Optimal DLI for High PPFD Day Neutral Plants

Temperature and humidity are two of the most critical factors in creating the right growing environment, especially in high-PPFD conditions. It’s crucial to adjust these variables according to the plant’s growth stage, ensuring optimal health and yield.

Here’s a breakdown of the ideal temperature and humidity ranges for each phase of your plant’s development:

1. Germination (Time to Sprout: 2 to 7 Days on average):

• Temperature: 78 to 82°F (25 to 28°C)
• Humidity: 90% or higher
• Why: During germination, seeds typically take 2 to 7 days to sprout, depending on environmental conditions. High humidity is critical during this phase, helping to keep the seed’s environment moist and ensuring that roots develop properly. The temperature should be warm enough to support metabolism but not so high as to stress the seed.

2. Seedling Stage (Day 1):

• Temperature: 80 to 85°F (26 to 29°C)
• Humidity: 70% to 75%
• Why: Once the seedling breaks through the soil, day 1 of the plant’s life officially begins. During this stage, maintaining higher humidity helps keep the young plant hydrated while avoiding the need for frequent watering. The temperature should stay in the low to mid-80s to encourage rapid root and leaf growth.

3. Vegetative Stage (From Seedling through Pre-Flower Stretch):

• Temperature: 80 to 85°F (26 to 29°C)
• Humidity: 65% to 75%
• Why: As the plant enters its vegetative stage, it focuses on building strong stems and leaves. A balance of slightly lower humidity and consistent temperatures will keep the plant growing rapidly while preventing mold or mildew from forming. This is the phase where you want to encourage robust structural development.

4. Start of Flower (Weeks 1 to 3 of Flowering):

• Temperature: 75 to 77°F (24 to 25°C)
• Humidity: 55% to 65%
• Why: When transitioning to flowering, lower humidity helps prevent mold or mildew from affecting the buds. Keep the temperature slightly lower than during the vegetative stage to focus the plant’s energy on flower production. While the humidity decreases, you do not want to lower the temperature too much during this stage.

5. Late Flowering and Maturation (Final 3 to 5 Weeks):

• Temperature: 68 to 72°F (20 to 22°C)
• Humidity: 40% to 55%
• Why: As the plant matures, lowering the temperature helps to preserve the quality and density of the flowers. Reducing humidity at this stage minimizes the risk of bud rot or mold. At this point, the flowers are dense, so keeping air moisture low is essential for avoiding potential issues.
• Note: In warmer climates or during the summer, maintaining lower temperatures can be challenging in the final weeks of flowering. To adapt, many growers reduce the DLI (Daily Light Integral) to around 40-45 mol/m²/day during the last 1-3 weeks. This helps manage heat stress while still allowing the plant to finish maturing, especially in environments where temperature control is limited.

Avoiding Drastic Temperature Drops:

• It is important to avoid allowing your nighttime temperatures to drop more than 5°F below your daytime temperatures. Large temperature swings can shock the plant, slowing growth and increasing the risk of mold. Keeping a steady climate ensures a smoother transition between light and dark cycles and promotes better overall plant health.

By following these temperature and humidity ranges throughout the plant’s life, you create a balanced environment that supports healthy growth and maximum yield potential at each stage.

Daily Light Integral (DLI) is one of the most important factors to consider when growing high-PPFD day-neutral plants. It refers to the total amount of light a plant receives over the course of a day, measured in moles of light per square meter per day (mol/m²/day). Proper DLI management helps you avoid overstressing your plants while ensuring they receive the optimal amount of light for healthy growth.

Here’s how to manage DLI effectively throughout each stage of growth:

1. Germination:

• DLI: Minimal light, just enough to guide seedlings out of the soil or starter cube, best practices is to use a germ tray with a dome, single led diode, and starter cubes, something like Root Riot, or Rapid Rooters.
• Why: During the germination phase, seeds don’t need much light. Their primary energy comes from internal reserves until they break the soil. The goal is to provide just enough light to help guide them upward toward the surface. Once they sprout and break the soil, start at 13 DLI to support their early development without overstressing them.

2. Seedling Stage (Day 1 to Day 15):

• DLI: Start at 13 mol/m²/day and gradually increase to 16 mol/m²/day by the end of week 1, and potentially up to 20 mol/m²/day by week 2.
• Why: Once the seedling has broken the soil, day 1 of the plant’s life officially begins. At this stage, start with 13 DLI and gradually increase the light as the seedling develops. If the plant is healthy and responding well, increase the DLI to around 20 mol/m²/day by the end of week 2. This careful progression helps the plant adjust without overwhelming it.

