I am growing my cannabis plants in dirt and for some reason the leaves are turning yellow and some have rust on the leaves. Can you tell me what I need to do in order to get the leaves to stay green 'til they are ready? - Gordon G.

Unfortunately, the yellow leaves and rust spots could be caused by numerous issues. If the yellowing is happening in the last couple of weeks of flowering, I wouldn’t worry about it. It is natural for the large fan leaves to yellow or become rust colored at the end of the ripening stage. Like other annual plants, cannabis will use all of its stored energy in a last effort to reproduce as it reaches the end of its life cycle. In the case of female cannabis plants, the remaining energy is put into flower production. When this occurs, the large fan leaves will become discolored and, in some cases, even start to become brittle and fall off the plant.

If the yellowing your plants are experiencing is occurring while the plants are still in the vegetative stage or in the early flowering stage, it could be caused by a few different things. One common issue that causes yellow leaves with rust spots is pythium. Pythium, also known as root rot, is a very common pathogen that affects a wide variety of plants, including cannabis. Pythium is anaerobic, meaning it thrives in an environment depleted of oxygen. When growing in soil, a gardener must amend his or her soil mixture to make sure there is a good ratio of water retention and air retention. Overwatering is also a common mistake made by novice cannabis growers. When a soil container is overwatered, the soil doesn’t have a chance to dry out. If the soil doesn’t get to dry out, the mixture will eventually become stagnant and depleted of oxygen. This creates the optimal environment for pathogenic fungi like pythium.

Nutrient deficiencies, more specifically calcium or magnesium deficiencies, can also cause yellow and rusted leaves in cannabis plants. Be sure to check your base nutrients or your soil composition to make sure you have ample cal-mag. In some cases, nutrient manufacturers do not include enough calcium in the base fertilizer because it is assumed that the grower is supplementing a calcium-specific additive.

Finally, I would suggest getting out a microscope and closely examining the leaves. Spider mites, thrips, russet mites, and fungus gnats can all cause yellowing and the discoloration of leaves. If pest insects are attacking your plants, the first step is to positively identify which particular pest insect is making its home in your garden. Once identified, an appropriate treatment plan can be implemented.

A well-balanced soil will normally provide the proper nutrition to keep your cannabis plants’ leaves green until ripening. That being said, as a cannabis plant grows larger, its nutrient requirements increase. Many soil growers, especially those using soil containers, will supplement a well-rounded fertilizer in addition to a well-balanced soil to ensure their plants receive the proper nutrition throughout their entire life cycles. I hope this answers your question.

Keep on Growing,
Lee G. Lyzit