Step by Step to Healthy Plants in Containers

The following plant care guide is a work in progress. The emphasis is on indoor or houseplants but the basic principles apply to both indoor and outdoor plants growing in containers. I plan to add videos in the near future.

In an effort to improve the clarity of this guide, it will be most helpful if you would make constructive suggestions or ask questions either by comment or email me at urbangreenscaper [AT] gmail.com. Thank you on behalf of other readers.

The first three rules of container plant care…measure, measure, and measure

It’s easy to understand why so many houseplants look unhealthy and why so many people feel they have a black thumb.

The undeservedly popular drench and drain method of watering is tricky to learn because it’s all art rather than science. It’s based on feel and estimating rather than measurement. It might be okay for the greenhouse but not in your house where light is scarce and the environment sometimes harsh. 

Poking your forefinger in the soil to feel the soil moisture and using window direction to estimate the light level is hardly scientific and highly subject to error.

For most of us, human judgment about measuring light and soil moisture is woefully inaccurate. Human judgment may work for plants in the sun when photosynthesis is at its peak but not in low light building interiors. The margin for error is quite small.

What follows applies to any plant species whether grown for foliage or flowering decoration or for food production. The only difference in these categories is the level of light required, vegetables being at the top of the chart. Sub-tropical foliage plants require the least amount of light but even they will die without adequate light.

The key to success is to follow the golden rule of light-water relationships

Understanding the critical light-water relationship of plants growing in containers is critical to success. This light chart is a good starting point to understanding this all-important relationship. Click on the chart to enlarge it.

Once you know the light level is adequate (as measured with a digital light meter), you can proceed with measured sub-irrigation maintenance. If the light level is inadequate, the plant will die no matter what you do. A digital light meter therefore is one of the most important tools you can use. The cost of digital meters has come down dramatically in recent years. eBay is a good source. Since you only need it when you locate plants, share it with a friend or in a group you belong to.

It is important to note that a digital light meter is an incident meter. It measures the light falling on the plant. The same sources that push drench and drain watering often recommend the use of a camera’s light meter to measure light. These photographic meters are reflective meters that measure the light reflected by the plant rather than the light it is receiving. This is a significant difference.

Measured Sub-irrigation – the scientific path to understanding light-water relationships

The most precise way to measure soil moisture and water consumption of a container plant is a scale (preferably a digital scale). Commonly available digital scales measure to one tenth of an ounce accuracy.

Many drench and drain advocates recommend the use of cheap soil moisture meters. It’s highly debatable whether they are any better than poking your finger into the soil. First of all the construction is flimsy and highly susceptible to damage. Meters in the consumer market are definitely not professional instruments.

Perhaps more important is the fact that they do not really measure moisture. They measure the galvanic effect of the salts in the water. You can test this by inserting the meter in deionized or distilled water. The meter will give a dry reading because the salts have been removed from the water. These meters will also give a false reading when the soil is very dry but has a high salts content. The meter can inaccurately read this as moist soil.

The notion of “self-watering” is bogus, but sub-irrigation is free

Of all the sub-irrigation planters I’ve worked with over the past 30 or so years the best one is free. I’ve discovered that recycled plastic pop bottle planters have all the features you need from both a horticultural and educational standpoint. Granted that they are only usable for small plants up to 6” pot size but they are the best sub-irrigation teaching aid I know of. If you learn the scientific principal of sub-irrigation using these pop bottle planters, you can maintain plants in any size sub-irrigation planter.

The only other tool you might need is a soil probe to test soil moisture in larger planters that are not practical to weigh. I strongly recommend measuring soil moisture rather than water level in a sub-irrigation planter.

The inverted neck of the bottle forms the capillary channel. The bottle bottom creates the reservoir. Water feeds by capillary action up through holes in the cap into the soil. All of this is visible through the clear plastic. You can see both the foliage and the all-important root system. It’s that simple.

An important question is how much water to add to the reservoir. Now is a good time to explain my strong objection to the tern “self-watering”. Users of these so called self-watering planters are led to believe that the plant controls how much water it takes up. The claim by providers and others is that the plant drinks what it needs. That is simply not true.

The term self-watering in the consumer market has led to two popular negative opinions. Both of them are false and are only due to ignorance of how capillary action works along with ignorance of basic plant physiology.

1. Self-watering planters over water.
2. Self-watering planters are for moisture loving plants only.

Plants have no intelligence to start and stop consuming water. Capillary action is a principal of physics. Water will rise until field saturation is reached (the soil can hold no more water). When the rate of photosynthesis is reduced in low light, the plant’s rate of water usage is also reduced. When there is too much water in the media, oxygen is forced out of the pore spaces and the plant dies from drowning.

The critical measurement is soil moisture rather than the level of water in the reservoir. If you keep the reservoir topped off in reduced light, you will most probably kill the plant from over watering. It’s that simple.

Remember that you are the ‘self’ in self-watering. You must determine the soil moisture and only add more water (preferably measured) when the plant needs it.

The Internet is full of false claims about automatic or maintenance free self-watering. A current favorite on the web is a little planter called a Grobal. Here are just a few of the many false claims about the Grobal the current container plant gadget du jour in the blogosphere.

“The Grobal self-watering planters are pots designed to water your plants automatically.”
“The "grow chamber" draws water out of the reservoir when needed, therefore keeping the plant alive and not wasting water in the process.”
“These vases are exactly what I need. You just make sure it’s always filled with water and it automatically makes sure your plants get enough water without overwatering it!”

