Guide to Caring for an Unknown Plant 3: The Issue of Watering

You’re on your way back from the plant store, an unplanned but pleasant detour after a day’s shopping. On a whim, you buy a plant you’ve never seen before, but which you really like. To make matters worse, the store doesn’t identify it. So how do you know how to care for it?

This article is a continuation of the other guides to caring for an unknown plant:

1. Plant type and light
2. Leaf color and light

Foreword

Now that we have a better understanding of our plants’ need for light, this third guide touches on the tricky question of watering. We’ll start with some basic considerations, then explore what observing a plant suggests about its watering needs.

The last article will cover all the other aspects of plant care (repotting, potting soil, fertilization, temperatures, propagation, etc.).

To illustrate the article’s objectives, we’ll take a look at this new mystery plant and, based on the information given in the article, draw some conclusions that will help us understand its watering needs.

Answers to the Practical Exercise (Guide 2)

At the end of the second guide, these three photos were presented and you were asked what the observation of the plants suggested about their light requirements.

1) Although the leaves contain green, the plant is prized for the multitude of other shades it adopts, suggesting a higher need for light than if the plant were entirely green.

Plant name: Codiaeum variegatum (indeed, they tolerate up to medium light, but put on their best colors under intense light with lots of direct sun).

2) With its growing height, emerging trunk and leaves with alternating green and yellow stripes (which can’t photosynthesize), we have two indicators of a high need for light.

Plant name: Dracaena reflexa ‘Song of India’.

3) This plant grows in a rosette and its leaves are entirely green. It should survive in average light. However, it needs more light to produce its pretty red bracts.

Plant name: Anthurium.

Watering and Water Requirements: A Few Basic Principles

Watering is much simpler than light requirements. In fact, this whole section could be summed up with the golden rule of houseplant watering, in the knowledge that plants recover much better from too little water than too much.

A few miscellaneous considerations are in order before delving into what plant observation tells us.

First of all, we try to water the plant according to the state of the soil. There are several ways of finding out how dry it is: first of all, the easiest and one of the most reliable is to put your finger in the soil. This doesn’t mean touching the top of the pot, but digging down to the second phalanx if possible (plus, it aerates the soil).

This isn’t always possible for all potting soils (especially compact ones) or for all plants (I don’t plan to plunge my forearm into my two-meter Radermachera sinica); you can then weigh the pot, bearing in mind that waterlogged soil is much heavier than dry soil. With practice and knowledge of your plants, you can achieve a certain degree of precision, especially when combined with the first method.

You can also try looking through the holes in the planter to get an idea of the condition of the soil at the bottom of the pot. It’s not always possible, but it can be informative.

Then, for gardeners like me, who observe their plants tenderly and frequently, there are several signs that they need water: curled leaves, soft petioles that hang sadly, yellowing basal leaves, etc. With a little attention in the first few months of care, it’s easy to understand the watering rhythm.

A Word About Moisture Meters

A tool is sold whose very purpose is to measure the moisture in the soil of our plants. I can’t suggest a particular model, as their reliability generally leaves a lot to be desired. My advice: don’t rely solely on your moisture meter!

Finally, light levels have a direct influence on the amount of water a plant needs: a plant in bright sunlight loses much more water through evapotranspiration than one hidden in dark, damp basement. This is obviously a dimension that needs to be taken into account.

Observations That Provide Information

Let’s think about the characteristics of plants that are easily observable and what they tell us about their water requirements. It’s mainly the plants’ degree of succulence that tells us. Succulence refers to plants’ adaptations to survive in arid environments.

Let’s take a look:

1. Swollen, waterlogged stems, like those of cactus or caudex plants.
2. Very clearly fleshy leaves, like those of Crassula ovata, thick and rather solid leaves, like those of Hoya carnosa, and thin but not very flexible leaves, like those of Ficus elastica; the thicker the leaves, of course, the better they retain water.
3. Subterranean organs: the rhizomes of Dracaena trifasciata (formerly Sansevieria) or the tuberous roots of Chlorophytum comosum.

All these water-retaining organs, and the combination of them, help the plant to tolerate drought. To a certain extent, the type of plant growth is also interesting: climbers and epiphytes are used to having a small root system, in a restricted space. They are therefore forced to tolerate drought better.

Photo left: here’s a fine example of a plant, a Beaucarnea recurvata, whose caudiciform stem enables it to withstand drought. Photo by Maja Dumat.

The middle photo, in addition to its beautiful colors, shows the fleshy leaves of small succulents. Photo courtesy of Pickpik.

The photo on the right shows the tuberous roots (in dire need of repotting!) of a Chlorophytum comosum. Such underground organs are less easy to see, but contribute to a plant’s longevity in the event of water shortage. Photo by Keith Williamson.

Different Plants, Different Water Requirements

After observing the degree of succulence of the plants, we could divide them into three sub-groups to better organize our observations:

1. Plants that have almost no way of surviving in an arid environment. This is the case of all plants with thin, flexible, paper-textured leaves. Examples include the delicate fronds of ferns, the thin leaves of Syngonium podophyllum or the highly ornamental, flexible leaves of Calathea cultivars. These plants need to be kept constantly moist.
2. Plants with at least one water-retaining organ, such as Scindapsus (slightly leathery leaves on a climber) or Tradescantia (swollen stems). Even if you don’t notice any obvious organs, it’s best to consider all plants as belonging to this category until proven otherwise. For example, Epipremnum aureum may not look very solid, but it retains water very well in its stems and leaves. Water these plants according to the golden rule mentioned above, when the soil begins to dry out.
3. Plants with multiple water-converting organs, clearly succulent plants and cacti. For example, Zamioculcas zamiifolia has thick, inflexible leaves, swollen stems and grows by means of rhizomes: that’s a lot of reserves for resisting forgetfulness! Water this plant when the soil is deeply dry (you can be a little more generous during the growing season).

What About Humidity?

It’s hard to paint a uniform picture of the moisture requirements of houseplants. The truth is, most commercially available plants tolerate humidity well in our homes, with a few exceptions (Calatheas, for example, which seduce us with their magnificent leaves, only to betray us with their inordinate need for moisture).

As we move away from common plants, needs vary: some have thin leaves and need a lot of moisture (e.g. Strobilanthes, Begonias rex, ferns), some have thin leaves and need a little moisture (e.g. Radermarchera sinica, Hibiscus), some have water-retaining organs and still need moisture (e.g. Cordilyne fructicosa, despite its thick trunks)…

In short, the general rule is that plants in desert climates don’t like humidity; all others benefit from it. So there’s nothing wrong with increasing humidity.

Lack of humidity is usually indicated by two signs: dry leaf tips (which can be caused by other things, see the article Why Do Leaf Tips on Turn Brown? and the presence of spider mites.

Applied Example: The Mystery Plant

Based on the information we have about the type of plant, what can we say about its water requirements? How do you plan watering?

Observation of the mystery plant reveals slightly fleshy, inflexible leaves, surmounted by thick petioles. It should be watered when the soil begins to dry out. By watching it closely for signs of water shortage in the first few months, you can check whether it needs more frequent watering. Such signs would be the collapse of the usually tough petioles.

Practical Exercise

In the next article, we’ll talk about other aspects of plant care (repotting, potting soil, fertilization, temperatures, propagation, etc.). In the meantime, what can you tell about the water requirements of the following plants by observing their characteristics? Feel free to write your answers in the comments. Answers will appear in the next article.