During a recent visit to a local hydroponics store, I was presented with a purpling on new growth while going into the flowering phase on an outdoor grow. Now We've all been there - walking into our grow room or garden to discover those telltale purple leaves appearing on the new growth of our cannabis plants. While purple can sometimes be a desirable trait in certain strains, unexpected purpling of new growth often signals underlying issues that need addressing. In this guide, we'll break down the various causes of this phenomenon and provide practical solutions to get your plants back on track.

The Multiple Faces of Purple: Distinguishing Between Genetics and Problems

Before diving into troubleshooting, it's important to note that not all purple coloration indicates a problem. Many cannabis strains naturally express purple pigmentation due to their genetic makeup. However, when purpling appears suddenly or primarily affects new growth, it's typically a response to stress or deficiency.

Genetic Purpling vs. Stress Response

In our experience, genetic purpling usually presents as an even distribution of color across the plant, often intensifying during flowering or cooler night temperatures. This is simply the plant's anthocyanins (natural pigments) becoming more prominent.

Stress-induced purpling, which we'll focus on today, typically appears unevenly and is often accompanied by other symptoms like stunted growth, curling, or wilting. This distinction is crucial for proper diagnosis and treatment.

Pathogen Factors: The Hidden Culprits

Leafhopper-Transmitted Pathogens

One crucial factor that growers often overlook is the role of insect vectors in spreading diseases that cause purple discoloration. Leafhoppers, in particular, can be significant transmitters of pathogens that affect cannabis health.

The beet leafhopper (_Circulifer tenellus_) deserves special mention as it transmits Beet Curly Top Virus (BCTV), which can wreak havoc on cannabis plants. When infected with BCTV, plants may develop purple veins along with leaf curling, yellowing between veins, and overall stunting. The purpling often starts in new growth areas, making it a prime suspect when young leaves take on a purple hue.

Similarly, other leafhopper species can vector various phytoplasmas (bacterial pathogens) that interfere with the plant's vascular system, causing purpling as a symptom of nutrient transport issues. These tiny, often colorful insects might seem harmless, but their feeding activities can introduce pathogens directly into the plant's vascular tissue.( Phytoplasmas deserves its own blog post stay tuned as I learn more about this facinating pathogen)

Fungal Infections and Their Impact

Beyond viral pathogens, certain fungal infections can also trigger purpling in new growth. Fusarium species, particularly _Fusarium oxysporum_, may cause vascular discoloration that presents as purpling in leaves, especially when the infection blocks nutrient transport.

This is because fungi like Fusarium infect the plant's vascular system, essentially creating a partial blockage that prevents proper nutrient movement. The plant's newest growth is most affected by this disruption, as these tissues have the highest nutrient demands for cell division and expansion.

Nutrient Imbalances: The Classic Culprits

Phosphorus Deficiency

When it comes to purpling leaves, phosphorus deficiency is often the first suspect on our list. While phosphorus deficiency typically affects older leaves first, under certain conditions—particularly cold temperatures—it can manifest in newer growth as well.

Plants suffering from phosphorus deficiency will initially develop dark green leaves that gradually take on a blue or purple hue. As the condition progresses, you'll notice slower growth rates and potentially see brown, burnt-looking patches developing on the purple leaves.

It's worth noting that phosphorus deficiency often appears when growing temperatures drop below 15°C (60°F). This is because cooler temperatures make it difficult for plants to absorb phosphorus, even when it's present in adequate amounts in the growing medium.

Magnesium Deficiency

While magnesium deficiency typically causes yellowing between the leaf veins rather than purpling, it can sometimes contribute to reddish-purple coloration, particularly in the stems and petioles (leaf stalks). This deficiency usually starts with the older leaves, but in severe cases or certain growing conditions, it can affect new growth as well.

Magnesium is a mobile nutrient, meaning the plant can relocate it from older to newer leaves when supplies are limited. However, when the deficiency is severe enough, even new growth will show symptoms.

Calcium Deficiency

Though less commonly associated with purpling, calcium deficiency can sometimes cause reddish-purple stems and petioles, especially in new growth. Calcium is immobile within the plant, meaning it cannot be relocated from older to newer tissues, so deficiency symptoms appear first in new growth.

This deficiency often manifests alongside curved or hooked leaf tips and stunted root development. Since calcium is crucial for cell wall formation, new growth suffering from calcium deficiency may appear distorted or malformed in addition to showing purple coloration.

Environmental Factors: The Silent Stressors

Temperature Fluctuations

We've found that one of the most common environmental triggers for purple leaves in new growth is temperature stress—particularly cold nights combined with warm days. When night temperatures drop below 65°F (18°C), many cannabis strains will respond by producing anthocyanins, the pigments responsible for purple coloration.

This is actually a protective mechanism by the plant. Anthocyanins help protect plant tissue from light stress when phosphorus uptake is limited by cold temperatures. It's essentially the plant's version of putting on a coat when it gets chilly.

Light Intensity and Spectrum

High-intensity light, particularly those rich in UV-B radiation, can trigger protective purple pigmentation in new growth. This is the plant's equivalent of getting a tan—producing compounds to protect sensitive new tissues from potential light damage.

