Garden Detective: Lemon tree’s monster fruit has them puzzled

01/25/2014 12:00 AM

01/24/2014 10:54 AM

We have been puzzled over the last two years. We had a beautiful lemon tree, probably 9 years old now and 10 by 10 feet. Something happened and now the tree basically produces only gigantic lemons or pomelos. They do not have a good taste. We are really disappointed because this tree used to produce 100-plus lemons each year. This year we have 20 to 25 nuclear “lemons.” These “lemons” are now the size of a person’s head. First question, what happened to cause this change? Second question: Can anything be done?

Mike and Melissa Thompson, Sacramento

Your misshapen fruit may be growing on sucker wood, say UC master gardeners. Lemon trees almost always are grafted onto rootstock of another citrus to provide disease resistance and faster maturation along with other benefits. Root stock also may be used to dwarf the tree size, but typically it does not produce good fruit.

One way to tell the difference between suckers and the grafted (good) tree: Look at the leaves.

Dwarf citrus often are grown on rootstocks of trifoliate orange ( Poncirus trifoliata). Trifolate has a distinctive three-part leaflet and very long thorns, but bears very bitter odd-shaped little oranges.

Most likely, your tree’s rootstock is rough lemon ( Citrus jambhiri), which is very hardy to cold temperatures (and popular with Northern California citrus growers), or a variety of pomelo ( Citrus maxima), such as thick-skinned Cuban shaddock. The pomelo leaves look more rounded than the grafted lemon tree’s leaves. Rough lemon may have similar-looking leaves, but its fruit looks like gigantic deformed lemons.

From your description, the tree has developed a huge sucker from the rootstock that must be removed. That sucker can basically take over the grafted lemon tree, which seems to be the case with your lemon. If you can remove the sucker, you may save the grafted tree and get it back to producing good lemons.

Look at the base of the tree. There will be a distinct ringlike formation in the bark on the lower trunk. Above the ring is the grafted variety; below is the rootstock. The graft may look like a large lump or swelling on the trunk, about a foot above ground level.

The bark above the graft scar will have a different appearance and texture from the bark below it.

If you find sprouts or branches that rise from the rootstock or trunk below the graft, cut them off. This can be a big job and somewhat alarming; these branches may constitute three-fourths or more of the tree. If so, they are stunting the grafted variety and preventing its fruit production.

Root suckers should be removed as soon as possible. They should be cleanly cut off next to the trunk. General pruning of citrus should be done in late spring after the last killing frost (usually mid-March in Sacramento).

Also remove “water sprouts” – vigorous shoots that grow straight up from branches or trunks. Water sprouts can be removed at any time and generally do not bear quality fruit. Prune them 6 inches below the top of the canopy, and they will gradually break into multiple branches and fill out the canopy.

After removing the suckers and water shoots, give your tree a deep watering and some balanced citrus fertilizer. That will help induce the grafted tree’s branches to start growing again – and hopefully bear lemons.

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