Lemons (Citrus limon) are produced on small evergreen trees native to subtropical Asia. There are only a few types of true lemons available in nurseries and these can grow in U.S. Department of Agriculture plant hardiness zones 9 through 11. The peak of production for most varieties is in the winter months. However, depending on the variety and growing conditions, it is possible to harvest lemons in any month of the year.
Variety
"Eureka" lemons can often be picked year round, but the primary season of harvest is from late winter to early summer. The "Lisbon" variety also produces fruit sporadically throughout the year, though the main season is in winter and early spring. The harvest of "Bearrs" lemons is heaviest in the months of July through December. "Meyer" lemons are a hybrid variety with sweeter juice, picked mainly from November to March, but is likely to have some ripe fruit at almost any time of year.
Flowering and Fruiting Cycle
Lemons ripen anywhere between four and 12 months after flowering. Flowers most commonly appear in spring, the fruit develops over summer, and then slowly turns from green to yellow in fall or winter. You can harvest green lemons whenever they reach 2 inches in diameter, at which point they will have the typical lemon flavor. You can use them this way immediately, though they will turn yellow if left to ripen indoors. The ripe fruit often hangs on the trees for a long period without rotting, further extending the harvest season.
Temperature
Though lemons tend to produce the most fruit in winter, the trees can have both flowers and ripe fruit at any time of the year. This tendency is somewhat dependent on temperature and other environmental conditions. The gentle temperature fluctuations in coastal areas supports a longer harvest season, while interior areas with a more pronounced seasonal variability tend to flower and fruit at distinct times of year. Also, winter harvests may be eliminated in marginal growing areas because frost easily damages the fruit.
Drought Stress
Water supply also influences whether the harvest spreads throughout the year or is more concentrated in the winter. If soil moisture is consistent, lemons will blossom and produce some fruit throughout the year. During times of drought, the trees are unlikely to flower, resulting in longer gaps between periods of ripe fruit. Commercial producers exploit this characteristic by intentionally causing drought stress followed by irrigation, forcing the trees to bloom at a time that will produce ripe fruit in the summer off-season.