Weak, floppy stems that can't hold the plant up

Likely causes, most likely first

Ranked for this sign on its own. Your stage, medium and readings change the order, so run the checker for your plant.

1. PossibleNot enough light

   LightLow urgencyRead: Stretchy, leggy cannabis: a differential

   Fix: Bring PPFD to stage targets: seedlings 100-300, veg 300-700, flower 800-1,200 µmol/m²/s. Yield rose linearly with PPFD from 120 to 1,800 in a controlled trial. Lower canopy buds under ~300-400 µmol/m²/s make larf: thin or lollipop the shaded bottom, or train flat.

   Recovery: New internodes shorten within a week; denser buds on the following weeks of growth.

   How to confirm it

   • Measure PPFD at the canopy top. Use a quantum (PAR) meter, or a calibrated phone app as a rough guide, at the top leaves in a 5-9 point grid. Compare with the stage target: seedlings 100-300, veg 300-700, flower 800-1,200 µmol/m²/s at ambient CO2. Bleaching and taco at the top above ~1,100 point to light; stretch below ~300 in veg points to too little.

2. Less likelyGeneral underfeeding (whole feed too weak)

   NutrientDeal with it this weekRead: Building a cannabis feed by stage: ppm targets per element and how to hit them

   Fix: Raise the whole feed in steps of about 0.2-0.3 mS/cm toward the stage target (veg 1.2-1.8, flower 1.6-2.2 mS/cm in coco and hydro). Stop any 'flush': plain water for weeks starves the plant, and pre-harvest flushing is a myth that costs yield.

   Recovery: Colour deepens and growth speeds up within a week; runoff EC tracks input.

   How to confirm it

   • Measure runoff EC against input EC. Water to 10-20% runoff, collect the runoff in a clean cup and measure EC of both with a calibrated meter (1.413 mS/cm standard). Runoff more than ~0.5 mS/cm above input means salt build-up; well below input means the plant is hungry or you are over-leaching. In hydro, compare reservoir EC with the EC you mixed.
   • Measure input pH and root-zone (runoff or reservoir) pH. Calibrate the meter with fresh pH 7 and pH 4 buffers, then measure the mixed feed and the runoff (containers) or reservoir (hydro). Soil and living soil should sit at 6.0-7.0, coco, rockwool and peat mixes at 5.5-6.5, hydro at 5.5-6.2. Track 3+ readings; one reading is not a trend.

3. Less likelyToo much nitrogen

   Too much of somethingLow urgencyRead: Nitrogen (N) in cannabis: nitrate vs ammonium, deficiency, toxicity and targets

   Fix: Cut feed N by 20-30% (or the whole feed EC by ~0.3-0.5 mS/cm); for a heavy overdose in containers, run a root-zone flush with pH-adjusted, quarter-strength feed to 20-30% runoff. In flower, taper N while keeping P, K, Ca, Mg, S and micros fed.

   Recovery: New leaves come in lighter green and flat within 5-7 days; clawed leaves relax partly.

   How to confirm it

   • Measure runoff EC against input EC. Water to 10-20% runoff, collect the runoff in a clean cup and measure EC of both with a calibrated meter (1.413 mS/cm standard). Runoff more than ~0.5 mS/cm above input means salt build-up; well below input means the plant is hungry or you are over-leaching. In hydro, compare reservoir EC with the EC you mixed.
   • Lift the pot and check moisture at depth. Lift the pot right after a full watering to learn its wet weight, then lift before each watering. Water when it is clearly lighter (about half the added water gone). Push a finger or a moisture probe 5 cm (2 in) down. Heavy for 3-4+ days with droop: overwatering. Very light with limp papery leaves: thirst.

4. Less likelyPotassium deficiency

   NutrientDeal with it this weekRead: Potassium (K) in cannabis: role, deficiency, excess and K-Ca-Mg balance

   Fix: Check runoff EC first: salt burn looks the same. If K is genuinely short, feed about 175-250 mg/L K (potassium sulfate, potassium nitrate or MKP) at a normal EC. A flowering DWC trial saw no yield response anywhere from 60 to 340 mg/L, so more is not better. Cut heavy Cal-Mag or sodium-rich water that competes with K.

   Recovery: Margin scorch stops advancing up the plant within 7 days; new leaves open with clean margins.

   How to confirm it

   • Measure runoff EC against input EC. Water to 10-20% runoff, collect the runoff in a clean cup and measure EC of both with a calibrated meter (1.413 mS/cm standard). Runoff more than ~0.5 mS/cm above input means salt build-up; well below input means the plant is hungry or you are over-leaching. In hydro, compare reservoir EC with the EC you mixed.
   • Get a water report (or test your water). Ask your water utility for the annual report or test well water: alkalinity (mg/L CaCO3), pH, EC, Ca, Mg, Na, Cl, B, and whether it uses chlorine or chloramine. Alkalinity above ~100-150 mg/L as CaCO3 pushes root-zone pH up; sodium above ~50 mg/L and chloride above ~70 mg/L are a problem; softened water is high in sodium.
   • Leaf tissue analysis. Send 20-30 most-recently-matured leaves (no petioles) from affected plants, and separately from healthy ones, to an agricultural lab. Compare with cannabis and hemp sufficiency ranges (for example N 3.3-4.8%, Mg 0.40-0.81%, K 1.8-2.9% dry weight in recent mature leaves). Results take 1-2 weeks; use it to confirm a chronic problem, not an emergency.
   • Watch new growth for 5-10 days after one change. Make one correction at a time, mark the top node with a twist tie and photograph the same leaves every 2-3 days. Damaged leaves never recover; judge only new growth and whether the symptom stops spreading. Mobile-nutrient symptoms stop climbing within 5-7 days; immobile ones show in new leaves within 7-10 days.

Look for

Bending or leaning stems, internode length and the light distance in one photo..