3. Vegetative Stage (Day 15 through Pre-Flower Stretch):

• DLI: Start at 20 mol/m²/day, and if all looks well, gradually increase by 3 to 4 mol/m²/day every other day. By the end of week 4, aim for 40 to 45 mol/m²/day.
• Why: By day 15, if the plant is growing well, increase the DLI to 20. Over the next two weeks, continue increasing the DLI by 3 to 4 mol/m²/day every other day, or even daily if the plant is stretching faster than wanted, some will begin low stress training around this time. By the end of week 4, you should reach around 40 to 45 DLI, setting the stage for vigorous growth and the potential to push into the stretch. Most strains can handle DLI levels up to 50 to 55 without issues, provided there is good air exchange. If everything else is dialed in, some strains can tolerate up to 60 DLI, but this is where CO₂ management becomes critical.

4. Flowering Stage (Weeks 1 to 3 of Flowering):

• DLI: 50 to 55 mol/m²/day, depending on strain.
• Why: As the plant transitions into flowering, maintaining a high DLI supports flower production. Many strains respond well to a DLI of 50 to 55. For plants that can tolerate more light, you may push up to 60 DLI, but ensure that your ambient CO₂ levels remain stable (above 420 to 430 ppm). If CO₂ levels drop, especially during high light conditions, growth can slow significantly.

5. Late Flowering and Maturation (Final 3 to 5 Weeks):

• DLI: 40 to 55 mol/m²/day, but beyond 60, elevate CO₂ levels.
• Why: In the final stages of flowering, higher DLI levels help the plant finish maturing. If you push DLI beyond 60, you’ll need to increase CO₂ levels to 1,100 to 1,300 ppm to avoid stunted growth. Keep an eye on CO₂ levels to ensure that they don’t dip below 420 to 430 ppm. Without sufficient CO₂, pushing higher light intensities will not result in increased growth and may stress the plant. If you are having trouble keeping temps in check towards the end, reduce the intensity down to 40-45 DLI the final week or two.

Managing Light Intensity and CO₂:

• As DLI increases, so do the plant’s demands for CO₂, water, and nutrients. If you push your lights to higher DLI levels, make sure you are adjusting CO₂ levels and keeping them in the optimal range (above 420 ppm for standard setups, and 1,100 to 1,300 ppm for setups exceeding 60 DLI).
• Without proper CO₂ management, increasing light intensity beyond the plant’s capacity will result in diminishing returns and can lead to light stress. Keep a close eye on environmental conditions, and only increase DLI when the plant is responding positively.

Avoiding Light Stress:

• Monitor plant behavior as you increase DLI. If you notice signs of light stress (leaf curling, discoloration, or stunted growth), it may be necessary to dial back the light intensity or make adjustments to your CO₂ and nutrient levels. Light stress can set back growth, so it’s better to increase DLI gradually than to push the plant too hard too fast.

By carefully managing DLI throughout the plant’s life cycle, you’ll create a balanced environment where light, CO₂, and other factors work in harmony to produce healthy, high-yielding plants. Keep in mind that higher light levels require proper support from the rest of the environment to ensure successful growth.

Photosynthetically Active Radiation (PAR) is a critical measurement for growers to understand because it directly relates to the amount of light your plants can use for photosynthesis. PAR measures the portion of the light spectrum (400-700nm) that plants utilize for growth. While many growers focus on light intensity (PPFD). It’s crucial to translate that intensity into DLI (Daily Light Integral) to truly understand how much light your plants are receiving over the course of a day.

Why PAR is Important for DLI:

• DLI is calculated by combining the PPFD (Photosynthetic Photon Flux Density), which measures the amount of PAR hitting your plants at any given moment with the duration of light exposure in hours. For day-neutral plants, which typically receive between 18 to 24 hours of light, the DLI can vary significantly depending on how many hours of light they receive each day.

To achieve the right DLI, you need to ensure your PAR levels (measured by PPFD) are appropriate for the light duration. In general:

• Fewer hours of light (closer to 18 hours): Requires higher PPFD levels to compensate for the shorter duration.
• More hours of light (closer to 24 hours): Can use lower PPFD levels since the plants are exposed to light for longer periods. This allows them to accumulate enough total light.

DLI Calculation Formula:

• DLI = (PPFD × Light Hours × 3,600) ÷ 1,000,000

Where:

• PPFD: is measured in micromoles per square meter per second (µmol/m²/s)
• Light hours: is the number of hours the lights are on per day

By plugging in your PPFD value and the number of hours of light, you can determine the DLI your plants are receiving.

DLI Ranges Based on Light Hours:

For day-neutral plants running on an 18 to 24-hour light cycle, the ideal DLI range depends on the balance between light intensity (PPFD) and duration. Here are general guidelines for DLI based on light hours:

• 18-hour light cycle:
  • To achieve 60 DLI: You’ll need a PPFD of around 925 µmol/m²/s.
  • To achieve 45 DLI: You’ll need a PPFD of around 700 µmol/m²/s.
• 20-hour light cycle:
  • To achieve 60 DLI: You’ll need a PPFD of around 800 µmol/m²/s.
  • To achieve 45 DLI: You’ll need a PPFD of around 600 µmol/m²/s.
• 24-hour light cycle:
  • To achieve 60 DLI: You’ll need a PPFD of around 700 µmol/m²/s.
  • To achieve 45 DLI: You’ll need a PPFD of around 520 µmol/m²/s.

