We like to judge the hydration of the soil on a six point scale:
Parched- Zero moisture, soil is dusty and takes effort to get it hydrated
Dry- Soil is dry halfway down the pot
Almost Dry- Soil is dry a quarter of the way down the pot, below that is cool and somewhat damp
Damp- Soil is very cool, and moisture is clearly present. Clumps when squeezed but falls apart easily
Watered- Soil feels wet all the way through and recently watered. Clumps when squeezed and holds its shape
Saturated- Soil is soggy and almost muddy, excess water is visible or pools just below surface. When squeezed the soil releases a lot of water
Almost all of the plants you’ll keep at home will be fine with having their soil checked once a week, and won’t need to be watered more than once a week (with a few exceptions during the summer months)). When we refer to a plant’s preferred watering, we’re usually talking about three things: how dry we let the soil get before watering each week, how wet we want the soil to be after watering, and how that changes throughout the year.
The most common guidance you’ll see is ‘moderate watering,’ and it applies to a wide range of tropical plants and some succulents. With moderate watering you let the plant get almost dry (dry in the winter) and then water it fully (or until damp in the winter). The cycle should be weekly, with the plant reaching the appropriate dryness every week, although in the winter time you may find yourself skipping a week every now and then.
Plants that need just a light watering tend to be more drought tolerant or slower growing. You’ll typically wait until they’re dry and only get them damp, or almost but not fully watered (although during the growing season plants at the higher end of their light tolerance may be fully watered). You should be extra careful during the winter, or if the plant is at the lower end of its light tolerance. You’ll still check these plants every week, but it won’t be uncommon for you to skip a week of watering. Take particular care with these plants after you’ve repotted them, when they’re more prone to overwatering.
While most plants enjoy the wet/dry cycle, there are some that are happiest staying damp at all times. Note that even these plants can suffer root rot if you keep the soil constantly watered, and usually they’re fine if they get almost dry. While these plants can still be maintained by checking them once a week, some may find it more beneficial to check twice a week or use a self-watering pot.
Many plants that require light watering, and some that require moderate watering, are also drought tolerant. Drought tolerant plants can often be dry or even parched for varying amounts of time without suffering. While some plants like cacti like to get parched in between waterings, just because a plant is drought tolerant doesn’t mean it enjoys drying out that much. Pothos can be quite drought tolerant, but if they get parched between every watering their growth will be sluggish and there’s a higher chance older leaves will drop sooner.
Some plants never want to dry out completely, or have a very difficult time recovering from drought. Their leaves are often delicate and/or prone to drying out easily. While many can get almost dry, careful attention should be paid that they don’t dry out completely.
Some plants are very aggressive growers, and have very active root systems that fill out their pot quickly (like Spider Plants) or maybe tend to be kept very pot-bound (like Peace Lilies). These plants might seem particularly “thirsty” and dry out quickly even after thorough waterings. Take care during the high growing season or very warm times of year.
Some plants have delicate leaves that are prone to trying out, or otherwise prefer very humid air. They do well away from dry, blowing air, and enjoy being misted or kept close to other plants to raise the relative humidity. Some people may use pebble trays filled with water or humidifiers to keep leaves from drying out.
Watering plants is dynamic. While a few might be fine with the same amount of water every week, year round, most plants will need to have their watering adjusted based on a number of factors, including the current level of hydration, root density, light intensity, time of year, and soil quality. When any of these things change, expect your watering to change as well.
Level of Hydration
Most people water on a schedule, checking their plants once a week, and every week the dryness of the soil may be a little different based on all of the qualities listed above. Some weeks a plant will need only a little moisture to last it the upcoming week, while other times an extra thorough watering will be needed.
The more soil there is, the more a plant is able to retain moisture (for better or worse). When a plant is first repotted the ratio of soil to roots is high, and when thoroughly watered the soil can stay hydrated quite easily. It may take a much longer time for the plant to use all of that moisture. After the plant grows and becomes pot-bound or root-bound, meaning the roots fill the pot fully and wrap around the side, the ratio of soil to roots is very low (often soil has eroded or been spilled out over time as well). Even when thoroughly watered pot-bound plants may use up the moisture in a short amount of time.
Each plant has its own range of light that it will tolerate. If it’s situated where it receives the upper range of light it will have more active growth, dry out quicker, and require more/more frequent hydration.
Time of Year
During the winter our homes receive significantly fewer hours of daylight, especially the further north you are. Our homes also tend to be cooler (especially by windows) and the natural growth cycle of most houseplants slows down. The result is a plant that needs less water, often less frequently.
Soils that are well aerated and drain quickly, like cactus and succulent soil, will hold less water than typical potting soil and dry out faster. Conversely, potting mixes that are high in peat moss or compost will retain more moisture for longer.