Jenny Harris, of Balclutha, asks :-
My lemon tree grows leaves and lemons but it also grows thorn type leaves on some of its branches. Is this normal?
Vincent Bus, a horticultural scientist at Plant and Food Research, responded.
Citrus, and in particular lemon trees are known to produce 'canopy sprouts' that grow out at 90 degrees from the stem and main branches. As well as being a nuisance, these thin branches, often showing many thorns, should be pruned off right at the base because they cause damage to the fruit, which in turn become entry points for diseases.
As is common with most garden and orchard trees, citrus trees are composite trees consisting of a scion cultivar grafted onto a rootstock, for example, sour orange. If below the graft union low to ground, the thorny branches may also be shoots sprouting from the rootstock, some of which have a very thorny growth habit since they are selected for their ability to control tree vigour and/or resistance to pests and diseases rather than their fruit quality. The shoots growing from these rootstocks should be removed, too.
Thorn-type leaves on the branches of the tree may also suggest that the tree is growing under sub-optimal conditions and may have health problems, particularly if this is associated with yellowing of the leaf margins. If there are only a few thorns, the problem may not be of concern, but more extensive symptoms would need to be addressed. The cause of the problem may well be underground, e.g. overwatering or diseases in the soil affecting the functioning of the roots.
Therefore, to prevent health problems, the trees should be planted in a fertile well-draining soil that is just kept moist rather than wet, and receive plenty of light. Yellow leaves may also be an indication of the tree receiving insufficient fertilisation.
The 'Meyer' lemon commonly grown in New Zealand usually does not have large thorns, but occasionally may do. It is also known as being one of the most cold-tolerant lemons, hence will grow in most parts of the country.