Using the DLI Calculator:

To make DLI calculations even easier, use our DLI Calculator to instantly check if your grow setup is optimized. Simply input your PPFD and light hours for accurate results tailored to your setup.

By understanding and managing PAR, you can ensure that your plants receive the correct DLI, maximizing their growth potential while avoiding overstressing them with either too much or too little light.

Vapor Pressure Deficit (VPD) is a concept used by some growers to fine-tune the balance between temperature and humidity in their grow environment. While VPD can be a helpful tool, many growers tend to overcomplicate its importance, especially if they’re already managing temperature and humidity within optimal ranges.

What is VPD?

VPD is the difference between the amount of moisture in the air and how much moisture the air can hold when fully saturated (relative humidity). It’s a measure of the “drying power” of the air, which affects how much water plants can transpire (lose through their leaves). The goal of VPD management is to optimize transpiration, which helps plants take up water and nutrients more effectively, promoting better growth.

Do You Need to Worry About VPD?

For most growers, chasing VPD charts is unnecessary. If you’re already maintaining optimal temperature and humidity ranges for your plants, they will be able to transpire efficiently without you needing to calculate and adjust VPD. In fact, trying to target a specific VPD can lead to unnecessary stress, both for you and the plants.

By simply ensuring your grow environment is in the correct temperature and humidity range for each stage of growth (as covered in previous sections), you’ll naturally achieve a healthy transpiration rate and avoid potential issues like mold, mildew, or plant stress.

When Should You Focus on VPD?

• You’re running a sealed grow room where controlling every aspect of the environment (temperature, humidity, CO₂) is critical.
• You’re aiming for maximum efficiency in nutrient uptake, and you want to fine-tune the environment to get every bit of potential out of your grow.
• You can’t run ideal temperatures for your plants. In cases where the temperature is higher or lower than recommended, using VPD charts to adjust your humidity may help optimize plant health.

Simplified Take on VPD:

• If your temperature and humidity are in the right range (as outlined earlier for each growth stage), you don’t need to worry much about VPD. Simply focus on maintaining those ranges.
• VPD becomes useful only if you can’t hit the optimal temperature range and need to balance that with humidity adjustments. But for most growers, following basic environmental guidelines will lead to great results without needing to chase VPD charts.

While red light is an essential part of the light spectrum for plant growth, particularly in promoting flowering and fruiting, over-reliance on it, especially in high PPFD environments, can do more harm than good. Many grow light manufacturers focus heavily on promoting red-heavy spectrums, but this approach can lead to a range of issues if not managed carefully.

Many manufacturers are hesitant to promote pushing higher PPFD levels, not because it cannot be done successfully in smaller indoor environments, but because most lights designed for these setups feature extremely red-heavy spectrums. These spectrums are cheaper to produce, not better, and are often more suited for greenhouse growing or leafy vegetables like lettuce, which tend to benefit from red-heavy spectrums and extra far-red. However, this is not the case for most other plants. In indoor settings, a red-heavy spectrum can cause negative results at high intensities.

The Problem with Far Red Light in Indoor Growing:

• Stretching: When far-red light exceeds 20 µmol during the light-on cycle, it can cause unwanted stretching. This is especially problematic in the vegetative stage, as it weakens the plant’s structure, leading to elongated stems and inefficient growth.
• Reduced Yields: In high PPFD environments, just 60 µmol of far-red light can reduce yields by up to 30%. This is because far-red light signals the plant to stretch, which diverts energy from bud production, leading to weaker and less dense flowers.

If you’re manipulating dark hours, consider using supplemental far-red bars for short periods, and make sure they can be completely turned off when the main light comes on.

Why Going Over 600 µmol of Red Light is Problematic:

Regular red light (660nm) is crucial for flowering, but it must be used in moderation. Ideally, the amount of red light should not exceed 40% of the total light output. Exceeding this threshold can lead to several issues:

• Overdependence on Red Light: Too much red light, especially above 40%, throws off the balance needed for healthy plant growth. It can limit the effectiveness of other critical wavelengths like blue and green, which are necessary for proper leaf development and overall plant structure.
• Spectrum Imbalance: While red light enhances flowering, relying too heavily on it without enough blue or green can result in leggy growth and reduced overall yield. This is why balancing the spectrum is so important for indoor growing setups.

