Growing Hibiscus In Pots

We have more concrete space than open grounds in our house, hence, the reason why we grow hibiscus in pots. Our garden is less than 25 square meters only, but once it held a beautiful assortment of perennials. That was before hibiscus came to our lives. Beginning with our first plant, Gelia Castillo, we frequented the different nurseries and every time we got a new plant we had to uproot a number of perennials. They disappeared gradually, and yet our hibiscus hunt did not stop there. Sometimes while waiting for our son from therapy, we would drive around the neighborhood in search for wood. As always, Vera’s charm was more effective so I leave the asking to her while I argue that someone should stay in the car. Soon our house was overrun with hibiscus scattered in the walkways, foyer, veranda, window sills, and eventually our room.

At the onset, we never knew that it is tricky to grow hibiscus in pots. Like those we planted in the ground, we thought they are hibiscus just the same. We came close to losing all our plants to something we did not even understand, three to five at a time.

It was convenient for us to say that it was damping-off disease, so we spent a lot of time on fungicides. When it failed to solve the problem, again we spent a lot of time on the quality of our water. But fate repeated itself over and over without telling us what went wrong. It was maddening to see the plants die, but that it would happen unexpectedly, leaving us completely clueless was what frustrated us more. We could hardly advance in fortitude. To make matters worse, Vera and I are physically apart for the most part of the year due to work, and discussing hibiscus problems over the phone or through web chat adds a layer of complication. If not for people who patiently answered our emails one after another, who almost guided us every step of the way, who engaged us in interesting discussions, like Allan Little, Matt Martindale, Don Mixon and many others who do not grow hibiscus but specialize in other nursery crops, probably to this day we would still be overwhelmed by frustration.

Our email discussions, followed by further reading, were engaging. It opened our eyes to many new things like learning earth science all over again, only this time we were eager to learn. Things got more exciting where, for seemingly complicated problems, we discovered simple and practical solutions. Where conventional off-the-shelf products failed us, something as simple as pot-in-pot saved us. As we gained new insights slowly but progressively, one at a time, brick by brick, we moved in the direction of looking for, in every challenging moment, a common sense explanation. As always we are thankful to the many people who helped us, whose account we have taken note of, and share it here in case someone may find it useful.

Potting Mix

There is only one commercially prepared potting mix in the Philippines, which we have tried and the result was deplorable. We lost many seedlings to poor aeration. It was initially exciting to see the seedlings sprout from seed, then later it would be maddening to see them wilt and die. It took us such as long time to figure out that our potting mix is not porous enough, the perched water table (wet feet condition) is high, and too much water is retained.

So we thought that if our potting mix has poor tilth and conducive to wet feet condition, the efficacy of fungicides is momentary, as soil-borne pathogens develop anew when the strength of fungicides dissipate. We decided to make our own potting mix recipe and put it to the test. We dropped perlite and vermiculite because local suppliers could not confirm that it is asbestos-free. As for pine bark fines, we had none, so Vera suggested the use of organic materials available to us. After making over 50 recipes, we found the best result in Recipe #36 and #38 for the following reasons: first, it allows the plant’s roots to anchor on and support plant growth; second, it has sufficient amount of empty spaces to allow water movement in the potting mix; and third, it stores nutrients.

These three functions are termed as anchorage, porosity and nutrient sink. Anchorage and porosity are mutually exclusive—they oppose each other. If you improve porosity, the less compact the soil. If you improve anchorage, the less draining the soil. As for nutrient sink, we learned that it is not the natural absorbent capacity of organic matter but the porosity and capillary function of the potting mix that plays a significant role in nutrient deposit. Porosity also plays a key role in making nutrients available to plant’s roots through water movement in the potting mix. A good potting mix should, therefore, strike a balance between anchorage and porosity for effective nutrient sink.

