Indoor gardening: Watering and humidity
The importance of watering
Without water, plants die—quickly in the case of moisture lovers, such as peace lilies (Spathiphyllum) and some ferns, or over months, even years, for some drought-tolerant cacti and succulents. But over-watering commonly kills plants, too.
Routine watering on a set day or at regular intervals can lead to over or under-watering, because needs vary according the time of year or stage of plant growth. It is much better to check all plants regularly and learn to judge when they need water and how much to give.
- Test the compost by feeling if it is moist to the touch.
- Lift the pot to see if it feels light and needs water, or is heavy and does not.
- Use a moisture meter, or watering indicator that changes colour to give a warning reading.
- Look for early signs that water is short, such as dull or pale leaf surfaces or slightly drooping buds and stem tips. Do not wait until plants wilt and stems become limp, as these are more advanced symptoms. If in doubt, wait a day rather than risk over-watering, which will unduly stress the plant. Roots in waterlogged compost will not be able to absorb oxygen from the atmosphere, which is needed for growth. In time the roots will start to rot and the plant will die.
Top tip: Double potting helps to keep a plant moist. Select a container with a diameter about 5cm (2in) bigger than the pot in which the plant is growing. Half-fill the larger pot with gravel or vermiculite and then plunge in the planted pot and top up the gravel to the inner pot‧s rim. Regularly top up the water level in the gravel so the bottom third is damp.
Humidity is a measure of the amount of moisture in the air at a given temperature. In centrally heated rooms, the air can be very dry, with humidity levels as low as 15 per cent.
Conditions like these are tolerated only by dry-air plants such as cacti and succulents, billbergias, spider plants (Chlorophytum comosum) and figs (Ficus). To prevent their leaves from drying out, most houseplants prefer humidity of 40 to 60 per cent—a range often found only in a bathroom or steamy kitchen.
Fortunately, it is easy to increase humidity levels locally around a plant, by using any of the following techniques:
- Misting: Use a hand sprayer with a fine nozzle to mist leaf and stem surfaces in the morning, so they dry before nightfall. Do this daily in warm dry conditions. Misting will also discourage dust, as well as problems from red spider mite.
- Evaporation: Group plants that enjoy the same conditions in a tray or bowl filled with a layer of pebbles resting in a little water. As the water evaporates, it will increase the humidity around the plants.
- Double potting: The evaporation effect can also be achieved by double potting: plunge the potted plant in an outer pot filled with moist gravel or vermiculite (see below left).
- Humidifiers: Installing a humidifier will raise the air moisture level in the whole room. A cheaper method is to stand a dish of water near a radiator and your plants.
Top tip: Plants can be grown together if they thrive in the same conditions. Begonia rex, syngonium, button fern (Pellaea rotundifolia), mind-your-own-business (Soleirolia soleirolii) and pogonatherum all like moist but well-drained soil and moderate humidity levels.
How to water
Most plants prefer a good soak occasionally rather than frequent dribbles that do not wet all the compost and encourage the formation of roots close to the surface. Water the compost from above until excess begins to drip from the drainage hole.
If water runs quickly through the pot, the compost is very dry and you should stand the plant in water until it is thoroughly moistened and bubbles cease to rise.
Always consider whether a plant has special watering needs, such as the following:
- Plants that are resting, or dormant, need very little water compared with those in flower or active growth.
- Plants in cool rooms need less water than those in high temperatures.
- Hairy leaves on plants such as gloxinias (Sinningia speciosa) and african violets (Saintpaulia) are damaged by water so a specific watering technique is needed.
- Cacti and some other succulents should be watered only sparingly.
- Bromeliads such as aechmeas and vriesias like to have their central 'vase' filled with water.
- Bog and water plants such as cyperus and venus fly trap (Dionaea) need constant moisture.
- Air plants absorb water vapour in humid air and so require regular misting instead of watering.
Watering tip: Lime-hating plants such as azaleas (Rhododendron simsii hybrids) are best watered with rainwater, so collect some in a garden water butt. They must not be watered with tap water unless it is very soft or has been boiled and left to cool. Also, avoid using water from water-softeners, because it contains harmful chemicals.
Top tips: Position a water meter half-way between the pot rim and the plant stems. Push the probe very gently into the compost until it is about two-thirds of the way down the pot, taking care not to damage any of the plant‧s roots or stems. Read the meter, then remove it and wipe clean.
When using a watering can to water hairy-leaved houseplants such as african violets, hold back the foliage as it dislikes becoming wet. Do this gently as succulent leaves are very easy to break.
Waterlogged plants can be slightly dried. Remove the plant carefully from its pot and wrap the rootball tightly in several layers of absorbent material, such as paper towels. Repeat until the excess moisture has been soaked up from the compost, then replace the plant carefully in its pot.