Even the best marijuana growers can run into trouble occasionally and yellow leaves are usually the first warning sign.
Senescence is when plants will naturally yellow off and die. During the final weeks of flowering do not be alarmed. A variety of factors cause chlorosis. Chlorosis in ordinary decent grower terms means loss of chlorophyll. In order to effectively apply treatment, you must know what you’re dealing with. We can help you correctly identify most of the common causes for yellow leaves. The following is not a definitive list of the causes for chlorosis. It is such a common symptom, that sick plants display, we could write a book. We have focused on the most common causes for yellow leaves based on grower experiences.
NOT ENOUGH LIGHT
During photosynthesis, leaves take in light and carbon dioxide (CO2) and convert it into plant energy. Without enough light, leaves will begin to yellow and eventually slow growth to a standstill. Common incandescent house bulbs are severely insufficient, and fluorescent lights must be kept quite close to plants to be remotely effective.
The Fix: Increase the amount of light the plant is getting. This could mean lowering an existing grow light to the proper level above your plants’ canopy or investing in a stronger lighting unit. I highly recommend using HID (High Intensity Discharge) lighting, such as MH (Metal Halide) or HPS (High-Pressure Sodium) lighting for growing pot plants indoors. LED (Light Emitting Diodes) and Compact Fluorescents are a decent, if not perfect, alternative if heat or power usage is an issue.
Indoor cannabis plants grown under artificial lights must be perfectly positioned and lamps must be in just the right distance above the canopy for best results. When lamps and reflectors are too close to the flowering tops, buds and leaves will be yellow and brown as the light is too intense. This is very much a top down problem. In contrast, too little light is usually a problem, that is most evident from the bottom up. Light deficiency causes shaded leaves or leaves too far below the light penetration capability of the lamp to yellow and drop off.
The Fix: Make sure to hang your grow lights as per the manufacturer’s instructions. Adjust as plants progress and use a tape measure for precision. These days there are so many new grow light technologies on the market with their own nuances, you really must stick with the manufacturer’s guidelines to position the light in the sweet spot.
UNDER OR OVERWATERING
Cannabis plants will only thrive with an effective wet-dry cycle. This means you must water the right amount at the right time, with every watering. So what’s the best way to tell if your marijuana is thirsty? Lift up the containers when they are dry. Then again after watering, so you will know the difference. Perhaps it’s not possible to easily lift plants? Then you can purchase a humidity meter to insert into the pot. Hydro growers need to ensure pumps, air stones, reservoirs and timers are set up correctly from the start of the grow. Check twice, and then check again.
The Fix: Monitor the plant’s post feeding behaviour closely. Use one of the measures above to more accurately gauge when to water to prevent recurrence. Too much water can lead to root rot. Too little water will stunt growth. Keep a grow diary if you must and try to get into a routine as the grow progresses.
pH or potential hydrogen is the measurement on a scale of 1-14 of the acidity or alkalinity of a soil mix or nutrient solution, with 7 being neutral. Soil pH should be kept between 6-7, while hydroponic pH should be 5.5-6.2. Fluctuations outside these parameters will lead to nutrient lockout, preventing your roots from being able to take in food. Often misdiagnosed as a deficiency of nitrogen or iron, an undetected pH imbalance can compound problems further when more nutes are added. This creates an over-abundance of plant food in your root zone that your plants cannot absorb.
The Fix: Use a pH meter to measure the level of acidity or alkalinity of your soil and nutrient solution. Adjust using pH up or down accordingly. Bear in mind that these solutions come in concentrated form, so add them sparingly to raise or lower pH incrementally.
OVER or UNDERFERTILISATION
Again, plants need just the right nutrients at the right time. Read the dosage instructions on the label of every bottle of fertiliser before adding to water. Furthermore, we highly recommend you consult the website of the brand of nutrients you are using as most feature custom charts and feeding schedules.
The Fix: As you might have guessed, begin treatment with a flush of pure water. Then follow up with the manufacturers recommended dose at the particular growth/flowering stage. A good rule of thumb is to increase doses incrementally and monitor plant behaviour before further increasing the strength of the solution.
Temperatures below 10°C will cause the leaves of the majority of cannabis leaves to yellow. Some will even take on purple and bluish tones. Sure, this is a fantastic bonus in the final phase of bloom with some strains, but to be avoided early in the grow.
The Fix: Indoors simply turn up the heat or add more lights to the grow op and turn a negative into a bumper harvest. Outdoor growers might need to take plants inside at nighttime or at least invest in a basic DIY greenhouse.
Heat waves can cause cannabis plants leaves to yellow and dry out indoors and outdoors. Temperatures above 28°C prevent cannabis plants from efficiently photosynthesising and cause buds to form more loosely and airy.
The Fix: Indoor growers need to turn up the air conditioning and ensure intake and outtake fans are powerful enough for the size of the grow room. Again, we strongly advise consulting the fan manufacturer’s website for performance specifications. Outdoors the best growers can do is construct a simple shade and hope the heatwave is short-lived.
PESTS AND PLAGUES
Bugs and microorganisms can easily hitch a ride on your clothes, clones gifted from a pal or on a household pet and invade your grow op. Outdoors quadrupedal furry pests of all shapes and sizes must also be taken into consideration and regarded as hostiles.
The Fix: The cannabis growing area must be sealed off from intruders in so far as practicable indoors or outdoors. Naturally an outdoor grow is impossible to 100% quarantine, so you do what you can. Grow in a quiet sunny spot and use chicken wire to protect plants from nibblers like deer and rabbits.
The leaves on your cannabis plants are like biological solar panels. They are essential to photosynthesis. Removing all the yellow leaves is not recommended surgery, especially for young plants. Correct identification of the source of the problem and swift remedial action can restore marijuana.