March - Tī Kōuka /Cabbage Tree - Native Plant of the Month
Tī Kōuka / Cabbage tree
The little tree that gives so much. Cabbage trees, or tī kōuka to use their Māori name, are arguably our most underrated native tree. Tī kōuka are ubiquitous, so common that we are perhaps guilty of not noticing their extraordinary qualities. These trees might be more modest in size than more celebrated forest giants, but they sure punch above their weight in terms of the many ways that they help support people and other native species.
Have you ever wondered why cabbage trees are called cabbage trees? They certainly don’t look anything like a cabbage you would buy in a veggie shop. We can thank Captain Cook for this rather inaccurate name. As a seasoned sailor he knew the importance of eating vegetables to ward off the dreaded scurvy. Cabbages store well and were a common vegetable brought on long journeys. Captain Cook took every opportunity on his voyages to seek out local ‘cabbages’ in order to encourage his sometimes reluctant crew to keep up their vitamin C levels. The first records of ‘cabbage trees’ in Aotearoa actually describe nīkau (which is also edible, but in a very final way for the tree). These records were followed not long after by descriptions of harvesting the ‘cabbages’ from what we now think of as cabbage trees to supply vegetables for sailors. Why this name stuck (especially as it was used in such a generic way to describe different trees in different countries) we don’t know.
The Māori name for cabbage trees; tī kōuka, has also caused some confusion, mostly among early European settlers and visitors. There are many, very specific names for the different parts, uses and varieties of cabbage trees and the name tī is the generic name for the tree itself, which prefaces each of the more specific names. Tī kōuka is now commonly used as the name for the tree, but it more specifically describes the vegetable head that can be harvested from branch ends. In the past tī was the general name for the trees and this was also used by early Europeans, who called them ‘tī trees’. Because Māori is an oral language there was some variance in how the name was written down. This meant that there was overlap with plants given the generic term ‘tea-tree’. Tea trees included mānuka and kānuka (as well as a range of Australian species and generally anything that could be boiled to make a drink that at least looked like tea). Tī and tea were pronounced almost identically and so were also written down with an overlapping range of spellings, making contemporary study of early writing about our native plants hard to decipher.
Cabbage trees are found throughout Aotearoa, primarily in open, lowland habitat. Cabbage trees are extremely hardy and will grow in a huge variety of conditions, from exposed coastal sites to dry inland ridges. Their only requirement is high light levels and this preference, coupled with their ability to grow in waterlogged soil, means that they are commonly seen growing in wetlands. The cabbage trees that you will commonly see growing around Kaipātiki are one of seven species of cabbage tree in New Zealand - five of which are native. There are also two, very similar-looking, species of cabbage tree that have been introduced to New Zealand. One from the Pacific Islands and one from Australia. Neither are invasive and one, known as tī pore, has an important history in New Zealand. This was one of the precious early food plants brought by the first Pacific navigators who arrived in Aotearoa and settled to become Māori. It was thought that tī pore had become extinct in New Zealand, but a remnant grove was rediscovered in an old pā site in Northland in 1999.
Cabbage trees belong to the Asparagales Order of plants - which also includes asparagus. This order evolved in Gondwana and the descendents of these original plants can be found throughout former Gondwanan landmasses. This group of plants are sometimes described as the ‘tree lilies’ and it is likely that you will have heard of some of them - yuccas, aloes, dracenas (dragon-blood trees), joshua trees, Australian grass trees and agaves are all tree lilies, with a strong family resemblance. Tree lilies generally have a slender trunk with a spiky mop of foliage near the top. Their flowers grow on a special floral branch (or bract) out of the top of the plant and the flowers are usually a mass of white cup- or bell-shaped blooms with six petals.
