Our mystery seed last week was indeed from a grapefruit, Citrus × paradisi (hybrid).
Grapefruit are interesting plants for a number of reasons.
Grapefruit are called trees, but when allowed to go natural are more like large shrubs.
Here in Arizona they flower in the spring, usually in March.
The flowers are favorites of honey bees, which are important pollinators.
The green fruit grow through the summer and then begin to turn yellow on the outside in late October. If not picked, the fruit may hang on the tree for up to a year. or even longer.
Another surprise: The seeds may begin to germinate within fruit that has been hanging on for long periods.
Although citrus trees look similar when not in fruit, there is one way to tell what kind it might be. Lemons have a bare petiole (where leaf attaches), as seen on the left. Oranges have small wings on the petiole (center). On the right is the grapefruit leaf, which has such large wings on the petiole it resembles another small leaf.
As Sarah at Share It! Science is finding out, it is possible to grow a citrus tree from a grapefruit seed.
The grapefruit featured above is from our yard and we are very fond of it. Do you have a favorite kind of citrus?