Answer: It sounds as though your lemon tree is suffering from weather stress. These plants will tolerate an occasional frosty night, but temperatures in the 20's are just too cold. I suspect that's why your tree is losing leaves and fruits. You may want to take it indoors overnight when temperatures threaten to drop down to the 40's.
Here are some general care guidelines for your citrus tree:
SOIL. Provide good drainage. If your soil is clay, mix organic matter - peat, compost, leaf mold - into the soil before refilling the planting hole. Or plant in containers or raised beds.
If your soil is sandy, mix organic matter into it to increase water retention while keeping good drainage.
WATERING. Citrus trees cannot stand to be overwatered. Water a newly planted tree (or a tree in a container) thoroughly about twice a week in normal summer weather. Once it is established, water a tree in the ground every two weeks.
When you water, fill the basin so that the entire root zone is moistened several feet deep. Don't water when the top of the soil is still wet.
FERTILIZING. Use packaged citrus food that contains nitrogen, phosphorus, and potassium, plus iron and zinc, crucial to citrus fruit production.
Leaf yellowing indicates either overwatering or a shortage of trace elements, especially for trees in containers. If you are watering correctly, apply a chelated iron product containing trace elements, following label directions.
PRUNING AND THINNING. Prune anytime to shape the tree, remove dead wood, or thin the branches. Remove any suckers arising from below the trunk's graft union. Varieties that bear heavily in alternating years sometimes need to be thinned during the heavy years. For those varieties, after any fruit has dropped naturally, thin developing fruits to clusters of two or three.
As fruits mature, prop up the branches if necessary to keep them from breaking.
SUN. Lemon trees need full sun, but in areas with very hot sun, citrus bark burns. Wrap the trunks, paint them with tan or brown latex paint, or leave the lowest branches in place to shade the trunks.
HARVESTING. Flavor, not color, is the only reliable indicator of ripeness. Pick one fruit and taste it - if it's not ready, wait. Use pruning shears to nip the fruit with a bit of stem. Don't pick by hand or you'll risk breaking a branch.
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