aerial roots

[hohcs]

another beginner's question:

how does one know if a root is aerial or not? i have phals and when i repot them i just bury all the roots, does it matter? i do not think i bury the plant too deep, just that some plants start to grow lopsided and when i straighten them in repotting, one side where roots were exposed is now covered. right?

c.s.

[uncasteeb]

how does one know if a root is aerial or not?

Aerial roots don,t go into the medium so if they are outside the pot they,re aerial.

[Jmoney]

burying aerial roots may cause their demise if the medium is dense enough. unless the plant doesn't have any regular potted roots, it's probably better to leave the aerial roots outside.

[Sue]

With Phal aerial roots, if you're using a nice open, well-draining mix, I'd think that they'd do all right being buried. If you use a dense, mud-type mix, then it might be a problem.

Or in other words, I agree with the comments above.

[LJA]

Ditto to what was already said. If the aerial roots are in active growth, they'll have a better chance at surviving a dense mix, otherwise.... It's best to repot when you start to see roots emerging from the stem: that way you know that the roots are actively growing, and any aerial ones you bury can form branches into the new medium.

[hohcs]

thanks for your replies. my plants devleop aerial roots or roots above the medium maybe because the previous medium was too wet or decomposing. now they are in a mixture of prime agra and diatomite in s/h. i buried the aerial roots or roots formerly above the medium because they seem to sit better that way. it has been a few days the base of the roots, as much as i can see through the stones still looks fat, firm and green. i think it should be fine, i hope. there is also no signs of the leaves going limp or yellow. so perhaps no roots are dying? true?

also these plants are developing new buds on old spikes and the new leaves continue to grow, all good signs, right?

the reason why i use diatomite and prime agra mix is because i am running out of prime agra and so i mix in some diatomite which i bought. i discovered that the hq of diatomite usa is here in buffalo and went down to his place to purchase 2 bags of it. so now i can getting rid of all peat kind potting mix from all the orchids and also my african violets and planting them in straight diatomite. the african violets seem to like it better.

diatomite is quite a special medium. it feels cool and moist like peat but not soggy or wet. i think it is ideal for all kinds of plants.

so now as a report to anyone who has been following my attempt at and conversion to s/h, i have now 2 phals in complete prime agra. one of which is not doing well because of the severe root trim i gave it during repotting, and the other is picking up very well growing new leaves really fast and also putting out i think either roots or spikes soon.

then a few days ago i repotted another 2 phals into the remainder of prime agra topped up with diatomite. the first of which has more prime agra than diatomite and the mix is evenly spread throughout the pot. the second has more diatomite than prime agra and i have the prime agra at the bottom of the pot in the reservoir and diatomite on top, because the diatomite is smaller and also they stay moist longer, i think, so it will keep the whole mix more evenly moist throughout the entire height of the pot. i hope these 2 settles in well because they are developing buds on the spikes and seem like i am getting flowers soon. tghe reason why i chose to repot them now is because they seem to be doing strong and also having new roots and also the potting mix seem to be rotting. when i took the plants out of their pots, the mix has a strong earthy smell and the mix of bark and peat seem to be powdery at some places. so instead of waiting for them to finish flowering which might take up to a few months for phals, i thought i should do it now. let's hope i am making the right move!

ok, so much for now. any comments on the above is welcome!

[LJA]

On monopodials, aerial roots can develop pretty much anywhere along the stem of the plant. With Phals, you might get a root growing out of one side of the stem, and a flower spike growing out the other side, at the same level in height. But there's a crucial difference in structure between a root that grows aerially, and one that grows submerged in the medium.

If you unpot a Phal that has both aerial roots and submerged roots, the first difference you can notice is the color. Since aerial roots are exposed to light, when they're wet, they can take on a greenish-grey cast, and their growing tips are often bright green or reddish-purple. They carry the same pigments the leaves do, and they can help the plant carry out photosynthesis.

Roots that have naturally grown submerged have no such color. Older ones are often dark brown, and newer ones are a clammy looking yellow-white.

What's not immediately apparent though is the difference in velamen structure. The whole point of velamen (the thick, fleshy or spongy tissue around the actual root thread) is the conservation of water. In aerial roots, the velamen's outer epithelial cells form a thick, basically one-way water barrier. When those cells get wet, they allow water to passively enter. When they dry off, they keep the water inside from getting out. Those outer cells protect aerial roots from dessication, and only aerial roots will develop that outer wall of dead, "shield" cells to such a great extent.

In submerged roots, the outer wall is very thin, and sometimes completely absent. The plant has no need to develop an "air-water shield" on submerged roots since moisture is a lot more abundant and easily obtainable. But with no shield in place, submerged velamen is also a lot more permeable to air. It has to be to carry out gas exchange in a potted environment that's relatively "air-poor."

When you go to repot, and you take roots that have developed aerially and submerge them into an environment that has a lot more water, you upset the original air-water balance that their structure has been built around: an original environment with lots of air, too much of which has to be kept out, and relatively little water, as much as possible of which needs to be absorbed. With their one-way shields designed to let water in freely but keep out drying air, water goes in, not enough air can *get* in, and the root thread inside the velamen drowns, dies, and rots.

The opposite is also true. If you take a plant that's grown its root system submerged, and you decide to mount it on a piece of bark where roots that have been submerged are now exposed to constant air movement, without that shield of dead cells in place around the velamen, those roots will die from drying out, often dessicating in a matter of days: *too much* air passes in, too much water is allowed to escape.

This is why it's best to do any kind of repotting--especially the kind that will submerge aerial roots or expose submerged ones--when the root system has just started the season's active growth, and new roots are developing. It won't change what will happen to the roots that are already there, but it will give the plant a chance to grow its new root structure to coincide with the environment you've placed it in, and the new root system can take over for the old roots that are going to die off because of the changes.

[cedric817]

Louis -

Your functional description of velamen and various types of roots was extremely helpful. I've always read to repot when roots were actively growing, and so I've done that. But, I never really thought about the 'why' behind the recommendation. A wealth of information, as always...

Christopher

[LJA]

Thanks Christopher. The actual goings-on are a lot more complicated than I described (it's a little like trying to explain an atom's electrons and nucleus in terms of planets revolving around the sun instead of quantum theory--not quite what's happening, but useful as a way of envisioning the process), so if anyone out there is a plant biologist, please forgive my liberties and feel free to jump on in with more details or a more accurate representation....