An orange tree full of ripe fruit is something every dooryard citrus grower works for. If your oranges are not turning that lovely orange color, it is a process of elimination to understand why. Physical, cultural and environmental causes can be the culprit for nonripening citrus. Some problems are not real problems, some are easy to fix and some cannot be fixed. The first step is to determine exactly what is causing your oranges to stay green.
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Varieties and Seasons
Oranges don't all ripen at the same time. There are early, midseason and late varieties. Early varieties, such as "Hamlin" and "Navel," ripen around November and December. "Parson Brown" is an early midseason variety that ripens from October through January. "Pineapple" and "Midsweet," two midseason varieties, ripen from December through March. "Valencia" is the latest variety, ripening from March to June.
Idiosyncrasies and Temperatures
If the weather is unseasonably warm or cold, the oranges may not ripen on schedule. Citrus needs a certain number of hours of the proper temperatures for fruit to fully ripen, so if the summer or winter lasts too long, ripening may be delayed. Valencia oranges have a temperature-based idiosyncrasy that may make it appear that the oranges are not ripening. If ripened Valencias are hit with a warm spell, the skin may reabsorb chlorophyll, resulting in ripe oranges turning partially green.
Huanglongbing, also called HLB or citrus greening, is caused by a bacteria carried by an Asian citrus psyllid, Diaphorina citri Kuwayama. The first symptom of citrus greening is an uneven yellowing on leaves, called "blotchy mottle," which spreads across the tree over a year or less. Infected trees produce few fruits, and the fruits they do produce are abnormally small and may be lopsided. The fruit fails to ripen properly, remaining green at the stylar, or stem end. Infected trees should be destroyed.
Choosing the right variety for your needs is the most important consideration in having oranges ripen when you need them to. Proper watering and fertilization can also be instrumental in ensuring that oranges ripen on time. These are the two things you control. If the tree was already on your property when you moved in, you may have to replace it with a more suitable variety. Earlier-ripening varieties are better for part-time residents in orange-growing states, and having someone care for your tree when you are not in residence may make all the difference.