(The original article was written for the Hong Kong Gardening Society)
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| Leucaena can reach more than 10m in height but rarely good-looking |
The media reported that a tree struck on and
seriously injured an elderly at Shatin on 22 May 2020. Various experts were interviewed and
commented that the tree was a Leucaena leucocephala (White Popinac /
River tamarid, 銀合歡) that has weak wood strength. Some opined that the tree should not be
planted at the beginning and can be removed without application.
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| each pods with up to 25 seeds |
The above comments are partially true. Amongst the hundreds of tree species recorded
in Hong Kong, Leucaena leucocephala in fact is the only tree species mentioned
in government technical circular that it is “undesirable” and can be removed with
the weedy nature of its own in departmental operation. However, submission to the authority is still
required for private lands! This nitrogen-fixing
legume is native to Central America but was introduced worldwide due to its speedy
growth rate and many other uses (including for fodder). It is listed as one of the 100 top invasive
species by IUCN.
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| Seedling even of this size is not easy to be pull up |
The weed was introduced to Hong Kong as a
pioneer species for revegetation back to 1970’s. It can grow to as tall as 6m within 3 years
in the condition of Hong Kong and the tree then colonize around quickly,
eliminating other species nearby. It
takes pity short time for the tree to flower and to seed. Large numbers of seeds (up to 25 seeds) are
usually produced per pod. The tree also develops
deep roots quickly once germination starts. So a seedling would become very difficult to
be pulled up when it has reached just 1 foot tall! It is quite adapted to cutting, so the stump would
resprout rapidly after felling is attempted. People was soon aware of its aggressive properties,
the tree was gradually banned in local revegetation projects back to 2000’s.
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| The wood is susceptible to breakage |
Leucaena leucocephala is
relatively short-life (20-40 years). It does
not invest too much in developing dense or strong wood. Long stems or branches are susceptible to
breakage. Hence, it is never a good
choice to have any of this tree in your garden.
They are not easy to be eradicated once colonized due to the speedy and
efficient reproductive strategy. It is
highly recommended to do regular checking of your garden to see if there is any
seedling, which should be pulled up as far as it can be. Otherwise some digging may be required. For larger plants, what you can do perhaps is
to cut them close to the ground, followed by applying herbicide with a brush
onto the cut surface, or covering (and tie up) the whole stump with a black
plastic bag. These actions suppress resprouting
but you still need to re-check afterward and repeat the action once new sprout
is discovered. Locally there is few seed
disperser for this weed and it relies basically on seeds propelling from dry pods. If you get one in your garden you should also
investigate whether there is a nearby seed source.
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| The tap root strategy in seedlings |
Although the young pods and seeds of Leucaena
leucocephala are consumed as food in come counties, the plant is believed
to have toxin to some extent. Eating may
be a way for weed control if more researches suggest that it is safe for
culinary purpose!
