Breaking Up is (Not) Hard to do
Depending on the orchid, it must, it really should be re-potted at least every two years. There are two contributing factors as to why the orchid should be re-potted, size of the orchid and state of decay of the potting medium.
Some orchids prefer, in fact they will not bloom unless they are root-bound and squeezed into a pot and that could be a pot smaller then you would normally think. Other orchids over time, as they mature and continue growing, do outgrow the pot they are in.
Organic potting medium, decays. It can turn from bark chips to a spongy smelly mess that retains water and it is this water retention that drowns an orchid. Drowned orchids are soon to be dead orchids. So re-potting replaces that old decaying organic medium with a fresh new medium.
Re-potting also gives you a chance to take a peek at the roots and see how they are doing. Healthy roots mean everything.
These two reasons are for the most part, why orchids get re-potted every two years
This presentation is specific to my Oncidesa (Oncidium) Gower Ramsey. and why I chose now to re-pot this orchid. For the most part the process be the same for any orchid. For many, dividing orchids might not be a factor in re-potting but again, the process is the same.
This orchid was definitely ready for re-potting as it has outgrown, and even over-grown the standard 1-gallon pot it was in. If I left it as it was, I risk breaking the growth beyond the boundaries of the current pot. However, there was a chance that it explode with numerous spikes and each spike have a multitude of blooms. If it bloomed for me last year, and I had the space in my mini-greenhouse, I would have strongly considered placing this current pot in a larger pot and back filling with potting medium. This would make watering a little difficult - as in a shower for about 15 minutes. So I made the decision that it is time to divide. I was prepared for 4 divisions, but turned out it became 6 divisions. 6 divisions in 6 uniform pots take up more space than a single 1-gallon pot, so I lost in that effort, but some of these divisions will be used for trade once established. Then I will gain in space.
Timing is everything: The Oncidesa (Oncidium) blooms in the spring. Mine is currently in the new growth stage of its annual growth cycle. The multiple new growths are or have already deeloped new roots. This is the time to re-pot this orchid. The developing new growths and their roots will easily adjust in the fresh medium perhaps minimizing set-back and shock.
This is a close-up view of new growth with its young roots on the outside of the pot. Most of the roots have a healthy green tip. Obviously the dark roots are older and not associated with the new growth and might be in the process of dying not from being exposed to the air, but rather extreme heat (in Sacramento). This is perfect timing for making adjustments so that the above new growth is seated in a potting medium.
Oncidiums produce "air roots" that extend into the air and not into the potting mix.
The Oncidium is an epiphyte and "Epiphyte orchids use their roots to absorb nutrients from the air. They also absorb moisture and get the carbon dioxide they need to thrive directly from the air. This unique root system uses the humidity around it to get the water and nutrients it needs to survive." (Source: Taking Care of Your Orchid: What Are Air Roots?)
They will not take to being buried in a potting medium as they are programmed to being exposed to the air. It is possible to remove these if you are a neat freak, but not sure if it harm the orchid.
When my epiphytes (oncidiums, phaleanopsis, catasetinaes, etc., etc.) produce these roots, I just mist them with a very very diluted fertilizer when I water the orchid. WHen it is hot in the mini-greenhouse, I just mist everything, including these roots.
Removing the Orchid from the pot, in one piece: If I had waited and allowed the medium to dry, it probably shrink just enough that the orchid slip out by banging the pot all around just below the top and then tip the orchid and let grairty do the rest. Seeing how it was over 100-degrees the day I worked on this and the day before, the medium was heavy with water, but my described method did work. It slipped out rather easily.
There have been times when you would need to seriously hammer on a pot to loosen the orchid for it to be removed. As a last result be actually cutting the pot carefully. (click image left for larger view)
It did not take long and only a little effort to remove this Oncidium from the pot. One of the difficulties is that Oncidiums do not come with "pull tabs". There is no simple way to grab hold of the orchid and pull because doing so will almost always result in it breaking in a way you are desperately seeking to avoid.
I used to water this orchid with the "dunk and drown" method. I would place the pot in a 2-gallon bucket, slowly pour water onto the orchid so that the water was almost to the top of the bucket - resulting in the top of the pot itself being about 2 inches beow the water line. I let it sit for about 15-minutes, pull it out and allow to drain before replacing in its growing area.
Looking at the condition of the roots, and their abundance, I can only conclude that my watering method for it was very acceptable. The potting medium, might be on the verge of starting to decay (it has been 2 years since last divided and potted).
Then I carefully flipped it to see the other side. I am not concerned that their is an area of black, dead roots, this may have been a result of the last division that I did not do. The abundant presence of healthy roots convinces me that everything is fine. (click image, right, to see full-sized view)
As I clean and remove the old potting mix, I suspect those dead black roots and other debris get simple washed away.
Removing the old potting mix: I used an old dish pan to capture the debris and gently banged the orchid to loosen up and allow the debris to fall away. I followed that by running my fingers to gently comb out the remaining potting mix and any loose material.
A few times when needed, I would dip into my sink full of epid water and swish further removing any loose material. This process perhaps took about 20 minutes, 15 minutes longer than getting the orchid out of the pot.
All the unwanted material still might not be removed, but I am only half way through this process.
Dividing the Oncidium: This is where skill and knowledge come into play. I will be the first to admit that niether my skill or knowledge can be compared to someone who has tended orchids for decades. However, I know the following:
Most important, I want clumps of bulbs to be no less than 4, 5 be acceptable as well. I would identify a new growth and in my mind sort of picture the four or 5 surrounding back bulbs and that be a division. This is where magic becomes involved. I would hold in one hand the division to be, and then in my other hand, bend back and away the rest of the orchid. A little twist, a little pull trying not to be scared of feeling the snap of they rhizome, hopefully my intended division separate itself from the main plant. Most the time I am lucky. Most, not all. Sometimes I get a bunch of broken pieces in my hand and on the floor.
I would pay very close attention to any new growths, because breaking those means a set back of 2 years minimum for the division.
A knew growth or growths, attached to a cluster of 4 or more bulbs, is the desired division. Back-bulbs are still providing nutrients and support to the new growths. Without those few back bulbs the new growth cannot thrive, or set back be a factor and it be a few years before that first bloom.
An Oncidium with 8 to 10 bulbs be easy to divide. Hold the intended division in one hand, let the other division slowly fall away, find the natural split, bend and twist. Oh and try not to freak out when you hear the rhizome snap or feel it break in your hand. The very first few times you experience this you will be worried. I repeat, I am no expert on this. I have done my fair share of reading, watching the many videos on the internet about dividing and perhaps the best and highly recommended way to learn is spend a few days helping an advanced grower with their collection.
About an hour later, pictured below is my 6 divisions and 3 loose bulbs. Not bad.
Once the dividing process was completed I rinsed all the divisions in water to remove any remaining undesired material. The root zone get a very heavy misting of physan20 and allowed to drain and dry. I use a soft bristle toothbrush to gently scrub the bulbs above the root zone looking for the freshly broken rhizome. Once found, if it can be found, I sprinkle cinnamon on the break as a natural fungicide, which makes it useful for protecting your orchid plant from fungus and bacteria.
By now my back is sore, the orchid is miffed, and it just makes sense to stop and allow for both it and myself get a rest and finish the process tomorrow.
Above is the divisions after a third round of good cleaning. In the lower right corner is a leaf attached to a bulb lying on some roots. This was the only piece of this orchid after dividing, that I tossed.
Why Aerial Roots?
I do not know why they are formed exactly, but I will lean on the following two possibilities as to why they are formed by epiphytic orchids (Oncidiums, Phaleanopsis, etc.). Vanadas are prolific at throwing aerial roots but that is because they are mostly kept bare root in a humid environment, just as they are found in nature.
Some suggest that epiphytes form these aerial roots because the condition of the pot (water content) is extremely high drowning the old roots because the medium is decaying.
I lean towards the idea that simply because these orchids are epiphytes, they have roots buried in some organic matter as well as in the air. Both of these roots perform the same function - trap and absorb nutrients from visiting water of the local environment. If an orchid notices the humidity is of a constant high level, transporting nutrients via sporadic placement of aerial roots means less distance for nutrients to travel within the plant if left to just roots in the medium. Transporting nutrients take energy.
There is a major difference between aerial roots and subteranean roots. Aerial roots have a more dense velamen. They will not adjust to being "buried". The harder covering of the velamen of aerial roots is a protection for the actual roots from bending to the point of breaking, exposure to the sun and wind. They absorb water, but at a slower rate then the buried roots. They also retain water from a slower rate of intake by the root, so gas exchange helps them dry, where as being buried, they can rot.