichomes do indeed contain this resin, hidden inside their heads, which are clearly visible when magnified with a microscope. A handheld microscope with at least 30x magnification or a macro lens for a mobile phone camera is sufficient.

The highest levels of active compounds and terpenes are found in ripe trichomes whose heads are milky and cloudy. Unripe trichomes have transparent heads, and in the final stage, when the active compounds and terpenes begin to degrade, the trichome heads turn brown. For maximum yield, it is best to harvest the plants when most of the trichome heads on the flowers are at peak ripeness.

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That is all for today. In the next and final part of Grow Three Plants Indoors, we will harvest the plants and learn how to dry, process and store them properly. You can find more useful advice and guides for growers on the Higarden blog for growers.