Sour oranges may come from rootstock

: I have five orange trees that stand about 12 feet tall. They were mature when I bought the house and I have taken care of them for the past four years. My problem is the fruit looks good and smells good, but tastes bad. I tried taking the fruit at different times of maturity, but there’s no difference. Are these just ornamental or can they be edible?

A: I am guessing that what you meant by “bad” is that they are very sour and inedible. If that is what you mean, then most likely you are tasting the fruit produced by the rootstock, not the original citrus tree that was planted.

Most citrus are grafted trees. They are grafted or budded on to a citrus rootstock that gives them a particular advantage over being on their own roots. Advantages of citrus rootstocks over planting citrus on their own roots might include tolerance to winter freezes, resistance to some diseases, resistance to nematodes and better tolerance to saline soils.

A very popular rootstock is “sour orange.” Sour orange is an orange but, as its name suggests, the fruit is extremely sour. The tree can withstand very low winter temperatures for a citrus so it is valuable as a rootstock in cold climates. If a winter-tender citrus such as navel orange is budded on a sour orange rootstock and winter temperatures reach below about 25 F, then it is possible for the navel orange to die (the top) but not the rootstock (sour orange).

In the spring, new growth sprouts from the above-ground part of the cold-tolerant rootstock while the tender navel orange is dead. One strong sucker growing from the rootstock takes over and the dead navel orange part of the tree is pruned out.

In a couple of years the sour orange flourishes and replaces the navel orange tree. You buy the house and now you are confused because you are expecting a sweet orange and your oranges are not good to eat.

You may be able to tell if it is a sour orange tree. The fruit, when it is mature, may be very juicy, sour but often hollow in the center. The rind is rough, bumpy and very aromatic. The leaves and flowers, everything about this tree is aromatic, much more so than sweet oranges so you will probably love this tree when it is flowering. In fact, the flowers are often distilled for orange blossom aromatics and used in perfumery and cooking and are highly prized in Mediterranean countries.

If this is what happened, then you do have a cold-tolerant citrus that is very aromatic but with inedible fruit and there is nothing you can do to improve the flavor.

Bob Morris is an associate professor with the University of Nevada Cooperative Extension. Direct gardening questions to the master gardener hot line at 257-5555 or contact Morris by e-mail at morrisr@unce.unr.edu.