I think you guys are confusing the ill effects of over-heated roots with sunburn. There is little question, since plumerias grow in full sun in equatorial regions that they will tolerate any sun load North America has to offer, including TX. What they WON'T tolerate is the inhibition of root function that comes with over-heated roots.
Photo-oxidation (aka sunburn) has little to do with ambient air temperatures. Sunburn is caused during high light intensity levels w/o adequate amounts of protective pigments present. This causes chlorophyll molecules to rise to a more excited state than normal. If light levels are high enough, the energy that is released as electrons in molecules return to their normal energy state may be sufficient to form oxygen (O2) radicals from regular O2. These O2 radicals are extremely reactive particles that readily destroy chlorophyll and other organic molecules. (This is the same O- radical that causes rapid oxidation [bleaching] when we apply H2O2 [hydrogen peroxide]). Evidence of the process is seen in a bleached or whitish/tannish/silverish appearance covering large areas of the leaves, and is also often referred to as peroxidation. The symptoms of sunburn occur randomly on the leaf. If you have burned leaf tips and margins, it's not sunburn.
There is also confusion about what a soilless mix is. A soilless mix is a soil that doesn't contain a mineral fraction, so MG, MGMC, and almost all other soils from a bag are soilless mixes. The only two things that would make one soilless mix "hotter" than another are it's color (darker mixes would have more passive solar [heat]gain), and it would have to be exposed to direct sun for that to occur; and evaporation, with the more highly aerated soils being cooler because of evaporative cooling. Taking this a step further, soils like the gritty mix, even the 5:1:1 mix are going to be cooler by several degrees, simply by virtue of the fact that more water evaporates from soils with higher rates of gas exchange.
So, it's not a technical issue that hinges on word play when we observe that Texas sun doesn't burn foliage, it's the heat (not the light). Everyone has their own little tricks they use to keep roots as cool as possible, and I won't elaborate because it's been discussed so frequently, but the goal should be to provide as much sun as you can while keeping root temperatures within a range that allows normal root function ..... which leads to some considerations about how not to keep root function at optimal levels. One way to ensure problems with foliage is to inhibit root function. Many growers seem to be too eager to give up aeration for the additional water retention offered by soils like MG and other bagged mixes that are exceptionally water-retentive.
There should be no reason to disagree with what's in the following two paragraphs because all the bases are covered. Growing in containers is about perspective and balance. There is no sense in my arguing a point from the plant's point of view when someone is approaching the issue from the perspective of grower convenience or their ability to provide ideal conditions. We all arrange our priorities in a way that allows us to best enjoy life, and who should judge how we arrange our priorities? If a grower has time or can only manage to water once per week, then the grower MUST use a soil that holds enough water to keep the plant's water needs met for the duration. That satisfies the grower's prioritization, but the plant isn't going to like it. That's simply a fact we need to acknowledge and go on from there.
Growing is also about balance. As mentioned, some growers THINK they need to use heavy soils that lack aeration and impede root function to get the water retention they need to get through the day. If they do, that's fine, but the fact is, these soils still do impair root function - even if they are completely necessary. Balance comes into play when we make conscious decisions based on our knowledge of soils, decisions that take into consideration how much inconvenience we're willing to tolerate in exchange for increased growth vitality in our plants. No one cares or judges you for the decisions you make re. how much time you can devote to your plants, but let's recognize there ARE decisions to be made and they DO affect our plants.
By adopting a highly water retentive soil, a grower is actually putting the goal of providing an adequate water supply to the canopy further out of reach. It may be necessary to make that compromise because of prioritization, but the simple fact is, heavy soils make it HARDER for the plant to move water to the canopy and cause spoiled foliage - starting mainly with the most distal parts - leaf tips and margins. The reason they (heavy soils) are chosen doesn't matter and doesn't change their impact on the plant - it is what it is.
I have no stake in what other growers use as a soil, but there are some things to consider. I mentioned this upthread, but it's important enough to repeat: Uptake of water and nutrients isn't a passive or energy-free process, it's energy driven and adequate aeration is a key ingredient, without which the root system cannot carry on transpiration, which is the energy-driven process on which the uptake of water/nutrients depends. From this we can clearly see that it makes good sense to try to maximize water retention while minimizing the negative impact that almost always has on aeration. We want the best of both worlds, but some combinations of soil ingredients simply make it impossible to have both. A perfect soil would be made of particles large enough that no perched water is held in macro-pores to inhibit root function, and would hold all of it's water inside the particles and in the interface area where particles contact each other. That's exactly the model the gritty mix is based on. If someone has a better model or can even conceptualize something better, I'd like to hear it. That's not me being bristly - just trying to get you to think before you disagree.
A few growers here have indicated they don't think they get enough water retention from the gritty mix, yet I have several friends in TX and SOCAL I've been helping with their figs, citrus, and other fruit trees in the gritty mix for years. This isn't a commercial for the gritty mix, but there is no reason it won't work in TX for plumerias if you take advantage of its adjustability for water retention and build more into it when you make it. The advantages are a much greater improvement in aeration and root function that in concert work together to help roots move water more efficiently. ANY well-aerated soil increases root efficiency and the plant's ability to move water/nutrients, and any poorly aerated soil, especially those that support significant volumes of perched water, inhibit that ability. Period. Wishing real hard doesn't change that fact.
No matter where a grower lives, the best shot at having containerized plants that grow as close to their genetic as possible includes a soil that maximizes aeration first, and then compromises a part of that aeration to provide the grower with watering intervals he's willing to tolerate. Plants in soils that REQUIRE watering twice each day have greater potential than plants in soils that require a single daily watering have greater potential than plants in soils requiring water every other day have MUCH greater potential than plants in soils that only require watering weekly .....
I truly hope this effort on the forum's behalf is viewed by all as helpful.
Al
