February 2013 – University of Bristol Botanic Garden

Each year at the Botanic Garden, the pool in the tropical zone glasshouse is drained of two thirds of its water and prepared for a new growing season of water-dwellers. The most dramatic of these, is without a doubt, Victoria – the giant water lily. The enormous round leaves, spanning as much as six feet (~2m) across, provide a floating habitat for insects that crawl from leaf to leaf, while anacondas and piranha lurk in the waters beneath. Penny Harms is the Glasshouse Coordinator at the University of Bristol Botanic Garden and she spoke to me this week about what’s involved with providing these native South American giants with the best habitat possible in a northern temperate climate.

As we stood beside the pond chatting, the surface of the water was constantly dimpled as the guppies rose to the surface in search of a meal. The water level is very low, revealing two large wooden planters that sit empty. The planter on the left, Penny tells me, will contain Victoria cruziana the Santa Cruz water lily – famous for its giant leaves with the strength to support small children.

In its native habitat, in the slow-moving backwaters of the Paraná Basin and in the Pantanal region, this giant lily grows as a perennial. However, here in the UK it is grown as an annual as the plant simply doesn’t tolerate the low light quality and short day length in winter, not to mention the low temperatures in the pool.

“It’s a large plant and very, very greedy,” explained Penny. “It grows at an incredible rate, so it’s easy to grow as an annual and most botanic gardens with tropical glasshouses that display this plant, do so over the summer months.”

Propagating Victoria

It is this time of year, in February, when the seeds of Victoria should be germinated. Propagators will keep the seed over the winter in distilled water and then sow them in small tropical tanks that Penny describes as “glorified washing-up bowls”, which are filled with a very rich loam.

“Really, you’re just trying to replicate what happens in the wild,” said Penny. “The seeds are scattered and are washed up along the edges of the river banks where they settle in very silty, very fine mud. They just sit there until the temperatures are perfect and then they germinate. So, you really don’t need to do much.”

Once the seeds germinate, the plants start growing quite fast. They will be planted out two or three times and when they’ve got two or three decent sized leaves and are in a 40cm pot or so, they are ready to be put into the pond planters. Penny prepares a special mix of soil for the planter that is predominantly loam-based, to which she adds well-rotted manure. She also adds a slow release fertilizer for these greedy plants, which will also be added every six weeks thereafter.

The water level in the tropical pond has been
dropped to expose the wooden planters that will soon
hold the giant water lilies.

The water level of the pond is then raised so that the leaves are resting on the surface and as the plant grows, the water level is raised again and again to accommodate the growing plant.

“Obviously in a pool this size we can’t let it get to the size it would in the wild,” said Penny, “so I remove about one or two leaves a week once it gets going. In this planter, the lily could have four or five leaves, all three to four feet across, but it would all get quite tangled. So instead I reduce it down to about three to four leaves allowing fewer leaves to stay on the plant longer and therefore get bigger.”

The leaves of V. cruzianaare a purplish red colour on the underside and have an impressive network of ribs that are lined with spines. It is thought that these spines offer protection from herbivores such as grazing fish and manatees. The leaves themselves can support up to 99lbs (45kg) of evenly distributed weight, which is facilitated by floatation provided by air that is trapped between the ribs on the underside of the leaf and its distinctive pie-dish rim.

Beetle trapping flowers of the night

Victoria cruziana starts producing flowers after about two months of growth. The flowers open only at night and cast a lovely pineapple smell out into the night air to attract pollinating beetles.

“It only flowers for two days,” said Penny. “The flower will open one late afternoon and be fully open over the night and then it will close during the day and open again a second night, and then that’s it. The little black beetles, which pollinate the flowers in the natural habitat, fly from flower to flower and often get trapped in the flowers when they close during the day.”

While beetle trapping may seem coincidental, it actually serves an important role in the pollination process. The first night that the flower opens it is white and it is its female reproductive parts that are mature and awaiting the pollen carried by the beetles attracted to its scent. However, when the flower is closed during the day it undergoes a sexual transformation and it is the male sexual parts – the pollen laden anthers – that mature. The flower also changes colour, from white to pink, during this process. The trapped beetles get covered in the flower’s pollen as they walk about their petal-walled cell. Then, when the flower opens on the second night, it releases the beetles, which will seek out a new fragrant white female flower. Essentially the plant has guaranteed its pollen delivery system by trapping it!

