Background

Lead tree, river tamarind, faux mimosa - Travel through just about any alleyway in the Phoenix metro area and you are sure to come across this tree. It will usually be loaded with reddish, thin seedpods and likely have many small lead trees around it. Native to southern Mexico, it was originally planted as an ornamental here during the 1970s and 1980s. Supposedly, the tree was promoted as a “miracle tree” with many uses for the pods and wood. The bark (like many trees) was used in folk medicine to help relieve pain. Some Asian countries (particularly Laos and Indonesia) imported it and incorporated its prolific seed pods into various food dishes. The pods have also been used as a high protein animal fodder - until it became known that it causes hair loss in donkeys and horses.

The trees grow unbelievably fast and spread quickly - trees less than a year old can produce copious amounts of viable seedpods that will grow wherever they touch soil. An advantage to wandering alleyways is that we are given the opportunity to really watch plants grow. I took the following picture of a small bit of lead tree brush growing in a dirt alleyway around 20th St and Camelback during a routine patrol, and marked it for removal.

Well, as it was a small bit of brush it was given low priority and not removed by tree crews when they came through the area. A few months later in response to a homeowner request, I found myself staring at a tree that looked familiar… This was growth during a very hot and dry summer - March 26, 2017 to September 22, 2017.

Look at that growth in six months! From barely taller than the cinderblock wall to reaching up and above the communication wires into the secondary power lines! The increase in diameter is significant as well - that’s easily an inch and a half of growth. And the canopy! Those two small little twigs flanking the main stem in the first picture became enormous! What makes this more impressive is that this was in an alleyway, with no supplemental water, over a summer where the highs stayed above 115F for weeks, against a cinderblock wall (heat radiator)!

You can see the beginnings of seedpods in the second picture, and flowers (barely) in the first. Given an opportunity these trees will rapidly outgrow and crowd out any vegetation. Most trees will slow down their growth after the first few years, but these guys will consistently grow ever more dense and taller - up to around 65ft (20m). The lead tree is listed as one of the 100 most invasive plants by the IUCN. It is damn near impossible to completely get rid of it once it becomes established - herbicides are mostly ineffective, and systemic herbicides will only kill the single tree… leaving the thousands of seed pods lying dormant ready to sprout and quickly take its place.

Identification

Leucaena leucocephala has alternate branching and twice pinnately compound leaves. Each leaf is generally 8-10 inches long and has around 10-12 leaflets. Each leaflet has an even number (usually between 20-34) of sub-leaflets, all with smooth margins.

The really easy way to identify this weed is by the overwhelming number of seed pods and litter. The seed pods are thin and brittle, effortlessly broken to reveal hard black seeds. The pods don’t decompose easily or quickly, and will usually be found in quantity around the tree. The pods will be found on the tree in clusters, the immature pods green and flexible and the mature pods reddish brown and brittle. From the time these trees start producing seed pods (around 7-8 months) they will almost always be flush with pods in various stages of maturity. Most larger lead trees will appear as a reddish brown mass, with a few speckles of green leaves.

The bark is generally smooth, young bark and twigs are green with long vertical gray patches of harder wood. Older bark is generally orange-gray and smooth. The wood smells like fresh pumpkin when going through a wood chipper…

The flowers on this tree are unremarkable - dirty white globular flowers as a single head. The species name for this tree “leucocephala” is from the greek “leuco” for white, and “cephala” for head in reference to the flowers. They are actually very similar in appearance to the flowers you would find on many acacia trees. Since the trees are in constant production of seed pods, flowers are pretty common. Honestly, the word flower with it’s beautiful connotations is done a disservice by associating this thing with it.

This tree is frequently mistaken as a mimosa (silktree Albizia juliibrissen) or as a lysiloma (desert fern Lysiloma watsonii). Again, the defining trait is the seed pods - both mimosa and lysiloma only have seed pods seasonally. Mimosa also has beautiful pink flowers in the spring, and a delicate form. Lysiloma has very long, fine twice pinnately compound leaves, and radically different seed pods.

A mimosa tree - Check out those beautiful flowers! In the last picture you can see seed pods on the tree:

A lysiloma from a neighborhood in north Phoenix: