Solutions for Yellowing Leaves

One of the biggest challenges that new growers face is the yellowing of cannabis leaves. Well, sometimes even experienced growers fail to catch this common problem early enough, leading to poor yields.

In this blog post, we will teach you more about the causes and solutions of yellowing. Not only that, we shall be looking at how to differentiate the various causes and proven preventive measures. Read along as we also answer the most frequently asked questions on yellowing cannabis leaves.

Understanding Yellowing Cannabis Leaves

Cannabis plants are primarily green. Some strains have purple and red hues, while others have bluish pigmentation. These primary colors add to the plant’s visual appeal and indicate the plants’ unique terpene and flavonoid content.

Green, the primary color of your cannabis, comes from a pigment called chlorophyll. During photosynthesis, this pigment helps the plant absorb light. It also helps in converting it into usable energy that aids the plant to grow and develop.

Yellowing of the leaves is a critical sign of deficiencies in metabolic processes. This deficiency, if left untreated, spreads into a serious decline in the plant’s health as it can not get the optimal energy needed for growth and development.

Primary Causes of Yellowing Leaves

There are many causes of the yellowing of cannabis leaves. We are going to break them into the different categories shown below.

1. Nutrient Deficiencies
2. Watering Issues
3. Light-Related Stress
4. Soil and pH Imbalances
5. Pests and Diseases

Nutrient Deficiencies 

There are two kinds of nutrients. Base nutrients include nitrogen, phosphorus, and potassium. On the other hand, we have secondary and micronutrients such as magnesium, zinc, calcium, and boron, among others.

Different weed strains have different nutrient requirements. For instance, sativa plants often need more fertilization to support their vigorous growth and high yield potential. Indicas may be generally conservative but also require a balanced nutrient mix.

Let us have a look at the specific nutrients that your marijuana plants need and their deficiencies.

Nitrogen (N) Deficiency

Nitrogen is the backbone of chlorophyll production. Even more, the compound helps the plant produce amino acids and proteins, which are key for plant growth. During the vegetative stage, nitrogen plays a key role in accelerating the maturity of your plants so they are ready for bud development.

Without nitrogen, older leaves are forced to turn yellow and start to fall off. This is because the plant shifts its focus to forming new foliage. It is critical as the plant loses vital mature leaves that could help with optimal light absorption.

Potassium (K) and Phosphorus (P) Deficiencies

Potassium is used by the plant to strengthen cell walls. It is also a vital cog driving transportation within the plant by regulating water flow. Low potassium leads to the curling of leaf edges. Eventually, the tips turn crispy brown after yellowing.

Cannabis plants depend on phosphorus for energy transfer and the development of a good root network. Phosphorus deficiency causes dark yellow patches that are sometimes mottled with dark colours. Prolonged phosphorus deficiency makes the stems and branches weak and slows down growth.

Secondary Nutrients and Micronutrient Deficiencies

There is a wide variety of secondary and micronutrients. Here is a breakdown of what they do and how their deficiencies affect your cannabis plants.

1. 1. Magnesium (Mg), just like nitrogen, helps the plant form chlorophyll. A deficiency in magnesium makes older leaves yellow mostly between the veins.
   2. Calcium (Ca) makes the cell walls stronger while supporting root health. Low levels can distort growth with spotting and crinkling.
   3. Sulfur (S) aids in protein synthesis, enzyme function as well as chlorophyll formation. It’s a key ingredient in a successful vegetative stage and overall metabolism. Lack of sulfur makes new leaves light green to pale yellow.
   4. Iron (Fe) is important for chlorophyll production. It also helps the plants form and use energy effectively.  Deficiency in iron causes interveinal chlorosis.
   5. Zinc (Zn) supports the plant’s enzyme activity, internode elongation, and hormone production. Inadequate zinc in your plants manifests as irregular yellow patches with light green pots. Leaf edges may also curl.

While there are many other micronutrient deficiencies, the above are the most common. 

Watering Issues

Water is involved in nutrient transport, maintaining cell wall integrity, cooling, and even cell expansion that helps the plants grow. It’s also a key ingredient in biochemical reactions.

Overwatering

This occurs when there is excessive water in your growing medium, making it soggy. It causes suffocation of roots as it prevents oxygen circulation in the root zone. 

Common causes of waterlogged growing medium are using too much water and poorly draining growing pots and medium. High humidity can also cause sogginess as it slows evaporation down, even with optimal watering and drainage.

Whenever your plants get exposed to waterlogged mediums, the leaves start to yellow, accompanied by drooping. If left for longer, the roots begin to decay, hampering nutrient absorption, and the plant may die.

Underwatering

If your cannabis plants don’t get adequate moisture to support their metabolic processes, it leads to dehydration. So what causes this? 

The most likely culprit is longer periods between watering’s. A dry and compacted growing medium can also prevent the even flow of water. Moreover, high temperatures can also increase evaporation, which depletes water before watering is done.

All these make the plant start to wilt. The leaves become crispy starting with the tips and edges. As this happens, the cannabis plants turn on survival mode at the expense of growth and development, leading to stunting.

Light-Related Stress

Light provides the energy needed to break down nutrients. However, inadequate and excessive light can harm your plants, leading to yellowing of leaves.

Light burn

Placing artificial lighting too close to the plants causes heat stress and scorching. You should also beware of prolonged lighting without dark hours. Cannabis plants need to relax in darkness, just like in their outdoor natural habitat. This helps them use up their energy to help them grow.

Light burns show up as yellowing starting at the tips and edges, which dry out in severe cases. To differentiate this yellowing from nutrient deficiencies, light burns start with the leaves that are closest to the light source. The rest of the plant may appear healthy and unaffected.

Insufficient light

Too little light can be a result of poor light intensity or inadequate light spectrum. This develops into low chlorophyll production and action.

Unlike light burn, insufficient light affects lower leaves that are often further from the light source. The leaves turn yellow and eventually fall off as they are unable to maintain energy levels that sustain them. Over time, the plants will also show uneven growth and development that is biased towards the light source.

Soil and pH Imbalances

The quality of your soil or growing medium and its pH are directly proportional to the health of your cannabis plants.

Incorrect soil pH

Marijuana plants thrive best in more neutral pH ranging between 6.0 to 7.0. A high pH signifies alkalinity, and a low pH translates to acidity. A highly alkaline or acidic growing medium locks out nutrients that are essential to plants.

Since yellowing due to incorrect soil pH is steered by poor nutrient uptake, it manifests similarly to nutrient deficiencies. An easy way of catching it is the plants not responding to optimal fertilization.

Poor soil quality

The quality of your soil affects the health of your weed plant’s roots. Here is what we mean by poor soil quality.

• • Compactness leading to poor aeration, nutrient, and water flow.
  • Poor drainage due to compactness or by the nature of the specific growing medium. 
  • Inadequate organic matter leading to poor microbial activity and nutrient availability.

Poor soil quality is also expressed with yellowing caused by nutrient deficiencies.

Pests and Diseases

Pest and disease attacks can progress to the yellowing of your weed plant’s leaves as a result of damage or infections.

Common pests

1. 1. Spider mites pierce through the leaf tissues to extract sap. The sap drainage over time shows as discoloured spots. Check below the leaves for their webs to tell them apart from other pests.
   2. Aphids also pierce the leaves for sap. Unlike spider mites, aphids can be noted by the sticky honeydew left behind on their trail.
   3. Whiteflies often pierce through the underside of the leaves. These pests can also cause viral infestation, which is hard to treat. They can be seen as tiny flying insects when the plant is disturbed.
   4. Thrips are hard to see with the naked eye. However, they scrape the plants to leave behind silver to bronze-coloured streaks. Their damage causes pale-looking leaves.
   5. Mealybugs are small white clumps often seen at the base of your leaves and stems. They also feed on sap and can spread fungal infections.