In our tests, Recipe #22 has the highest total porosity at 81.82%; whereas, Recipe #36 and #38 both have 63.64% only. But with Recipe #22 the roots could hardly settle. It was too loose, water drained so fast, and like Recipe #28 the overall result was not good enough. Recipe #20 has a total porosity of 72.73% which is good, but it was largely water than air.

A potting mix is made up of air, water and solid particles, where each one comprises a portion of the container volume. The space occupied by air and water are empty spaces (pores), without which it would be compact. We measure porosity in our garage using four things: (a) plastic pot, the size we use for growing matured plants; (b) water; (c) water pan; which you can find in your laundry; and (d) measuring cup, which you can find in your kitchen. The Ministry of Agriculture, Fisheries and Food of Canada (1996) explains the procedures for measuring porosity.

1. Seal the drainage hole in the pot and fill it with water. Measure the volume of water and record it as ‘container volume.’
2. Empty and dry the pot and fill it with potting mix. Slowly saturate the potting mix by gradually pouring water onto the surface. Continue adding water over a period of time until the growing medium is completely saturated (i.e. the surface glistens). Record the total volume of water added as ‘total pore volume.’
3. Place the pot in the water pan and remove the seal from the pot’s drain holes. Allow all the free water to drain out (this make take several hours). Measure the amount of drained water and record it as ‘aeration pore volume.’
4. Compute total porosity, aeration porosity, and water holding porosity as follows:

Water-Holding Porosity (%) = Total Porosity – Aeration Porosity

According to J. S. Annan and R. K. White (September 1998) there are many ways to compute porosity and each one has its own limitations. But for our purpose, the method explained above is sufficient enough. The same method was used D. M. Jeyaseeli and S. P. Raj (2010) in their study of the porosity of coco peat.

Large particles of coco peat improve drainage. But if it is very coarse, its water-holding decreases and the lateral movement of water and nutrients within the potting mix is restricted. On the contrary, when the particles are fine to very fine, the potting mix is dense, restricts root growth, and encourages the development of soil-borne pathogens. Our type of coco peat has a total porosity of 63.6%, which is good. However, it is 55.6% water and only 8% air. This shows that we are using coco peat that has a lot of fine to very fine particles.

To improve aeration, we added large particles such as coco chips and pumice. Coco chips are our local equivalent of bark fines or the closest we can get to a bark fine, while pumice became abundant to us after the June 1991 volcanic eruption of Mount Pinatubo. We were particularly excited with the pumice because it had natural pores. However, the results were not encouraging. Small particles packed around large particles, clogging the pores, such that the improvement in porosity does not justify the cost. But when we employed rice hull into the recipe, we realized a significant increase in porosity. In terms of particle size, rice hull is smaller than coco chips and pumice. Unlike pumice, it has no natural pores. But what makes it so porous and well draining is its irregular shape (Al Fassezke, March 2005).

Perched Water Table (PWT)

Al Fassezke (March 2005), who lives in Bay City, Michigan and spent over 20 years studying soil, noted that “there is, in every pot, what is called perched water table. This is water saturated at the bottom layer of the potting mix that will not drain. It can evaporate or can be used by the plant, but physical forces will not allow it to drain. This is the area of the pot where roots seldom penetrate and where root problems begin due to lack of aeration.”

In hibiscus terms, PWT means wet feet. PWT is present in every pot regardless of how many drain holes there are in the pot, and regardless of how porous the potting mix is. It is also present in ground soil, except that it is too deep for the roots to reach. This explains why hibiscus planted in the ground suffers from wet feet to a lesser extent. But just the same, there is PWT in ground soil.

