Yellowing leaves around week 4 of cannabis growth can be alarming, especially when plants appear otherwise healthy. At this stage—typically late vegetative growth or early transition into flowering—your plants are actively building structure, increasing nutrient demand, and preparing for bud production. When leaves begin to fade from vibrant green to pale yellow, it’s often a signal that something in the grow environment, feeding schedule, or root system is out of balance.
Understanding why this happens requires looking at multiple factors rather than jumping to one conclusion. Cannabis plants communicate stress through their leaves, and yellowing is one of the most common signals. The key is identifying whether the issue is nutritional, environmental, or biological. In this guide, we’ll break down the most likely causes of yellow leaves in week 4 and how to correct them before they impact yield and plant health.
Nitrogen Deficiency: The Most Common Cause
One of the most frequent reasons cannabis leaves turn yellow during week 4 is a nitrogen deficiency. Nitrogen is essential for chlorophyll production, which gives leaves their green color and allows plants to perform photosynthesis efficiently. When nitrogen levels drop, older leaves begin to yellow first as the plant reallocates nutrients to newer growth.
This typically starts at the bottom of the plant and gradually works its way upward. The yellowing is usually uniform across the leaf rather than appearing as spots or patches. If left uncorrected, affected leaves may eventually dry out and fall off. While some leaf drop is natural later in flowering, week 4 is too early for significant nitrogen depletion, especially in the vegetative phase.
Correcting a nitrogen deficiency involves adjusting your feeding regimen. If you’re growing in soil, consider supplementing with a nitrogen-rich fertilizer or top-dressing with organic amendments like worm castings. In hydroponic systems, check your nutrient solution’s EC and ensure it aligns with the plant’s growth stage. However, avoid overcompensating—too much nitrogen can lead to dark green, clawed leaves and slow bud development later on.
Overwatering and Root Oxygen Deprivation
Another major cause of yellowing leaves in week 4 is overwatering. Cannabis roots require oxygen as much as they need water, and when the growing medium stays too wet, oxygen levels drop. This leads to root stress, reduced nutrient uptake, and eventually yellowing foliage.
Overwatered plants often display drooping leaves that feel heavy and swollen rather than dry and brittle. The yellowing may appear inconsistent, sometimes affecting both older and newer leaves. In severe cases, root rot can develop, further damaging the plant’s ability to absorb nutrients.
To fix overwatering, allow the medium to dry out between watering cycles. Lift your pots to gauge weight—light pots indicate it’s time to water again. Improving drainage with perlite or using fabric pots can significantly enhance airflow to the root zone. Establishing a consistent watering rhythm is crucial at this stage, as fluctuating moisture levels can stress the plant and slow growth.
pH Imbalance and Nutrient Lockout
Even if you’re feeding your plants correctly, a pH imbalance can prevent them from absorbing nutrients. This condition, known as nutrient lockout, is a common culprit behind yellowing leaves in week 4. Cannabis plants have a specific pH range in which nutrients are available—typically 6.0–6.5 for soil and 5.5–6.0 for hydroponics.
When pH drifts outside this range, essential nutrients like nitrogen, magnesium, and iron become less available, leading to deficiency symptoms even when nutrients are present in the medium. Yellowing caused by pH issues can appear similar to nutrient deficiencies but often affects multiple parts of the plant simultaneously.
Testing and adjusting pH should be a routine part of your cannabis grow. Use a reliable pH meter to check both your water and runoff. If levels are off, flush the medium with properly pH-balanced water to reset the root zone. Maintaining stable pH levels ensures that your plants can access the nutrients they need during this critical growth phase.
Magnesium and Micronutrient Deficiencies
While nitrogen gets most of the attention, magnesium deficiencies can also cause yellowing leaves in week 4. Magnesium is a key component of chlorophyll and plays a role in enzyme activation and energy transfer within the plant. A deficiency often presents as interveinal chlorosis—yellowing between the veins while the veins themselves remain green.
This issue typically starts on older leaves and can progress quickly if not addressed. In some cases, calcium or iron deficiencies may also contribute to discoloration, especially in hydroponic systems or when using reverse osmosis water without proper supplementation.
To correct magnesium deficiencies, many growers use a Cal-Mag supplement or add Epsom salts (magnesium sulfate) to their feeding routine. It’s important to follow proper dosing guidelines to avoid nutrient imbalances. Ensuring your base nutrients contain adequate micronutrients can prevent these issues from arising in the first place.
Lighting Stress and Environmental Factors
Environmental stressors, particularly lighting, can also lead to yellowing leaves. If your grow lights are too intense or positioned too close to the canopy, plants can experience light stress or even light burn. This often causes the upper leaves to turn pale or yellow, sometimes accompanied by curled edges or bleaching.
Temperature and humidity also play a role. High temperatures can increase transpiration rates, leading to nutrient imbalances and stress. Low humidity, especially during vegetative growth, can further exacerbate these issues by forcing plants to lose moisture too quickly.
Adjusting your light distance and intensity is a simple but effective fix. Ensure your grow space maintains temperatures between 70–85°F and relative humidity around 50–70% during vegetative growth. Proper airflow and ventilation help stabilize these conditions, allowing plants to thrive without unnecessary stress.
Natural Leaf Senescence vs. Real Problems
Not all yellowing is a cause for concern. Cannabis plants naturally shed older leaves as they grow, particularly those that receive little light. This process, known as senescence, is a normal part of plant development and doesn’t necessarily indicate a problem.
However, timing is everything. In week 4, especially if your plants are still in the vegetative stage, widespread yellowing is not typical and should be investigated. If only a few lower leaves are yellowing and the rest of the plant looks healthy, it may simply be natural aging.
The key is observing patterns. Healthy new growth, strong stems, and vibrant upper leaves usually indicate that the plant is functioning well overall. In contrast, rapid or widespread yellowing signals an underlying issue that needs attention.
How to Diagnose and Fix Yellow Leaves in Week 4
Diagnosing the exact cause of yellowing requires a systematic approach. Start by examining where the yellowing occurs—lower leaves, upper leaves, or throughout the plant. This can help narrow down whether the issue is related to nutrient deficiencies, lighting, or environmental stress.
Next, review your feeding schedule, watering habits, and pH levels. Small inconsistencies can compound over time and lead to visible symptoms. Keeping a grow journal can be incredibly helpful for tracking changes and identifying patterns.
Once you’ve identified the likely cause, take corrective action gradually. Sudden changes can shock the plant and worsen the problem. Whether it’s adjusting nutrients, improving drainage, or fine-tuning your environment, consistency is key. With proper care, cannabis plants are resilient and can recover quickly from early-stage issues.
Final Thoughts
Yellowing leaves in week 4 are a signal—not a death sentence. In most cases, the issue stems from correctable factors like nutrient imbalances, overwatering, or environmental stress. By paying close attention to your plants and responding thoughtfully, you can resolve the problem before it affects growth or yield.
The most successful growers treat these moments as learning opportunities. Each issue you diagnose and fix builds your understanding of how cannabis plants respond to their environment. Over time, this knowledge translates into healthier plants, bigger harvests, and a more efficient grow overall.