Infections

To understand infections in cannabis plants, let us break them down into three. 

Fungal

Fungal infections are often caused by dampness.  The most common are root rot and leaf septoria.

You can tell root rot by assessing the damaged roots after the yellowing of leaves from poor nutrient absorption. On the other hand, leaf septoria causes yellow patches with older leaves showing spots and a necrotic center.

Bacterial

A soggy growing medium usually causes bacterial infections. Physical injury to the plant can also damage its defence system as it forms an entry point for bacteria.

A good example of bacterial infection is bacterial wilt, which is shown as wilting starting at the base of the plant. The leaves turn brittle and fall off. Another example is soft rot, which makes the plant mushy and yellow with a bad smell.

Viral

Viral infections are typically spread by pests and cause yellowing of leaves by disrupting cellular functions.

The mosaic virus causes yellowing of the leaves in a mosaic pattern alongside curling and mottling symptoms. It is more rampant in younger plants.

Effective Solutions to Treat Yellowing

Tackling the root causes of leaf yellowing is the most effective solution to yellowing leaves. It is important to identify the specific problems and promptly employ effective interventional strategies.

Nutrient management

Adjust your fertilization schedule to match the plant’s genetic requirements and also the growing stage. You can feed the plants for 1-2 weeks or depending on the plant’s needs.

1. Seedling stage (0-2 weeks)– Minimal nutrients are required as the plants actively utilize energy in the seed. Heavy feeding can burn the delicate roots.
2. Vegetative Stage (3–8 weeks)– This phase has rapid growth which is supported by higher nitrogen. Phosphorus and potassium, alongside secondary and micronutrients, are vital in lower amounts.
3. Flowering Stage (9–12 weeks)– Lesser nitrogen at this stage slows down growth and helps the plant focus on bud development. Gradually increase phosphorus and potassium for high resin production and denser buds.

Optimizing Watering Practices

1. Your watering schedule should follow the plant’s size and environmental conditions.
2. Ensure the soil is loose for optimal water flow.
3. Choose a growing pot with holes for draining excessive water.
4. Only water your plants when the moisture content drops, typically when the top layer starts to dry out.
5. Water your plants evenly and sufficiently. To ensure the watering is deep enough to cover the roots thoroughly, you should be seeing any excess start coming out at the bottom.

Light Adjustments

1. Regularly check the distance between your plants and the lights and maintain a distance of about 50-100 cm between them.
2. LED and HPS lights are known to have the right spectrum for growing healthy cannabis plants
3. Use low heat emitting lights such as LEDs.
4. Rotate your plants for even growth in cases of fixed light sources.
5. Prune and train denser canopies for optimal light penetration. Using reflective material lining your grow room helps with even distribution to lower leaves.

Soil and pH Corrections

1. Use clean water with a neutral pH to avoid offsetting your growing medium.
2. Apply agricultural lime to balance the acidic growing medium and sulfur for alkalinity.
3. Enrich your growing medium with organic compost, teas or perlite for improved aeration and microbial activity.

Controlling Pests and Diseases

1. The introduction of beneficial insects such as ladybugs help to lower pest populations organically.
2. Pesticides, fungicides, and bactericides can help fight attacks effectively.
3. Improve air circulation and optimize drainage to prevent dampness.

Preventative Strategies for Healthy Cannabis Plants 

As always, prevention is better than cure. Here are crucial tips that will help you evade yellowing leaves altogether.

• Build a stable growing environment with temperatures of about 70–80°F (21–27°C) and a humidity of 50-60%.
• Routinely inspecting your plants is crucial to identify any potential causes of yellowing leaves early enough.
• Before growing your cannabis plants, research strains that favor your growing skills, experience, and environment. Also, check for specific genetic traits and work out a growing process that supports them.

 

Frequently Asked Questions About Yellowing Leaves