After Freeze, Is the Tree Alive or Just the Rootstock?

A Louisiana State University (LSU) Ag Center article explains how to determine if new growth sprouting from a citrus tree damaged by freezing weather is from the tree’s scion or rootstock. Edited excerpts follow:

With spring — and warmer weather — upon us, you may be noticing new growth sprouting from a citrus tree that sustained freeze damage this winter. It’s important to figure out whether these shoots are coming from the citrus cultivar or the rootstock.

A quick lesson on grafting is in order. The vast majority of modern citrus trees are grafted, which means a section of wood from a desirable cultivar, called a scion, is cut and joined with a separate species known as a rootstock. This creates a new tree — one that produces fruit on the scion wood on top and that has a hardy, more vigorous root system.

Rootstocks allow citrus cultivars like lemons, oranges, satsumas and kumquats to better thrive in certain climates and handle stresses such as diseases. While most kinds of citrus are still quite vulnerable in cold temperatures, they’d be even less hardy if grown on their own roots. Rootstock also can help maximize the quality and quantity of fruit.

Trifoliate orange (Citrus trifoliata) is one of the most common rootstock species used in the citrus trade. Flying Dragon (Poncirus trifoliata) is popular for dwarf citrus trees.

These tough species can survive subzero temperatures and work well as rootstock. But when allowed to grow as full-sized trees, rootstock species don’t produce tasty fruit. In fact, their fruit tends to be extremely sour — even bitter — and full of seeds. And, unlike other types of citrus, their branches are covered with unsightly thorns.

So, take a close look at your citrus tree. Do you see thorns on the new growth emerging from the base? If so, that’s the rootstock, and it’s possible that the desirable citrus cultivar died during a freeze. If you’re sure that you only have rootstock left, it’s best to cut down the tree and replace it with a new one.

If your preferred citrus cultivar still seems to be living but you see a few thorny rootstock shoots at the base of the tree, now’s a good time to trim those back to keep them from taking over the tree. They can grow quickly!

Source: LSU Ag Center