Every guide says 'water when the soil is dry' — how do I judge that reliably?

Surface dryness tells you almost nothing. The top two centimetres of any pot dry out within a day or two of watering while the bottom third can still be saturated, which is why finger-in-the-soil advice sends so many plants to root rot.

Three methods actually work. The first is weight: lift the pot immediately after a thorough watering and again a week later. The difference is unmistakable once you have felt it twice, and it integrates the whole root ball rather than one spot. This is the method most professional growers actually use.

The second is a wooden dowel or a bamboo skewer pushed to within a centimetre of the bottom, left for thirty seconds and pulled out. Damp soil clings to it and darkens the wood; dry soil comes off clean. It costs nothing and reads the depth that matters.

The third is a moisture meter, with a caveat: cheap analogue meters measure electrical conductivity, not water, so they read high in freshly fertilised soil and low in peat that has dried and gone hydrophobic. Use one for relative comparisons on the same pot over time, not as an absolute number.

What you are aiming for varies by plant. Most aroids, ficus and calathea want the top third dry. Succulents, cacti and sansevieria want the entire root ball dry and then a few more days. Ferns, calathea and carnivorous plants want to stay evenly damp and never dry through.

Season changes everything. The same plant in the same pot may need water every five days in July and every three weeks in January, because growth has stopped and evaporation has slowed. A fixed weekly schedule is the single most common cause of houseplant death.