As to having a coarse layer at the bottom of the pot, Al Fassezke (March 2005) believes that it does not improve drainage. Instead, it elevates PWT close to root zone and reduces the space for roots to colonize. With Al’s permission, here is an excerpt of his account on “Water Movement in Soils,” which explains the development of PWT:

“There are two forces that cause water movement through soil—one is gravity, the other capillary action. Gravity needs little explanation, but for this writing I would like to note: Gravitational flow potential (GFP) is greater for water at the top of the pot than it is for water at the bottom of the pot. I'll return to that later. Capillarity is a function of the natural forces of adhesion and cohesion. Adhesion is water's tendency to stick to solid objects like soil particles and the sides of the pot. Cohesion is the tendency for water to stick to itself. Cohesion is why we often find water in droplet form - because cohesion is at times stronger than adhesion, water’s bond to itself can be stronger than the bond to the object it might be in contact with; in this condition it forms a drop. Capillary action is in evidence when we dip a paper towel in water. The water will soak into the towel and rise several inches above the surface of the water. It will not drain back into the source. It will stop rising when the GFP equals the capillary attraction of the fibers in the paper.”

PWT occurs when the capillary pull of the soil at some point will equal the GFP. When this happens, water will not drain—it is perched.

Pot-in-Pot

We spent a lot of time addressing the issue of damping-off disease by testing different types of fungicides. Unfortunately, as our plants were constantly sitting on wet feet (PWT), soil-borne pathogens developed anew as soon as the strength of fungicide dissipates. We observed the effect of fungicide as momentary due to constant presence of wet feet.

The method we tested and found very useful in curbing PWT is called, “pot-in-pot.” As compared to money spent on fungicides, pot-in-pot is not only effective, it is also very cheap, and the result is remarkable. Pot-in-pot is as simple as placing an inner pot (the pot where the plant is growing) inside an outer pot (a much larger pot), with the inner pot buried 3-4 inches below the potting mix of the outer pot. Because the outer pot serves as an extension to the inner pot, this method tricks PWT to go down to the outer pot.

Consequently, the outer pot is constantly wet, therefore, should be treated with a good fungicide. We do not want soil-borne pathogens to climb to the inner pot, though this has not happened to us even when using untreated medium. It is also important to have as much medium in the outer pot, enough to cover the drain holes, to ensure that the outer pot is absorbing the water from the inner pot.

In our experience using pot-in-pot, roots tend to grow fast. In no time, they occupy the entire pot and, when left unchecked, they even extend to the outer pot. Sometimes, we had to break the outer pot just to save the roots. Vera also noticed that pot-in-pot is effective in insulating the inner pot from the high temps of our concrete floors.

If the roots are not too many (above right), we pull the roots back to the inner pot through the drain holes as gently as possible. We water the roots to remove excess potting mix, and examine whether the roots can still be pulled back through the drain holes. It is painstaking, but that’s the price we have pay if we leave it unchecked. If the amount of roots is too much, we break to save the roots. When this plant (right) came to us, it was just enough for a 4-inch pot. With pot-in-pot, this particular plant outgrew its pot in just three months. The same thing happens when you leave a pot sitting in the ground for a while. By the time you pull the plant, the roots are sticking out. Sometimes you can even hear it break.

Pot and Pot Sizes

The type of pot and the size of the pot matters, and it matters a lot. In fact, the first step to reducing PWT is choosing the right pot and pot size.

There are two types of pot: tall and squat.

In his study of soils, Al Fassezke (March 2005) noted that the height of PWT in each pot, having the same potting mix, is the same, regardless of the height and diameter of the pots. Tall pots are advantageous because it has bigger growing region. With squat pots, roots have less space to colonize. For the same reason, we grow our cuttings in plastic cups because its height is more than twice its width. However, Vera noted that tall pots loose balance easily and falls over, so she places them close to each other for support.

Choosing between tall or squat pots is not crucial, but tall pots are preferred because it has bigger growing region. Squat pots are just as effective as tall pots. When placed on a pot-in-pot, they produce healthy roots that sometimes extend to the outer pot such as the one shown below, where long roots had to be gently pulled back through the pot’s drain holes.

What is crucial is the size of the pot. Planting hibiscus on an oversized pot was, in our case, a deadly mistake. The picture (right) is our first Magic Moments in 2009 that we planted on an oversized pot. It came to us on a 4-inch square pot, and after it grew a few inches, we got so excited. It was alleluia to have Magic Moments so we transplanted it on an eight-inch round pot, thinking that we should give it more room to grow, but since then the plant never grew. We were lucky to have saved this plant, because we lost all others in the same batch.

