Short answer. Water by pot weight, not by the calendar. Lift the pot just after watering and again each day; water when it feels about half as heavy. In soil that is usually every 2–4 days, giving about 20–30 % of the pot's volume to light runoff. In coco it is daily or more, to 10–20 % runoff. Seedlings get a small ring of 50–250 mL. Overwatering, not thirst, is the most common first-grow killer.
Roots need air as much as water. A container mix that never dries fills its pores with water, the roots run short of oxygen, and the plant droops and yellows as if it were hungry [1]. Every watering rule below is about giving roots both.
What do you need to water well?
Watering well needs a way to measure volume and judge weight. A kitchen scale makes it exact.
- [ ] A watering jug or can marked in litres (or a measuring jug)
- [ ] A kitchen or luggage scale, if pots are too heavy to judge by hand
- [ ] Pots with drainage holes (fabric pots drain and dry fastest) and saucers you can empty
- [ ] A free-draining medium: potting mix with 20–30 % perlite, or coco with 30 % perlite
- [ ] A pH meter and EC meter for the water going in and the runoff coming out (pH made simple)
- [ ] Water at room temperature, 18–22 °C (64–72 °F); a bucket left out overnight works
- [ ] A turkey baster or old syringe to empty saucers
- [ ] Your grow journal, to log volume, runoff and the dry weight
How do you water by pot weight, step by step?
Water by pot weight in eight steps. Do this every time for the first few weeks and it becomes second nature.
- Weigh or lift the pot dry. Before planting, note the weight of the pot filled with dry mix.
- Water to saturation once and wait. Water slowly until 10–20 % drains out, wait 30 minutes, empty the saucer and lift or weigh again. That is the "full" weight, near field capacity.
- Lift it every day at the same time. Most growers check just after lights-on.
- Water when it is about half the full weight. The exact point is a feel you learn; roughly halfway between full and dry. The top 2–3 cm (1 in) of soil will also be dry to the touch.
- Water slowly and evenly. Pour around the whole surface, not just at the stem, in two or three passes a minute apart, so water soaks in rather than running down the sides.
- Stop at the right runoff. In coco, stop at 10–20 % runoff (100–200 mL out for every litre in). In soil, stop at the first trickle, or about 5–10 %.
- Empty the saucer within 30 minutes. A pot standing in runoff wicks the salts back up and stays waterlogged.
- Log it. Write down the volume, runoff, and (weekly) runoff pH and EC. If the pot now dries much faster, the plant has grown and the next watering needs more volume. See runoff testing.
Seedlings are the exception. For the first 10–14 days, water only a small ring of 50–250 mL around the stem every 1–2 days, widening it as roots grow. A seedling in a big wet pot sits in airless mix, the classic setup for damping-off [6].
How much and how often, by medium and pot size?
Water volume and frequency depend on the medium, the pot size and the plant's size. The table gives starting points for a healthy plant that fills its pot under 600–900 µmol/m²/s; check against your own pot weights.
| Pot | Soil: volume per watering | Soil: how often | Coco: volume per watering | Coco: how often |
|---|---|---|---|---|
| Seedling (any pot) | 50–250 mL ring | Every 1–2 days | 50–250 mL ring | Daily |
| 1 L (¼ gal) | 0.2–0.3 L | Every 1–2 days | 0.25–0.35 L | 1–2x daily |
| 3.8 L (1 gal) | 0.8–1.2 L | Every 2–3 days | 0.9–1.3 L | Daily |
| 11 L (3 gal) | 2–3 L | Every 2–4 days | 2–3 L | 1–2x daily in flower |
| 15 L (4 gal) | 3–4 L | Every 3–4 days | 3–4.5 L | Daily to 2x daily |
| 19 L (5 gal) | 4–6 L | Every 3–5 days | 4–6 L | Daily to 2x daily |
| 26 L (7 gal) | 5–8 L | Every 4–6 days | 5–8 L | Daily |
A large flowering plant can move 1.5–2 L of water a day through its leaves, so frequency rises sharply from mid veg to mid flower. Coco should never dry out fully: it is fed like hydroponics, with nutrients in every watering (coco week by week). Soil can dry more between waterings (soil week by week).
Does pot size matter?
Pot size matters, but it is a trade-off, not a fixed rule. Across 65 studies, doubling rooting volume raised plant biomass by about 43 % on average [2], and the GenetixBay house position is that bigger pots give bigger yields within reason. Root volume isn't the whole story, though: the founder has grown a 4 oz+ (QP+) plant from a 16 oz cup by watering and feeding very often, which is the logic behind high-frequency fertigation. Small pots need more frequent care; big pots forgive a missed day but are easy to overwater while the plant is small.
| Pot size | Best for | Watch out |
|---|---|---|
| Under 7.5 L (2 gal) | Small plants, SOG, high-frequency feeding | Dries in hours in flower |
| 11–19 L (3–5 gal) | First grows, most tents | Overwatering seedlings |
| 26 L+ (7 gal+) | Long veg, big plants, living soil | Very slow dryback when small |
What about dryback and drought stress?
Dryback is how much the pot dries between waterings, and a moderate dryback is healthy because it pulls fresh air into the root zone. A controlled study found that a mild, controlled drought in late flower increased inflorescence dry weight and cannabinoid content [4], and the founder uses it: some cultivars thrive when allowed to go a bit dry late. Every cultivar responds differently, and the line between "a bit dry" and wilting is narrow. On a first grow, avoid both extremes; deliberate steering is in drain-to-waste and crop steering.
How will you know it's working?
You'll know your watering is right when the plant and the pot follow a steady, repeating rhythm:
- Pot weight: swings from heavy to about half that weight in 2–4 days (soil) or about 1 day (coco), and the cycle shortens as the plant grows.
- Leaves: held flat or slightly up in the light, perking up within an hour of lights-on, never limp and never swollen-droopy.
- Surface: the top 2–3 cm dries between waterings in soil; no green algae or constant dark wet patch.
- Runoff: clear, at the target %, with runoff EC close to (within about 0.5 mS/cm of) what you put in.
- No fungus gnats. These breed in constantly wet media [5].
- Roots (if you peek at the drain holes): white, branching, with fine hairs.
What mistakes do beginners make with watering?
Nearly all watering mistakes are too much, too often.
- Watering on a schedule ("every second day") instead of by weight.
- Flooding a seedling's whole pot. Water a small ring.
- Leaving pots standing in runoff.
- Treating coco like soil: plain-water days and letting it dry out fully.
- Watering only at the stem, so half the pot stays dry and roots don't fill it.
- Cold water straight from the tap in winter, below about 15 °C (59 °F).
- Mistaking overwatering for hunger and adding nutrients to a waterlogged pot. Lift the pot first (drooping: overwatering vs underwatering).
- Ignoring drainage: a pot without holes, or a dense, peaty mix with no perlite [1].
- Wilting the plant on purpose on a first grow after hearing about drought stress.
Root rot and wilts love a wet, airless, warm root zone [7]; good drainage and dryback are prevention.
Evidence
- Container media physics and air-filled porosity: Established in horticultural texts [1].
- Bigger pots, bigger plants: Established across crops [2]; pot size as a trade-off against feeding frequency is the house position, backed by the founder's grower data.
- Pour-through runoff checks: Established extension method [3].
- Mild late drought raising dry weight and cannabinoids: Supported by one cannabis study [4]; house-endorsed, cultivar-dependent.
- Wet media and gnats, damping-off and root disease: Established [5][6][7].
- Water volumes and frequencies in the table: Grower practice.