I call these planters “looks 10, brains 0”. It may be the most accurate thing you’ll read about them. For those who understand measured sub-irrigation a free recycled pop bottle planter will perform as well or better.

Measuring soil moisture by weight is infallible

All you need are two data points. Weigh the plant today and let’s say a week from today. Depending on the container size, you may extend the watering interval to two or three weeks. The reduction in weight will reveal how much water the plant used plus evaporation. You could devise an experiment to measure evaporation but it doesn’t really matter because it is relatively constant year round. The variable is the light that controls photosynthesis and consequent water consumption. That is what is important.

Perhaps you’re wondering how much water to add at the time of first watering. Measure the water as you add it and add only as much water that will rise by capillary action to about ½ to ¾’ from the soil surface. Do not saturate the soil. You will only provide a moist habitat for the propagation of fungus gnats (Sciarid flies). Their larvae must have moist soil to live. Dry soil, no gnats.

Today
Step 1. Weigh the plant and planter before you add water. Don’t weigh the reservoir. Put the reservoir on the scale and press the ‘tare/zero’ button. You’ll need to do a manual tare adjustment on an analog kitchen scale. Insert the planter back into the reservoir and now you’ll be weighing just the plant, soil and the planter housing it.
Step 2. Weigh the water you’re adding to the reservoir. You now have a new total weight for the plant, the planter and the water.

One week later.
Note: For educational purposes, you may decide to monitor one or more plants on a daily basis and relate the daily water consumption to the daily weather (light availability for that day). Students could use the digital light meter and record the weather and the daily light meter reading next to the daily water consumption. This will vividly tell the story of the light/water relationship of plants.

Step 1. Weigh the plant before you add water and compare the weight today with the total weight from the prior week. If it weighed 28 oz last week and this week it weighs 18 oz and the reservoir is empty, the plant used 10 oz or 1.4 oz/day.
Step 2. Add 10 oz of water to the reservoir.

Two weeks later.
Step 1. Weigh the plant again. If it weighed 28 oz. last week and this week it weighs 22 oz. and the reservoir is empty, the plant used 6 oz. or .9 oz./day. This reduced amount of water used is feedback that the light level was lower this past week than it was the prior week. The plant is now able to ‘communicate’ with you. You’re now able to observe and measure the all important light/water relationship.

Step 2. Add 6 oz. of water to the reservoir.

Comments

[Cassie]

Thanks for the information. I would like to try this with a soil-less medium such as perlite. I've seen you have some of yours in the expanded clay pellets. How do you do the nutrients?

[Greenscaper]

Sorry to have overlooked your question Cassie. I have used a constant feed method for the past 10 years or so. I use Dyna-Gro (both Foliage Pro and Bloom) at the rate of 1/2 teaspoon per gallon of water. Dyna-Gro is widely used in the interior plantscaping industry.

[Jessica]

When you describe how to determine how much water to use, you say that if the resivor is empty you should add as much water as the change in weight. What if the resivor isn't empty, should you wait until it is empty before adding more water?

[Greenscaper]

Good question Jessica. Suppose I added 8 ounces of water at the last scheduled watering day (let's say 7 days ago for this example). If there is still some water in the reservoir (let's say 1 ounce) it means that I added too much water. Perhaps it's been a week of cloudy weather.

I simply dump the remaining water and probably add only 7 ounces this time. I have been watering on a fixed schedule this way for going on 15 years now. It works!

Did I understand your question correctly? If not, ask another question.

[Sandra]

How do we compensate the weight of the plant. Growth produces more biomass, the plant itself gains a few ounces. In time we must correct the "ideal startpoint weight". Right?

[Charlotte]

Had the same question.

[Jeremy]

I believe he responds to this issue on an other page by pointing out that you are growing plants small enough that the extra weight due to new growth is negligible compared to the weight of the growing media(soil).

[Simon Green]

Normally I would check to see if the soil is dry around the plant itself, I haven't considered trying a moisture indicator.

[Harmony]

Your explanations are the most helpful I have found regarding subirrigation. I wonder if you could do a series on vegetables? I started looking into this subject because I have used a "self-watering" seed starting tray with good results, but then transplanted my tomatoes into 4" pots and wondered what the difference was between the constantly moist soil the seedlings were in when sitting on a capillary mat (with water always in the reservoir) and the constantly moist soil they would be in if I left the larger pots in 1/2" of standing water (which everyone says emphatically not to do).

Anyway, these are not houseplants and they are not in low-light conditions (they're under a grow light). I wonder if I can get away with leaving some water on them all the time, but I am curious what your knowledge and experience says about this situation. I'm mostly interested in finding an effective way to keep the plants watered without having to pour from above every day or two, but it can't be too complicated when I'm dealing with dozens of transplants.

Thanks for the information you have already shared!

[J]

With some bottles i have a hard time guessing what the ideal weight is.
I Cant clearly see if 1/2 inch from the top is dry or if it is more. I think i presses the soil to tight in these bottles, it was very hard to distribute the soil evenly around the ficus roots. I was afraid that any place without soil would prevent good Capillary action.

Is there an aprox ideal weight you could give for a small ficus benjamina in a 1,5 liter soda bottle? It currently weights 322 grams or 11.4 ounces. I think its a little bit to dry allready and the ideal weight should be heavier but i cant really tell.

Thank you.