The Ideal Light Spectrum Balance for Indoor Growing:

Indoor grow setups require a balanced spectrum that avoids an over-reliance on red and far-red light. Here’s what you should aim for in a well-balanced spectrum:

• Blue Spectrum (400-500 nm): Accounting for 15-20% of a light’s output, the blue spectrum lays the groundwork for healthy leaf and stem development. It’s not just about preventing stretching. It’s about building the robust framework that will support the plant through its life cycle.
• Green to Yellow Spectrum (500-600 nm): This spectrum segment should form 35-45% of your light’s output. It plays a critical role in driving photosynthesis throughout the entire plant structure, ensuring that even the lower leaves contribute to the plant’s energy production.
• Red Spectrum (600-700 nm): Comprising another 35-40% of the light output, this spectrum supports flowering and fruiting, directly influencing yield quality and quantity.

Proper CO₂ management is essential for ensuring optimal growth, especially when growing under high DLI conditions. While some sources suggest that passive ventilation is enough to supply the CO₂ plants need, this approach can fall short in high-intensity environments. CO₂ plays a critical role in the photosynthesis process, and as DLI increases, so do the plant’s CO₂ requirements. Without adequate CO₂, plants can’t efficiently use the light they receive, leading to slower growth and reduced yields.

CO₂ Levels Below 60 DLI:

For most home growers operating under 60 DLI, maintaining ambient CO₂ levels typically found in homes and outdoor environments is sufficient. Here’s a breakdown of what to expect:

• Average Home CO₂ Levels: Most homes have CO₂ concentrations between 600 and 900 ppm. This range is generally adequate for grows operating at lower to moderate DLI levels (under 60). Passive ventilation and regular air exchange through exhaust fans are usually enough to keep CO₂ levels stable in this range. As long as other environmental factors like temperature and humidity are well-controlled, plants can thrive without additional CO₂ supplementation.
• Average Outdoor CO₂ Levels: The average atmospheric CO₂ concentration outdoors is around 420 to 430 ppm. For many home growers using exhaust systems, this will be the baseline level of CO₂ inside their grow rooms when there is no active CO₂ supplementation. While this is enough for moderate DLI levels, it limits the plant’s ability to fully utilize light as the DLI increases.

CO₂ Levels Beyond 60 DLI:

When pushing DLI beyond 60, maintaining adequate CO₂ levels becomes critical for maximizing growth. Plants require more CO₂ to keep up with the increased light exposure and avoid bottlenecks in photosynthesis.

• Elevated CO₂ for High DLI (Beyond 60): When operating with high DLI levels (60 and above), plants can significantly benefit from increased CO₂ levels, typically between 1,100 and 1,300 ppm. This elevated CO₂ concentration allows the plants to process the extra light efficiently, promoting faster growth and higher yields. If you’re using high-intensity lights and pushing for maximum performance, elevated CO₂ levels are essential.
• Without Elevated CO₂: If you’re running over 60 DLI but don’t have a way to increase CO₂ levels, the plant will struggle to process the extra light. Growth will slow down, and you’ll see diminishing returns from the high light intensity. Without proper CO₂ levels, the plant can experience stress and may not reach its full potential in terms of growth and yield.

CO₂ Monitoring and Adjustment:

• Passive Ventilation (for under 60 DLI): For most home setups with DLI under 60, passive ventilation through exhaust fans should be enough to maintain healthy CO₂ levels. Just ensure that air exchange is consistent and that the grow room is not sealed, allowing fresh air to circulate regularly.
• Sealed Rooms with CO₂ Supplementation (for beyond 60 DLI): For growers aiming to push beyond 60 DLI, sealed rooms with controlled CO₂ supplementation are the best approach. Using CO₂ tanks or generators with a CO₂ controller ensures that the concentration stays in the optimal 1,100 to 1,300 ppm range. In this setup, it’s also necessary to manage temperature and humidity closely, as CO₂ supplementation is most effective when the environment is well-balanced.
• Avoid CO₂ Dips: At any DLI level, maintaining stable CO₂ levels is important. If CO₂ levels dip below 420 to 430 ppm, growth will slow down significantly, regardless of light intensity. This is true even at lower DLI levels, making regular ventilation or supplementation essential for consistent growth.

The Balance of Light and CO₂:

It’s important to understand that light and CO₂ work together to drive photosynthesis. Simply increasing light intensity without managing CO₂ levels will not result in better growth. As DLI increases, so do the plant’s demands for CO₂, water, and nutrients. If these factors are out of balance, you won’t get the full benefit of your lighting setup, and the plant may suffer from stress or inefficient growth.

By managing CO₂ effectively, you can ensure that your plants are getting the most out of the light you’re providing. For most home setups with DLI levels under 60, passive ventilation is enough to maintain CO₂ levels. However, for higher DLI environments, supplementing CO₂ becomes a key factor in driving higher yields and faster growth.