It was in one of our conversations with Matt Martindale, though it was on the subject of when to repot, that we realized the connection between water retained in the pot (water-holding), and the ability of the plant’s roots to absorb water. Though we said earlier that it is important to keep water holding below 45%, this is true on condition that the plant is not in an oversized pot. If the pot size is eight inches and the rootball is only four inches, even if water retained is 35%, this may still be too much water—way more than what a four-inch rootball can absorb. A plant sitting on a wet feet will either succumb to damping-off disease or it may survive but will look under-nourish and stunted such as what we did to this almost fourteen-month old beauty.

Therefore, it is important that the size of the pot should be just enough for the plant’s rootball, with a little extra room for growing up. Otherwise, what is the point of applying fungicide if, in the first place, the plant is constantly sitting on wet feet. On the contrary, if the pot size is just enough for the rootball, there is almost no need for pot-in-pot; or, if the plant is already on a pot-in-pot, it need not stay there for long if the rootball has grown up.

Even when we know our kid is growing up, we should never buy him an oversized shirt. We should buy what fits him well with a little room for growing up.

When is the time to repot?

When to repot the plants is not a matter of time, but whether the rootball is already too big for the present pot size. This is subjective though as plants can stay within the same pot for long periods of time. For example, given our limited space, in the last two years we restricted our plants to seven-inch tall pots (less than three-fourths of a gallon), but sustained them with water and daily feeding for over a year. It is also possible to prune the roots accompanied by pruning of the branches just enough for the roots to sustain in order to keep them in the present pot. The size of the plant vis-à-vis the size of the pot is another factor to consider, as tall plants on a small pot will frequently fall over.

When to repot the plants is always a subjective discussion between Vera and me, proof that our learning curve is still on a wide turn. But over the years we acquired a few tips from the hard lessons learned, so our discussions are either academic or due to certain targets we want to achieve. As a general rule in our house, we do not repot unless the following conditions are true:

1. The rootball is already too big for the pot, that is, the roots occupy 60% or more of the container volume, which we will easily know because the amount of water we give them is measured.
2. As suggested by Matt Martindale, the potting mix dries out in less than a day which means that the water holding is already less than the water needed by the rootball, or you can water them twice a day and keep them in the same pot.
3. The size of the plant is already too big for the pot.

Unlike soil that regenerates its fertility, texture and structure over time, soilless potting mix such as coco peat degenerates. Heterotrophs, for instance, will cause the gradual decomposition of soilless medium causing the potting mix to lose its texture and structure. It is hard to tell how long this process takes as it depends on a confluence of factors but, as a general rule, soilless medium should be changed at least once every three years.

References

The Ministry of Agriculture, Fisheries and Food of Canada (July 1996), “Importance of Aeration in Container Media” (adopted with permission). Retrieved from www.al.gov.bc.ca/ornamentals/floriculture/aeration.pdf

Jason S. Annan and Richard K. White (September 1998), “Evaluation of Techniques for Measuring Air Filled Porosity in Composts of Municipal Biosolids and Wood Chips.” A Paper presented to the Composting in the Southeast – Proceedings of the 1998 Conference (Athens, Georgia). Retrieved from http://infohouse.p2ric.org/ref/12/11558.pdf

D. M. Jeyaseeli and S. P. Raj (2010), “Physical Characteristics of Coir Pith as a Function of its Particle Size to be used as Soilless Medium,” American-Eurasian Journal of Agricultural & Environmental Science, 8(4) : 431-437

Al Fassezke (March 2005), “Container Soils and Water in Containers”. Excerpts reprinted by permission of Al Fassezke. Retrieved from http://forums2.gardenweb.com/forums/load/contain/msg031557203792.html

Matt Martindale, Website: www.hibiscusplace.com, Email: matt@hibiscusplace.com