Plants should never be watered on a schedule, because their water needs are different depending on the type of plant you have, the type of potting mix you use, and many environmental or seasonal factors. Simply pay attention to your plant’s changing needs and adjust accordingly.
Finding Balance
You want to find a balance between how dry you let the potting mix get and for how long. The ideal state for a houseplant are leaves that are plump and full of water, and roots that are touching a moist substrate with adequate airflow (air flow to the roots is directly related to the type of potting substrate you use).
An overly dry plant's leaves generally lose turgor (water pressure) and are more flexible to the touch. They may wilt. The potting mix will be dry all the way to the bottom. A plant in a dry state for too long can inactivate it's roots, resulting in a plant unable to take up water unless properly re-rooted.
An overly watered/wet plant's leaves can also lose turgor and may drastically wilt or suddenly yellow. The potting mix is soggy or bog-like, which drowns the roots. A plant in a wet state for too long can rot the roots and crown, resulting in a soggy, squishy, dead mess.
Once again, you're seeking balance.
So, how do I know when to water?
Short answer: When your plant is somewhere between overly dry and overly wet.
Long answer: You cannot water houseplants on a set schedule. Some plants need the potting substrate to thoroughly dry whereas some plants need consistently damp potting substrate. However, most plants fall somewhere in between. How quickly a plant goes from wet to totally dry is dependent on many factors, including:
What potting mix it's in (soggy soil kills!)
What kind of pot it's in
What size of pot it's in
How much light it gets
What time of the year it is
Temperature
Air flow and humidity
The maturity of the plant
Active growth vs. dormancy
All of these variables make it difficult to have a "rule of thumb" about watering plants, as all of our growing conditions and individual plants can be very different.
Water "groupings" help me decide when to water
You should aim to balance how dry you let the plant's potting mix get and for how long. Research the plant's natural habitat to understand what watering conditions it likely prefers, and adjust according the environment you provide the plant.
Group 1: Dry and stay dry
These plants store a lot of water in some way; in their leaves, in their root structure, or a trunk/caudex. They are likely adapted to very dry conditions or inconsistent rainfall.
Potting mix needs: They should be potted in a mix that drains exceptionally quick and retains a small amount of moisture in between watering.
Assessing dryness: Check the soil for dryness with your finger or feel the weight of the pot, and feel the turgor of the leaves.
Plants in this category: Cacti, succulents, sansevieria, dracaena, many hoya, some peperomia, caudex plants, plants adapted to arid conditions
Group 2: Moisture-lovers
These plants prefer to stay moist. The mix shouldn’t be soggy like a dripping sponge, but damp like a sponge that has been rung out.
Potting mix needs: They should be potted in a mix that retains a good amount of moisture but still contains air pockets that help root gas exchange.
Assessing dryness: Let the top 25-50% of the potting mix dry out before watering again and gauge this with your finger or by feeling the weight of the pot. Some of these plants will droop or wilt when thirsty, but if the potting mix is overly wet at the same time these could indicate a root issue.
Plants in this category: Calathea, maranta, fittonia, ferns
Group 3: Somewhere in between
These plants like their potting mix to dry out most of the way (80% or so) between watering and not let dry for too long. Every so often it’s helpful to let them dry thoroughly in alternating dry periods.
Potting mix needs: They should be potted in a mix that drains very well and holds a moderate amount of moisture in between watering.
Assessing dryness: I will check the soil down to the bottom of the pot with my finger, feel the weight of the pot, and look for signs the plant is thirsty such as drooping or wilting
Plants in this category: Aroids such as philodendron, monstera, anthurium; rhipsalis; jungle cacti; many hoya
Most houseplants benefit from alternating dry periods to help prevent crown and root rot, prevent fungus gnats, give the roots an oxygenated break, and more. However, do not let plants dry out in intense heat.
What if my potting mix just doesn’t dry out?
Plant roots need gas exchange in addition to water, so if your potting substrate gets soggy (like a dripping wet sponge), the roots will suffocate. Soggy potting mixes lead to crown rot, root rot, increased fungal/bacterial spread on the soil or leaves, fungus gnats, and more.
I recommend a well-aerated potting substrate composed of 50% orchid bark, 25% potting soil, and 25% perlite. This mix is ideal for most houseplants. I'll adjust the bark size depending on the plant. Succulents and sansevieria get 50/50 soil/perlite.
You should be mindful of the light requirements of your plant. Many plants are light-deprived indoors; these plants are not able to use the amount of water provided to them. Check out my blog on Light for more.
What are the different ways to water?
There are so many options to watering plants! Water the potting substrate thoroughly, including the edges of the pot. Never put a plant in a pot without drainage!
Flushing the soil
This is by far my preferred method. I keep every plant in a plastic nursery pot so when it comes time to flush the soil I'll slip it out of the cache pot and pop it in a sink or bathtub. I then run water thoroughly through the soil so that it flushes from the top to the bottom for a minimum of 5 seconds before moving on to the next plant. Leave the plants to drip dry and return to their cache pots/cover pots.
Do this at a minimum twice a year.
Benefits: (1) Water doesn't pool at the bottom of the pot. (2) Excess minerals/salts are flushed from the soil restoring proper pH. This method is particularly useful if you fertilize weekly, even in heavily diluted amounts.
Soaking / bottom watering/"butt chugging"
Place a pot with drainage holes in standing water and allow the soil to wick up water. Let them soak for a half hour or so. This works particularly well with terracotta because you can visibility see the wicking in action.
Benefits: Plants that are susceptible to crown rot do best with bottom watering (e.g. African violets, cacti, succulents).
Measured watering
Learn approximately how much water your plant needs and provide just the right amount. Use a shot glass, measuring cup, watering can, or squeeze bottle— whatever suits you best. Very slowly water the plant starting from the edges of the pot; aim to saturate the soil but not let any water come out the bottom. This is a learned skill and depends on the hardness of your soil. I have learned how many seconds to squeeze from my squeeze bottle per plant if using this method- 1 Mississippi, 2 Mississippi...
I never recommend putting plants in non-draining pots but if that's what you have, be sure to measure the amount of water or you risk drowning the roots.
Does type of water matter?
Some plants are sensitive to tap water additives such as fluoride, but here in Tacoma I have not experienced any ill effects of using the municipal water. I drink it straight from the tap and find it to be some of the best in the world! However, some folks swear by only rain water or distilled water for their houseplants. A sign your water is irritating your plants includes “crispy tips” on leaves when humidity and watering are adequate. If you have a fish tank use the fishy water on your plants and watch how they thrive!
The bottom line
Watering correctly comes with time and practice.
Let the soil dry according to your specific plant’s watering needs. I recommend keeping plants with similar watering needs together in your home.
Your potting mix should be drying out- if it isn’t you need to change the potting substrate, increase light levels, or check the roots for rot
What great advice! After 2 years of collecting plants, I am finally getting to really know the watering needs of my plants. Like you, I group same type plants together and check them on a schedule. In between, I am observing them for any “off” signs. I love to touch my plants and feel what the leaves are telling me!