By John M. Chater
Huanglongbing (HLB) has changed the citrus landscape in Florida in many ways. One important consideration is rootstock selection.
HLB affects the growth rate of a citrus tree substantially. Trees grow at a much slower rate on average compared to healthy trees. In the citrus under protected screen (CUPS) environment, trees can grow faster than a healthy outside grove. These different conditions can affect a grower’s rootstock selection.
Many older Florida groves were producing very large trees on wide spacing with somewhere around 124 trees per acre (10-foot by 35-foot spacing). Other groves were planted at less than 100 tree per acre. Some of these older groves were known to have up to 35-foot middles (drive rows). This would cause the grove to have very large trees that could produce many boxes of fruit but required ladders for harvesting.
Now, in the era of HLB, it is a challenge to grow off such a large tree, with the effects of the disease and other pest and disease pressures exacerbating the slower tree development. In recent years, considering it is difficult to produce trees to fill these large grove spaces, growers have opted for higher planting densities with over 200 trees per acre and in some cases 400 or more trees per acre. Having more trees per acre allows for the possibility of greater yields, but high planting density increases the cost to plant the grove because more trees cost more money.
The top rootstocks currently used in Florida include US-942, Swingle citrumelo, sour orange, X-639 and US-812. The top rootstock for the last several years, with the exclusion of the 2024–25 season, has been US-942, which grows off a small- to intermediate-sized citrus tree. Swingle grows an intermediate-sized tree. The other top rootstocks typically grow off large trees.
With rootstocks that grow small trees, it is often the best strategy to plant at a higher density because the trees will likely not be able to fill their space in the grove. A 10-foot by 20-foot grove spacing would allow for 218 trees per acre. A tighter in-row spacing, such as 8-foot by 20-foot, would allow 272 trees per acre.
In CUPS, where trees grow quickly, a size-controlling rootstock is probably the best decision a grower can make. US-897 and C-35 are selections that have worked in CUPS. Something very small or slow-growing like Flying Dragon does not do too well in terms of production, but fruit quality can be excellent with that rootstock.
If mechanical harvesting becomes common, it is likely that smaller trees will be used. The mechanical harvesting equipment available typically travels over the trees when harvesting, so larger trees would not be optimal.
In summary, if you wish to have a high planting density, choose a size-controlling rootstock. Lower planting densities will probably benefit from rootstocks that grow a larger tree.
John M. Chater is an assistant professor at the University of Florida Institute of Food and Agricultural Sciences Citrus Research and Education Center in Lake Alfred.
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