During flowering, most cannabis plants perform best with nutrient solution PPM in the range of 1,000 to 1,500, measured on the 500 scale. That range shifts depending on which week of flower you’re in, what growing medium you use, and whether you’re growing photoperiod or autoflowering varieties. The numbers aren’t static: they ramp up through mid-flower, then drop sharply in the final weeks before harvest.
Week-by-Week PPM Targets by Growing Medium
Flowering isn’t one phase. It’s at least three: early flower (weeks 1–2), mid-flower (weeks 3–6), and late flower (weeks 7–8+). Your plants’ nutritional demands peak during mid-flower when buds are actively putting on mass, then taper as you approach harvest. All of the following values assume a starting water baseline of 0 PPM (reverse osmosis filtered) and use the 500 scale.
Soil starts at around 1,150 PPM in week one, climbs to 1,300 by week two, and peaks at 1,400 PPM through weeks four to six. In week seven you drop to roughly 1,050, and the final week before harvest falls dramatically to about 140 PPM as you flush.
Coco coir runs higher overall because coco doesn’t hold nutrients the way soil does. Expect to start around 1,200 PPM, build to 1,400 by week three, and peak at 1,500 to 1,650 PPM during weeks four through six. Late flower drops to 1,200 in week seven and around 550 in week eight.
Hydroponics generally requires the lowest concentrations of the three, since roots have direct contact with the nutrient solution. Begin around 1,050 PPM, climb to 1,100–1,200 through mid-flower, then reduce to 900 in week seven and 600 in week eight.
If your flowering cycle runs longer than eight weeks, repeat the week-six values for any additional weeks, then follow the normal week-seven and week-eight taper at the end.
Your Starting Water Matters
These targets assume you’re mixing nutrients into water that reads close to 0 PPM. If you’re using tap water, you need to know its baseline before adding anything. Tap water in the 100–150 PPM range is generally manageable. At 200–300 PPM, you may need to cut back on calcium and magnesium supplements since your water already contains dissolved minerals. Above 300 PPM, consider switching to reverse osmosis (RO) water to avoid stacking unknown minerals on top of your nutrient mix.
Whatever your tap water reads, factor that number into your total. If your water starts at 200 PPM and your mid-flower target is 1,400, you only need to add 1,200 PPM worth of nutrients.
Autoflowers Need Less
Autoflowering plants are smaller, have shorter life cycles, and require less nutrition overall. A safe approach is to start at roughly 50–75% of the photoperiod values listed above. Autoflowers are more sensitive to overfeeding, so it’s better to start low and increase gradually based on how the plant responds rather than hitting full-strength targets from day one.
Why pH Determines Whether PPM Even Matters
You can dial in perfect PPM numbers and still starve your plants if the pH is off. Nutrient availability is directly tied to the pH of your root zone. In soilless media like coco or peat, aim for a pH of 5.8 to 6.2. In soil, shift that range up slightly to 6.0 to 6.5, with field soils tolerating up to about 6.7.
When pH drops below 5.0, micronutrients like iron and manganese become overly available and can reach toxic levels. When pH climbs above 7.5, micronutrients get locked out entirely, even if they’re present in the solution. Research from North Carolina State University found that cannabis can tolerate a surprisingly wide pH range of 5.0 to 7.0 without showing obvious deficiency symptoms, but staying within 5.8 to 6.2 gives you the most margin for correction before problems appear.
Signs Your PPM Is Too High
Nutrient burn is the most common result of running PPM too high. The earliest sign is brown, crispy leaf tips. If you miss that warning, symptoms progress inward from the edges: leaves turn an unusually dark green, begin curling downward into a claw shape, and overall growth slows. Unlike light burn (which hits the canopy closest to the light), nutrient burn typically appears first on lower, older leaves and works its way up.
If you spot these symptoms, reduce your nutrient concentration by 10–20%, flush with plain pH-adjusted water, and watch for improvement over the next few days.
Signs Your PPM Is Too Low
Underfeeding during flowering starves the plant of the nutrients it needs most: phosphorus, potassium, and magnesium.
- Phosphorus deficiency shows up as purple stems, bluish-green leaves, and dark copper or purple blotches on lower foliage. Buds stay small, and flowering can stall.
- Potassium deficiency causes rusty brown edges on younger leaves, weak stems, and noticeably slowed bud development.
- Magnesium deficiency creates yellowing between the veins of older leaves, with irregular rust-colored spots on margins and tips. Affected leaves curl upward and eventually drop.
Any of these showing up mid-flower is a signal to bump your PPM up gradually, checking runoff to make sure nutrients are actually reaching the roots.
Using Runoff PPM to Fine-Tune
Measuring what goes in is only half the picture. Checking the PPM of your runoff water tells you what’s actually happening in the root zone. If your inflow is 1,200 PPM and your runoff reads significantly higher, salts are building up in the medium and your plant isn’t absorbing what you’re providing. That’s a sign to flush or reduce feed strength. If runoff PPM is noticeably lower than inflow, the plant is consuming heavily and may benefit from a slight increase.
A common practical approach is feeding around 1,000–1,200 PPM going in and checking that runoff comes back in the 800–1,000 range. The exact numbers will vary with your medium, but the relationship between inflow and runoff is what matters most.
The Pre-Harvest Flush
In the final week or two before harvest, most growers flush with plain, pH-adjusted water to clear residual nutrients from the medium and the plant. The goal is to bring runoff PPM down to around 50, or at least close to the PPM of the plain water you’re flushing with. Early flush runoff can read as high as 1,300 PPM, so it takes multiple waterings to pull those numbers down. In soil, the final week target drops to roughly 140 PPM. In hydro and coco, growers typically bring it down to 550–600 PPM before cutting to plain water entirely.
High-Intensity Lighting Changes the Equation
Plants grown under powerful LED or HPS setups with light levels above 1,000 PPFD (a measure of light intensity at the canopy) photosynthesize faster and consume more nutrients. Commercial growers running up to 1,800 PPFD during flower often supplement CO2 to 1,000–1,200 ppm in the grow room air, which further accelerates metabolism. Under these conditions, plants can handle PPM levels at the higher end of the ranges listed above, or slightly beyond. If you’re growing under modest lighting without CO2 supplementation, stick to the mid-range of your targets. Pushing PPM high without the light and CO2 to match just leads to nutrient burn.