Despite the best efforts of cannabis growers to ensure that no pollen gets into their growing area, the odd phantom fertilisation event does still occur, resulting in bud that is inexplicably full of seeds. In most cases, this will be caused by the presence of a hermaphrodite plant – or even a sneaky male, if the grower has been a bit sloppy – although at other times the source of this mysterious invading pollen can be surprisingly distant, originating in someone else’s field several miles away.
That’s because cannabis pollen is exceptionally good at sailing on the wind, and can travel huge distances if the conditions are right. Given that bees are not attracted to female cannabis flowers, wind pollination is the plant’s only natural means of reproduction, and, like all living things, it has evolved to maximise its chances of passing on its genes. A single male flower will therefore generate about 350,000 pollen grains[i], and when you consider that a plant can often contain hundreds of flowers, that adds up to a hell of a lot of cannabis spunk in the air.
There’s Something In The Air
A study that ran from 1992 to 1996 in Omaha, Nebraska, found that cannabis pollen accounted for an incredible 36 percent of all airborne pollen during the annual marijuana flowering season, which runs from late July to mid-September[ii]. For this reason, industry regulations often require commercial growers to maintain a considerable distance between themselves and the next field if growing outdoors. In Canada, for example, an isolation distance of five kilometres is required for all commercial hemp fields, exceeding the obligatory isolation distance for all other crops.
Yet many industry experts recommend maintaining a distance of at least ten miles between cannabis fields, given that marijuana pollen has been found to travel extraordinary distances. Between 1991 and 1996, researchers noted that cannabis pollen originating in Morocco could be detected in southern Spain on days with favourable weather conditions[iii]. Each year between May and September, North African pollen was detected in the Iberian peninsula whenever the jet stream was at a high altitude, as this allowed the pollen to rise above the maritime layer of air and avoid being pulled down into the sea by convection. These conditions don’t occur every day, but the frequency with which Moroccan pollen travelled to Spain was still considerable, ranging from 15 to 29 days per year during the flowering season.
How To Avoid All That Pollen
As with all wind-pollinated plants, cannabis is naturally dependent on weather conditions to spread its pollen. A recent study found that the plant is able to distribute about six times more pollen in a downwind direction than in an upwind one[iv]. Obviously, wind direction is rarely constant and can change from day to day, so cannabis pollen will usually be distributed equally in all directions, although it’s still worth factoring the wind into your plans when setting up an outdoor grow site. For instance, if you’re based in a location that is highly affected by particular jet streams during certain months of the year, then you may be able roughly predict where most of the wind will come from during your flowering season, and may therefore want to avoid growing weed downwind from another cannabis field.
With all this in mind, there’s still no need to panic too much if someone else does start growing their own cannabis close by, as the chances of their pollen actually ruining your bud is never going to be that high. In theory, just one grain of pollen is all that is required to fertilise a female flower, although in reality multiple grains often have to land on a stigma for an ovule to become fertilised. This is particularly true if the pollen has had to travel for a distance, as the viability of cannabis pollen decreases quite dramatically over time.
A study found that when it is released from the anthers of a male flower, cannabis pollen has a viability of about 70 percent, but that this drops to less than five percent within three days[v], meaning that many of the grains that reach a female in another field are likely to be duds.
In spite of this, many growers decide not to take any chances, and cultivate their bud indoors. Doing so is seen favourable as it allows for complete control over all atmospheric conditions, including light, temperature and carbon dioxide levels – although it also has another, less obvious advantage, as it creates an impenetrable fortress that can’t be breached by foreign pollen.
[i] Faegri, K., Iverson, J., Kaland, P. E. and Krzywinski, K. 1989. Textbook of pollen analysis. , 4th edition., New York, NY: John Wiley & Sons.
[ii] Stokes JR, Hartel R, Ford LB, Casale TB. Cannabis (hemp) positive skin tests and respiratory symptoms. Annals of Allergy, Asthma & Immunology. 2000 Sep 1;85(3):238-40. – https://pubmed.ncbi.nlm.nih.gov/11030280/
[iii] Cabezudo B, Recio M, Sánchez-Laulhé J, Trigo MD, Toro FJ, Polvorinos F. Atmospheric transportation of marihuana pollen from North Africa to the southwest of Europe. Atmospheric Environment. 1997 Oct 1;31(20):3323-8. – https://www.sciencedirect.com/science/article/abs/pii/S1352231097001611
[iv] Small E, Antle T. A preliminary study of pollen dispersal in Cannabis sativa in relation to wind direction. Journal of Industrial Hemp. 2003 Mar 1;8(2):37-50. – https://www.tandfonline.com/doi/abs/10.1300/J237v08n02_03
[v] Bassani M, Pacini E, Franchi GG. Humidity stress responses in pollen of anemophilous and entomophilous species. Grana. 1994 Jun 1;33(3):146-50. – https://www.tandfonline.com/doi/pdf/10.1080/00173139409428991