The glasshouses are definitely worth a visit right
now as the orchids are putting on a wonderful display.

Though the University of Bristol Botanic Garden hasn’t yet propagated its own Victoria cruziana, Penny hopes to do so in the future. Part of the challenge has simply been finding room in the already full propagation house. Proper heaters would also be required to maintain the temperatures needed to germinate the seed. For now, Penny and the horticultural students are already incredibly busy with the preparations needed to plant out the tropical pond.

“There is a lot to do and we have to get things done by a certain date,” said Penny, “because if things aren’t planted out by a certain time, their growing season is shortened. The later you put things in, the longer it takes them to get going and ultimately we want to have things ready for when the public comes in at Easter.”

The glasshouses are of course open to the public now and it’s a good opportunity to see work in progress and to also check out the orchids in the sub-tropical zone, which are putting on a lovely show right now!

We have all been drawn in by that scent carried on a spring breeze – something sweet or fruity, maybe even spicy or with a hint of citrus. If we’re lucky enough, we might even find the source and bury our noses among the petals in order to fill our head with the aroma. We, and ancient cultures before us, have been besotted by the chemical allure of plants.

Recently, I wrote about the ‘Scent of winter’ in the Botanic Garden and how winter blooming plants tend to be incredibly fragrant in order to attract pollinators at this time of year. I immediately wanted to dive into the science behind floral fragrances, but quickly learned that this was a discussion all on its own…perhaps even a tome.

The scents associated with plants are produced by a mixture of chemical compounds known as volatile organic compounds (VOCs). They are described as volatile because they have a low boiling point, which means they are gases at room-temperature. It is these VOCs that are extracted as essential oils for aromatherapy or medicinal purposes, and that are carried on the wind in order to attract pollinators from long distances.

No two scents are the same

Due to the incredible variety of compounds and small differences in the relative abundances of these compounds, no two scents are exactly the same. In fact, insects are able to use these individualised scents to discriminate between individual flowers of a single species. This can be very important for pollinators as they can distinguish flowers that might offer a greater reward by having more nectar than others.

Scent is particularly important for plants that are pollinated by night-feeding animals like moths or bats, and it is also clearly important in winter when there are fewer pollinators about. Generally speaking, sweet smelling flowers tend to attract bees and flies, while strong musty, spicy or fruity odours attract beetles.

Specialised chemical cocktails

However, the chemical allure of plants is far more complex than just a sweet smell wafting on the breeze. Some species have become so incredibly specialised with their chemical cocktails, that they mimic the natural chemical signals of animals – pheromones – in order to attract pollinators.

One of the most successful groups of mimics is the orchids and within this group, perhaps the best known for their trickery are the bee orchids (Orphys). Not only have the flowers evolved petals that look like a female bee or wasp resting on the flower, the petals also give off an enticing scent that mimics the female’s pheromones. The male bees are lured in by the smell and “mate” with their deceptive petals, with nothing more to show for it than a dusting of pollen. The male will then carry the pollen to the next flower that tricks him.

Another orchid, Dendrobium sinense has a scent that mimics the honey bee alarm pheromone, which attracts hunting hornets as pollinators.

It’s not always about attracting pollinators…

Of course, the scent given off by plants isn’t always about attracting pollinators. Some plants use scent to attract predators and parasitoids of herbivorous insects. When the leaves of these plants are under attack by the herbivores, they will produce more VOCs, which not only bring in the predatory recruits, they also signal neighbouring plants to take action. For example, Lima beans (Phaseolus lunatus) will begin to secrete extrafloral nectar if a neighbouring Lima plant is under attack. This amplifies the call for help and attracts even more natural predators into the battle zone.

…and it’s not always a nice smell

I spent nearly 20 years on the west coast of Canada, the home of Lysichiton americanus. Anyone who has encountered the fragrance of this plant while walking in the wet coastal forests, knows where it gets its common name, skunk cabbage. The distinctive odour might be offensive to humans but for scavenging flies and beetles, which pollinate the plant, it signals the smell of something rotting and definitely worth investigation.

So, while I don’t recommend burying your nose in the flower of a skunk cabbage, it is definitely worth taking a moment to stop and smell the roses…just be sure a bee isn’t in there!

Month: February 2013

Preparing the pool for Victoria