Growing Streptocarpus: Watering

Watering may be the hardest part of growing streptocarpus well. Not because they are difficult to water but because there is a lot of confusion and misinformation about their care. The goal of this post is to describe some watering experiments and provide clarity about watering.

Common Watering Advice

I was given the following information when I first started growing streptocarpus, also known as Cape Primroses.

• Don’t get the leaves wet or they will rot.
• Don’t overwater or the roots rot and the plant dies.
• Don’t plant in a large pot, or it stays too wet and the plant dies.

The second two pieces of advice contain a kernel of truth but are wrong. The first one is just dumb advice.

Myth: Don’t Get the Leaves Wet

When I hear such advice I can’t help visualizing plants growing in nature where leaves get rained on all of the time and they don’t rot. Even though this advice seems wrong, I decided to test it.

I took a plant with a fairly flat leaf and put water on it every day for 3 weeks. The plant was fine and the leaf never rotted.

I water from above and regularly get the leaves and the crown of the plant wet. It does not harm the plant.

Don’t Overwater

The common watering advice is to water plants and then leave them alone until the soil starts to dry out. Then water again. Overwatering means that you water too often and before the soil starts to dry.

This makes perfect sense and is good advice but are streptocarpus harmed by more frequent watering? The reason I ask this question is that some people grow them with wick watering. Using this method, a cotton wick is inserted in the soil so that some of it dangles out the bottom of the pot. The pot is then set over a water reservoir so that the wick is constantly in water. The wick absorbs and moves water into the soil thereby constantly watering the plant.

Numerous good growers use this method so it certainly works and if it works, why do others suggest that streptocarpus should not be overwatered?

A critical fact of the wick method is that extra perlite, up to 50%, is added to the potting media making it more porous than the media used for overhead watering.

Does Too Much Water Kill Streptocarpus?

I see this comment a lot online. Watering too often will kill the plant. Is that really true?

I decided to test this – see Experiment #2 below.

Clearly, watering every day for 4 weeks does not kill the plants. They actually grow quite well with such treatment. I am not suggesting that you water every day, but it clearly won’t harm the plant provided the plant has enough drainage and porosity.

Warning: don’t put stones or pebbles in the bottom of the pot because it does not work and it will create a perched water table.

Overwatering vs Being Too Wet

The two terms, overwatering and being too wet may sound as if they are the same condition but when it comes to soil or soilless potting media they are quite different.

Consider this example. Take two pots, one containing sand and the other clay soil. Water both every day.

The container with sand drains well and in a few minutes, it has a lot of air gaps because excess water has run out. The sand never gets too wet even when it’s overwatered (i.e. every day).

The clay pot on the other hand gets very saturated and there is very little air between soil particles. Air only enters the soil after a few days as the water starts to evaporate. Overwatering this kind of soil never allows it to drain enough to allow air to enter the soil and it remains constantly wet.

Plant roots need access to both air and water which they can easily get in sand. The same plant roots growing in clay can’t access enough air and die.

The relative amount of air and water in a pot is a function of the watering frequency and the porosity of the soil. A very porous soil can be watered daily and still not be too wet for plants. A less porous soil may not drain fast enough to allow air into the soil with daily watering.

What about streptocarpus? In their native habitat, they grow on rocky slopes that provide good drainage and I think this is a key to their needs. Their roots need good exposure to air as well as moisture. When they are planted in low-porosity potting media, watering frequency becomes critical. When it is too frequent, roots don’t get enough air and die.

Using perlite to increase porosity creates a soil that always has enough air in it, even if the plant is watered every day or wick watered.

I now grow streptocarpus in peat-based media that is about 40% perlite.

Experiment #1: Dry Conditions

The advice given to people who present pictures of struggling plants routinely advise that the plant has been overwatered. I think that in some cases the opposite is true. The plants are actually underwatered. I set out to document the symptoms shown by a plant that was not watered enough.

I took a couple of plants and watered them just enough to keep the leaves from losing all their turgor.

Over a couple of weeks, the older leaves started to develop a gray coloration which started at the leaf tips and gradually moved toward the crown of the plant. Eventually, the whole leaf was gray and dry. During this process, the plant seems to remove moisture from old leaves and allocate it to new leaves that are just developing. Once all of the older leaves are dry, even these new leaves become gray and dry.

Experiment #2: Daily Watering

This experiment tests the notion that frequent watering will harm streptocarpus. I selected 6 flowering seedlings and put them on a special tray that had a false bottom to ensure that they did not sit in water. Each plant was watered every day for 4 weeks until some water ran out of the bottom of the pot ensuring that the potting media was constantly wet.

The test seedlings were not perfect specimens and had been set aside to be discarded or given away. They represent a variety of seed genetics coming from different mother plants. All are hybrids. They continued to receive the same light as other seedlings. The media was my standard mix of Promix with 30-40% perlite added.

The water used was my standard mixture which I use for all of my streptocarpus, both seedlings and mature plants. It contains an NPK ratio of 3-1-2 with a nitrogen level of around 100 ppm. This is my standard recommendation for all houseplants.

The above pictures were taken 4 weeks after the experiment started. All of the plants look healthy, are continuing to flower and show no sign of rot.