Cabbage trees are fast growing and their fondness for plenty of light means that they are excellent colonisers of bare ground and slip areas. Generally they will grow for the first few years straight upwards with a single straight trunk. When they first flower this will trigger the first branching, with the growing bud dividing in two from the base of the flower. From this point onwards each flower bract will prompt a stem to branch. This means that you can see a trace of a cabbage tree’s flowering history in its form. The trees do not flower equally every year and some will flower more than others. The trees flower in late spring/early summer and the massed flowers are highly fragrant. This scent attracts huge numbers of insects for pollination and to feed on the sweet nectar at the base of each flower. It is a delight to sit and watch what visits a cabbage tree in bloom on a still, sunny day. You are likely to see both native and introduced bees, beetles, wasps, flies and, at dusk, moths. If you are very lucky you might even see a gecko attracted to the nectar.
Because cabbage tree flowers are not particular about who pollinates them they have a very high success rate and flower bracts will quickly form into thousands of seeds. These ripen over roughly two months within small, round fruit that become white, or pale blue colour. Cabbage trees are as generous with their seed dispersal as they are with their pollinators. Many different birds eat cabbage tree fruit and spread their seeds. Tūī, silvereye, kereru, kākā, blackbirds, starlings and saddleback all enjoy cabbage tree fruit. The seeds germinate fast and can come up so thickly in areas that they look like newly sown grass! They will naturally thin themselves out with the strongest and best-adapted seedlings out-growing their siblings.
Cabbage trees are also attractive to wildlife because of their mop of leaves. Younger trees tend to hold a skirt of dead leaves on their trunk under the living leaves. This is rich habitat for our native invertebrates - from wētā, to spiders to slaters and even leaf-veined slugs. Because so many things shelter and live amongst these dead leaves, cabbage trees are often sought out by insect-eating birds like fantails and ruru (at night). During the nesting season you can often spot tūī rummaging about hunting spiders to build up their protein and feed their chicks. If you look closely at cabbage tree leaves you will usually see that most have uneven edges with chunks missing. This is the work of our endemic cabbage tree moth caterpillars. These caterpillars live in the young leaf shoot. Very young caterpillars eat strips down the length of leaves, leaving a brown stripe. As they get older the caterpillars chew the edges of leaves, which gives them the distinctive notches. Cabbage tree moths hide during the day amongst the dead cabbage tree leaves. They are a pale brown with parallel stripes running over their wings. They sit with these stripes perfectly aligned to the dead leaves they sit on, giving them perfect camouflage.
Cabbage trees might start their lives by growing fast, but they are surprisingly long-lived plants. A tree in good conditions could live up to 400 years. It is still possible to see large, weathered looking cabbage trees in farm paddocks around the country. These are often remnants of the original bush that has since been cleared. As they age, cabbage trees will often form split or hollow trunks. These provide a safe shelter for our native bats, geckos and cave wētā. Unfortunately these days hollow trees are more often occupied by possums. People too have put these hollow cabbage tree trunks to use for a long time. There are records of some hollow trees being so large that they were fitted with a door and used as sheds! Cabbage tree trunks are fire-resistant and early settlers often fitted hollow trunks into their huts to use as chimneys.
People have a long history of using other parts of cabbage trees as well. Cabbage tree leaves are tough and fibrous. They contain compounds that slow their decomposition. This makes them ideal for weaving and making ropes. Dead leaves are very flammable (in contrast to the trunks), which means they make excellent kindling or fire-starters. They can also be bound together at the end of a stick to make a torch. The spike of furled new leaves is edible (the famous ‘cabbage’). It is the tender base of this growth shoot that gets eaten and is removed from the plant by twisting and pulling the central spike of leaves. This might seem like it would kill the tree, but cabbage trees are tough and removing growth tips like this only causes more shoots to form. Cabbage tree leaves don’t have unanimous 5-star gastronomic reviews, as they taste quite bitter, but their roots are another story. Cabbage trees develop huge tuberous tap-roots that anchor them firmly. These are full of carbohydrates that can be converted into edible sugars through long cooking. This was an important food-source for Māori around New Zealand for centuries until crops requiring less effort to harvest and prepare were introduced. Historical accounts describe people eating cooked tī kōuka roots like sticks of sugar cane, with a comparable delicious sweetness. European settlers saw this resource as an opportunity to expand on their list of Things That Can Be Made Into Rum. The alcohol that was made by these enthusiastic brewers was so potent that it was responsible for shipwrecks due to enthusiastic sailors dipping into supplies as soon as they were loaded.
