What REALLY causes root death?

[ScottMcC]

I'm making a foray into the advanced forum briefly.

I got to thinking this morning about what actually causes roots to die. I've heard a lot of conflicting stories, some of which likely old wives' tales, some of which backed by science, and some of uncertain veracity.

Anyway, the most popular story is overwatering leads to rot. Some would clarify and say that keeping the roots too wet, without allowing them to dry is the culprit.

Others contend that roots need access to good air flow, and that overwatering only causes problems if it blocks air flow (and hence cellular respiration).

Some people say that a certain percentage of roots are going to die off regardless, and that this is a natural part of the plant's growth cycle.

Anyway, here's what got me thinking. When transplanting into semihydro, one of the main tenets is to do so during a time of active root growth, because only actively growing roots are able to adapt to that environment instead of standard culture. I've noticed that especially with cattleyas, a good portion of the old roots are going to get soggy and die. This typically isn't a problem though, because the new roots take over pretty quickly.

Based on this observation of the old roots dying, I've come up with a new hypothesis that someone should test. Roots can die from overwatering, not just because of lack of air flow, but because of edema. This edema is a result of roots which were previously adapted to dry conditions, and so are very efficient at absorbing any available water. Unfortunately, this means that they absorb too much when water is more available, and then become fluid overloaded and unable to transport nutrients and waste products appropriately, and so die. The newer roots, however, adapt to the wetter conditions as they are forming, but as a result need constant moisture or they will dessicate and die. This would also seem to explain why aerial roots should remain aerial, and underground roots should remain underground when repotting.

Like I said, the above explanation makes sense to me, but is as of yet untested. I'd like to hear any comments, and if someone has some actual scientific data on this topic, I'd love to hear it.

[Ross]

I definitely am no expert on this subject but have a couple observations of my own. As for aerial roots should remain aerial, when I repot my Phals that have roots hanging over the edge of pot, I always put them back in the pot and cover with bark mix. They grow big, fat and green. No longer aerial. Don't know what this means except roots of some spp seem to adapt to new environment.

How would you propose testing this hypothesis?

[ScottMcC]

well I hadn't really gotten that far...

you could test whether root aeration is important pretty easily, by sealing the root ball in an anaerobic environment, but leaving the leaves exposed to air. but I am willing to accept that the roots need air to perform cellular respiration--I think that fact is well established.

I'm not entirely sure how to test my hypothesis itself though. I was thinking that one thing to do would be to pot things in primeagra (but not s/h) and then titrate the watering frequency, even up to several times a day. as long as there's a way of showing that they get enough aeration despite the high frequency of watering, that might work. bark wouldn't work for this, since frequent watering leads to poor aeration. I guess you could even just use mounted or bare root plants too...

of course, I'm a little reticent to perform an experiment where the goal is to kills off roots...so someone else should do it and report their findings to me.

[ScottMcC]

ooh, I had an idea. you could submerge the lower half of a root in water and see if it survives. by this I don't mean the distal half, I mean cross section...that way that root segment is still aerated, but has constant access to lots of water. if my hypothesis about edema is correct, that constant access will be enough to turn that root soggy and dead, despite the air. you could even circulate the water or add fresh stuff to show that it's not infectious from stagnant water.

[littlefrog]

I have always assumed (with absolutely no proof...) that much of the root loss you see is due to opportunistic fungi/bacterial infection. Also, orchid roots do exchange air gasses, so keeping them entirely submerged can literally suffocate them. Again, an assumption that I have always had, but no references to back that up...

All of that said, I have had some luck in the distant past with completely hydroponic culture. IE, a plant sitting with its roots in a jar of water. Took a while to adjust to that culture, but I had a few den-phals that went nuts in that system. I think that came from a grower who used to be on the internet when it was 'new' - Rod Venger. He is out of business now.

[ScottMcC]

opportunists are one thing, but that implies that the host is compromised in some way. what causes this compromise? my hypothesis is that it's related to the edema, but again, I await the data.

[littlefrog]

Seems as good an explanation as any... I do wonder a bit about how the totally submersed roots in my 'experiment' managed to avoid the edema. Your point about totally new roots in a new environment is a good one, I'm pretty sure that there are different types of root developed for different conditions. Perhaps 'water roots' are a possible phenotype for some orchids.

[Ross]

BUT... where does this reaction stem from? In nature, I am not aware of any situation where orchids are subjected to such conditions. Even the Spiranthes, a terrestial which grows in muck soils, needs air "pockets" in the soil to survive. Maybe the best clue is the natural size (girth) of the root? And natural rooting depth?

[littlefrog]

An old vanda grower's trick (some people may still do this) was to take a floral pick (like a single rose comes in), fill it with stale beer, and slip it on over a big vanda root. Supposedly this gave the plant a boost. Maybe it got the plant drunk, I don't know. Tried it once, but I was never a good vanda grower.

The roots did eventually die, I think...

I don't know where the full water adaptation comes from. Plants are remarkably adaptable, though.

[Mahon]

I think that root rot is caused by different factors and situations. There could be infectious bacteria or fungi which attack the roots (which just happened to my now-dead Paph. victoria-mariae), or it could be small parasites, or it could be edema. Sometimes, roots will die off naturally... basically I just repeated all that was said... =)

Now what about Habenaria repens? This species is almost always found in small ponds or drainage ditches that are filled all year with water... I think this species has developed an advanced root system, similar to other non-related aquatic plants. If you take this species out of the water and plant in a nice humus mix, it will look as if it is over-watered and die within a few days. If a plant of Habenaria repens is growing near the water and placed in a humus mix in cultivation, it will thrive.

Originally Posted by rsfrid

Even the Spiranthes, a terrestial which grows in muck soils, needs air "pockets" in the soil to survive.

I have found about 8 native Spiranthes species, including a new undescribed species, and none in situ have air pockets around the tubercles. Spiranthes praecox, Spiranthes sylvatica, Spiranthes ovalis, Spiranthes vernalis, Spiranthes longilabris, and Spiranthes brevilabris are usually found growing in sandy areas. The quartz sand in which they are found in are rich with silica. Very rarely will any of these species be found elsewhere than quartz sand, but I have seen them growing in pine needles, light humus, and outer edges of bogs. Spiranthes odorata is known for growing in humus or inside the bogs. The rare Spiranthes tuberosa is found growing in wet, sandy, limestone areas in full sun, or the south side of a road running east to west in partial sun.

I have propagated all of these species, and they grow well in cultivation. None have air pockets, most of them are growing in Florida quartz sand and added limestone. Spiranthes odorata has to be grown in an ideal Calopogon or Habenaria medium, with rich humus and added charcoal. Just a little info on the genus' growing habits...

This is a very interesting subject, I haven't thought much about root rot... =)

-Pat