My meyer lemon lost it's leaves. The branches are still green, and look ok, so I am hoping it comes back. The leaves dried and curled, so it might be cold soil - I used an instant-read thermometer and found the the soil temp was 58-60 in the morning, and 65-70 during the day. Doesn't sound like it's too cold. I have two options - move it to a CHC/coir mix, or move it outside. The goal is to get it outside eventually, and leave it there, so I'd like to move directly to outside. It's cool here right now - 38 at night, 45-50 during the day. If cold roots did cause the leaf drop, wouldn't it have the same issue outside? Where the soil is cool yet the weather sunny? Would it be too much of a shock? It's not thriving in the pot - I suspect the soil does not drain well enough. When I pull it out, I will have to examine the roots to see if they are still viable, possibly cutting them back a bit if there is damage.
If im not mistaken according to the veteran citrus growers if the soil temp is much below 65 F the rootstock becomes somewhat dormant and does not supply the tree with any added nutrients needed to sustain existing foliage let alone produce new growth. So if your outside ambient temps are still low as you mentioned you might simply repot in a new good draining medium, inspect the current rootstock and keep the tree inside for another month or so until your outside temps increase. I live here in South Texas with adequate outside ambient temps and I still havent mastered the tricks with this improved meyer lemon. Maybe one day I will. Regards & Good Luck !
NOTE: I answered this post before reading your other post, where you advise that the problem container is actually a rapid draining container. Therefore, some of this information might not apply, but I will leave this post as is, because it contains some valuable information concerning container citrus. Davidgriffiths, What is the growth medium (potting soil) mixture that the troubled Meyer is currently growing in? How large is the container, and how tall is the tree? Concerning root zone temperatures, at temperatures of 55.4F or below citrus roots cannot function. Therefore, at temperatures slightly higher, but close to 55.4F citrus roots still have a hard time supplying enough water when the tree is setting directly in the sun's rays. During the cold time of the year, it is best to maintain a root zone temperature not lower than 64F. The optimum temperature for root growth is 86F. At soil temperatures above 95F citrus trees discontinue growth. If your tree's leaves dried curled up and fell from the tree, than your tree's roots are not sending water to the foliage. There is a problem with the tree's roots. The #1 requirement for a containerized citrus tree is a growth medium with high air porosity, and good drainage. Citrus roots require a high level of soil Oxygen. If you suspect that your tree's medium is not fast draining than you need to make a change. Without enough soil oxygen, citrus root cannot absorb water, no matter how wet the medium is. The opposite problem is when the medium is to porous, and the drainage is so rapid that not enough water is absorbed. In one of your post you mentioned, that the Meyer Lemon having the problem, is in a larger container. One of the biggest mistakes is usually made not in the type of pot but by using too large a pot. People think that they are doing their tree a favor by giving it a lot of room. But if the tree doesn't grow fast enough to use all of the space, trouble can quickly occur. An over sized container can become waterlogged easily, and then the tree can suffer from lack of air. Chances, are that the problem is caused be either: (1) to large of a container, (2) medium with poor draining, (3) low medium air porosity, causing the medium to stay to wet. (4) low soil oxygen and high soil CO2. - Millet (1,399-)