While harvesting the tree’s tap roots is fatal, cabbage trees are remarkably resilient to almost any other form of damage. Unlike most of our native plants, cabbage trees are able to survive fire damage as their corky bark protects the growing tissue underneath and dead leaves draw fire upwards to quickly burn out. Epicormic buds - tissue under the bark that can develop into new growth shoots, get triggered by trauma in cabbage trees. This extreme example of what doesn’t kill you only makes you stronger means that cabbage trees can rise like a phoenix from disasters that destroy other trees. We might even go as far as claiming they are a vegetative version of the immortal jellyfish! You would be forgiven for thinking that this would give cabbage trees weedy tendencies, but somehow they seem to stay safe, just on the right side of the trouble line. They are resilient, but not invasive.
A cabbage tree regrowing from a cut stump and a split and hollow cabbage tree still thriving
Because cabbage trees are tough and long-lived they were sometimes used as landmarks and trail-markers. Trees were traditionally planted by Māori at entrances to significant places and along paths, knowing that these slender trees would continue to flourish in years to come. What is now known as Newmarket was once called Te Ti Tutahi after an enormous and sacred cabbage tree of the same name that once grew on the site that is now Newmarket Primary School. The tree was controversially destroyed in 1908, but sections of its rhizomes were taken by locals and planted elsewhere. One of these was taken and grown on at historic Highwic House, where we understand it is still growing. The elegant and exotic appearance of cabbage trees has led to their being one of our most popular native plants exported to international gardens. In the UK they grow particularly well in south England, where they are commonly known as Torquay palms.
Does a tree that can rise from the ashes of disaster face threats? Sadly, yes. Cabbage trees can suffer from a disease called sudden decline. Large numbers of trees began to die from sudden decline during the 1990’s, particularly in Northland and Auckland. For many years the cause of the disease was unknown, but it is now known to be a bacterium that is spread by introduced passion vine hoppers. This bacterium is native and was restricted to flax, which is its natural host species (and which it also damages) until the passion vine hoppers were introduced from Australia. The disease can also infect karamū, mamaku (black tree fern), kōhūhū, pūriri and introduced crop plants like strawberries. There is no cure for this disease and an infected cabbage tree will suddenly wilt and drop leaves, dying rapidly. On occasion a tree will resprout from the base after an infection.
As well as sudden decline, cabbage trees are also threatened with ‘slow decline’. This is caused by gradual damage to trees that are not protected from trampling by people or stock. Erosion, soil compaction and repeated damage to trunks will eventually take a cumulative toll on tree health. Just like all plants, cabbage trees will do best if the base of their trunk is protected from trampling and compaction. They will usually respond quickly to improved conditions.
Finally, cabbage trees have developed a poor reputation due to their conflict with lawn mowers. Their tough leaves have a habit of wrapping themselves around mower blades. There are some simple solutions to this issue: you can collect the leaves up as a resource before mowing and use them for weaving, making garden twine, kindling for fires, or as a mulch for other plants (and getting kids to collect bundles of leaves could be a good way to earn some pocket money). Putting in some additional planting around cabbage trees can help ‘absorb’ fallen leaves and make a feature out of the tree while protecting its base. And finally, as a slightly curmudgeonly nurseryman once said in our hearing - you could flip the problem on its head and get rid of your lawnmower! A no-mow area planted with low growing plants saves mowing and results in higher biodiversity. Be brave!
If you are interested in learning more about cabbage trees, or simply love nature books, we highly recommend Dancing Leaves: The Story of New Zealand’s Cabbage Tree, Tī Kōuka, written by Phillip Simpson and published in 2000.