have you changed the light availability (extended overcast weather)? Changed type of ferts? If its in a container, do the roots need a trim? Temps staying consistent? Did they get a winter rest period (dryish) or just growing nonstop?

Doubt if roots need trimmed at this point but we are in winter and sunny days are hit and miss. Probably did not give them much winter's rest. Difficult to mangage in containers. I can back off the water until they really need it I guess. It's almost like...why all of a sudden?

I have been observing this issue for a long time. Anytime when there is new growth drops off, that means the plants are not getting enough sun or the root is too weak to provide enough water for the leaves. The new growth is usually very tender, quite often pushed by fertilizers, the plant will use stroed sugar to push out a splash of growth, when the stored sugar is used up but finally can't be supported by enough sunshine or root system, some new growth will eventually drops off. The plant is adjusting itself to its environment. My trick is to ease off on the fert, keep the plant in dormant as long as possible(can't be done well if indoors) and use some artifical light during cloudy days. I have been using this trick on my fussy Osmanthus tree, last year it droped half of its new growth, but this year, less than 5%.

Quite often, we just ignore the importance of plant dormancy. But it's so important for sub tropical plants, if not getting enough, it usually means weak growth the following year. I usually put these plants in my unheated garage for at least a month. This works well for me so far.

Looking a lot more closely at the lychee last night, I discovered a few scale on the branches or in the crotches of the branch and stem. Light and peach colored. I can't see where these "few" could have caused leaves to drop.

Thinking back though...about a week ago, my humidistat locked open in the middle of the night and caused my fogger to kick on for...at least an hour or so. The moisture was so dense it even caused the CO detector to go off. It was wet! Now I'm wondering if the dense fog could have cut oxygen back far enough to cause the heater to burn inefficiently and send out harmful by-products? It was a few days after this that I noticed the top new growth of the longan turn completely black and start to fall off. Some new but mainly some older leaves of the lychee started turning real brown all around the edges of the leaves...these eventually started to fall as well.

Do you think this could be an affect from that night? I've never heard of leaves turning black like that. I know that harmful gasses can be put out of a heater is burning incorrectly. If this is the case, shouldn't it clear up soon?

OJ, you may already know this, but lychees are very sensitive to fertilization. I treat mine with a topdressing of well-rotted manure and dilute Miracid fert about 4 times a year, that's all (mine are in the ground bed). I was losing a lot of leaves when I was fertilizing more.

Are the plants in front of a fan? Last year I lost lots of new lychee growth in this manner, as the new growth is extremely tender and fragile.

I agree with yellow thumb about dormancy. I've been trying to lay off of the H2O a little bit this year and let the plant store up for spring. My older tree is siting quietly and my new tree has only one small new branch. We'll see.

I'll watch the fertilizers and actually steer clear of them till spring. I'm still wondering about the toxicity from that night.

Jay,

You have the right concern. The worst thing you can do to indoor plants is to cook (stir fry with heated vegi oil), smoke and run a gas engine in front of them. Many plants are more sensitive to air pollutants than us, they can't walk away from it. Some plants are tough to this, but many sub tropical plants reacts badly to this, leaves turning yellow, curl and dropping. Osmanthus will just knee down and drop all its leaves if exposed to air polluted with vegi oil gas. Camellia will drop its bugs like crazy. I have to be careful with my unheated garage, each time, I truned off my car, I will leave the garage door open for a minute or two to release the air pollution.

The only recommendation I can give you is give them lots of air circulation, humidity and ease off on fert.

Here is what I read:
Air pollutants mainly penetrate a plant through skin pores. These are in connection with the intercellular spaces in the leaves of a plant. Through the skin pores gasses can penetrate cell walls, to be absorbed by the cell fluids. Some of the gaseous air pollutants directly damage plant leaves when they penetrate plant cells. This can cause the leaves of plants and trees to lose their colour, or even die off. It can also cause plant growth to stagnate. The leaves and stems may slacken and curl up.

Besides the fact that lychee leaves have a limited lifespan altogether (actually, I think this is true of all of the Sapindaceous species we seem to be interested in - lychee, longan, rambutan), I've noticed that they drop their older growth following periods of stress as soon as new flushes with healthy leaves become available. This especially true of any situations wherein the stress has somehow affected leaf development, such as insect damage during the leaf's juvenile stage.

As for the possibility of harmful gas effects, I would check the other plants in your considerable collection for signs of similar damage - in particular, if the rambutans had the same reaction then you've found your culprit. Otherwise, I wouldn't worry too much about it; younger lychee and longan trees appear to drop leaves in some instances simply to accommodate new growth. As the tree becomes more established, the leaves should hold together better.

Thanks all. I'm hoping the affects wear off and I'll watch the watering and fert as well. Can't really tell if my rambutan was affected...remember? I COOKED ALL THE DAMN LEAVES!!

I'm not too worried about the lychee. I think it will be fine. The thing about the longan is that it has shown a tendency to lose older leaves on the bottom when there is new top growth...unfortunately it hasn't shown any effort into replenishing the areas where it has dropped it's leaves! Goofy tree. Seems like just when things give you a big boost of "thataboy", something comes along to give you a boot to the groin.

Jay, longans dont tend to replenish the older leaves on the bottom because they dont usually branch out until you do some pruning. Pruning will trigger the dormant buds to start growing. I once had a seedling that grew to about 5ft with just one trunk and stem until I prune it. Especially if your only getting vegetative growth and no flowering pruning may be necessary to get it to bush out.

I see your point but it seems that I would have to prun off the entire section of leaves and such to get it to start growing out lower! Maybe I could try that with one branch at a time and see what shakes loose. Just looks really odd this way and is definitely top heavy. I've even tried grafting some pieces down lower with absolutely zero success. I obviously suck at grafting! Might wait to see if it blooms this spring. If not, I'll start whacking away.