LED grow lights with high blue and UV light components are particularly known for bringing out purple coloration in cannabis. While this isn't necessarily harmful, it can sometimes mask other issues, so it's important to consider light characteristics when diagnosing purple leaves.

pH Imbalances and Nutrient Lockout

Perhaps the most overlooked factor in purple new growth is pH imbalance. Even when nutrients are present in the growing medium, they become unavailable to the plant when the pH is outside the optimal range. For cannabis in soil, this range is typically 6.0-7.0, while hydroponic systems function best at 5.5-6.5.

Phosphorus, in particular, becomes less available in acidic conditions (below pH 6.0), which can lead to purpling even when phosphorus is present in the growing medium. This "nutrient lockout" situation can be particularly frustrating because adding more phosphorus won't solve the problem—only correcting the pH will.

 Diagnosis and Treatment: The Action Plan

Identifying the Specific Cause

To properly address purple leaves in new growth, we need to carefully observe additional symptoms:

1. **If purple coloration is accompanied by leaf curling, yellowing between veins, and stunted growth**, suspect leafhopper-transmitted pathogens like BCTV.

2. **If purpling appears primarily on leaf edges and is associated with dark green leaves and slow growth**, phosphorus deficiency is likely the culprit.

3. **If purpling is accompanied by yellowing between veins on older leaves**, investigate magnesium deficiency.

4. **If purpling appears after cold nights but plants otherwise look healthy**, temperature fluctuation is probably the cause.

5. **If new growth is purpling along with distortion or malformation**, calcium deficiency or pH issues could be to blame.

Treatment Approaches

For Pathogen-Related Purpling:

1. **Implement insect control measures** - Organic options include neem oil, insecticidal soaps, or introducing beneficial predators like ladybugs to control leafhoppers.

2. **Isolate affected plants** to prevent spread to healthy specimens.

3. **Remove severely affected leaves** to reduce pathogen load.

4. **Strengthen plant immunity** with silica supplements and balanced nutrition.

For Nutrient Deficiencies:

1. **Phosphorus deficiency**: Apply a bloom formula fertilizer (higher in P) and ensure temperatures don't drop too low at night. A foliar spray with phosphorus can provide quick relief while root uptake improves.

2. **Magnesium deficiency**: Add Epsom salts (1 tablespoon per gallon of water) as a soil drench or foliar spray. Cal-Mag supplements are also effective.

3. **Calcium deficiency**: Apply a Cal-Mag supplement according to manufacturer directions, and ensure pH is in the proper range for calcium uptake (6.2-7.0 in soil).

For Environmental Causes:

1. **Temperature fluctuations**: Maintain more consistent temperatures, particularly preventing drops below 65°F (18°C) at night. Consider heating options for outdoor grows during cold periods.

2. **Light stress**: Adjust light intensity or distance from plants, particularly for new growth. Consider light diffusion methods for intense lighting setups.

3. **pH imbalance**: Test and adjust the pH of your growing medium and nutrient solution. For soil, aim for 6.3-6.8; for hydroponic systems, 5.8-6.2 is ideal.

 Prevention: The Long-Term Strategy

Like pruning, addressing purple leaves requires consideration of potential positives and negatives. While some purpling may be harmless or even desirable, preventing pathological purpling requires a comprehensive approach:

1. **Regular monitoring** of plants, particularly examining new growth for early signs of stress.

2. **Preventative insect management** to reduce the risk of pathogen-transmitting pests like leafhoppers.

3. **Consistent environmental control**, especially temperature management during day/night transitions.

4. **Proper nutrient program** with particular attention to phosphorus, magnesium, and calcium levels.

5. **Regular pH testing** of both irrigation water and growing medium to prevent nutrient lockout situations.

6. **Strain selection** - If growing in conditions prone to temperature fluctuations, choose strains less known for purpling responses.

## The Organic Approach to Prevention and Treatment

For those of us committed to organic growing methods, several options exist for addressing purple leaf issues:

1. **Compost tea** can boost beneficial microorganisms that help prevent fungal infections and improve nutrient uptake.

2. **Rock phosphate** or bone meal can address phosphorus deficiencies organically, though they work more slowly than synthetic options.

3. **Dolomite lime** provides both calcium and magnesium while helping to buffer pH—addressing multiple potential causes simultaneously.

4. **Companion planting** with insect-repelling herbs like basil, rosemary, or marigolds can help reduce leafhopper populations naturally.

5. **Row covers or insect netting** for outdoor grows can physically exclude leafhoppers without chemicals.

 Balancing Response with Restraint

We've found success in pre-emptively addressing potential purpling by maintaining balanced growing conditions rather than reacting to symptoms after they appear. Whether to treat, when to do it, and to what extent varies depending on genetics and environment. Generally speaking, potential positives and negatives should be considered.

Remember that not all purpling indicates a problem—some strains naturally express these colors, particularly in cooler conditions. The key is distinguishing between genetic expression and stress response by looking for accompanying symptoms like stunted growth, curling, or other leaf abnormalities.

By understanding the various causes of purple leaves in new growth—from pathogens to nutrients to environment—we can make informed decisions about when to intervene and how aggressively to treat. In many cases, slight adjustments to growing conditions will resolve the issue without the need for dramatic interventions.

What has your experience been with purple leaves in cannabis? Have you found particular strategies that work well for your growing setup? We'd love to hear your experiences in the comments below.