Category: propagation

Sowing Victoria

Posted on by andy.winfield

By Nicola Temple

A photo taken a couple of years ago – I
have a grasp on my son as he leans
over into the tropical pool to get a
good look. Victoria cruziana is in flower
as is the lotus above us.
Photo credit: Shelby Temple

For me, one of the highlights at the University of Bristol Botanic Garden is the giant waterlily (Victoria cruziana) that lives in the pond in the tropical glass house. Its enormous leaves, which can reach 2 metres in diameter, are studded with spines on the underside and always provide ample wow factor for visiting children (my own included).  

The plant is found in slow moving waterways in Brazil, Argentina, Paraguay and Bolivia – in places such as the Pantanal. Its pollination story is an interesting one in that it is pollinated by a beetle (Cyclocephata castaneal). Its white flowers give off a strong scent that attracts the beetles in the evening. The flower then closes around the beetles, trapping them in the flower overnight. The flower produces heat (thermogenesis), raising the temperature as much as 9oC above the ambient temperature outside, which means the beetles can maintain a high level of activity without using as much energy. It’s a thermal reward and the plant benefits as the active beetles will pollinate the flower. The pollinated flower opens the next evening, revealing a new light pink colouration to its petals. The beetles flee the flower and make their way to the next unpollinated flower.

Of course, this species of beetle isn’t found in the Botanic Garden, which makes pollination a bit more challenging. However, there are other insects in the Garden that have filled this niche and the plants have set seed over the last few years. However, this is the first year that staff at the Botanic Garden have tried to sow this seed and, so far, things are going well!
The seeds of Victoria cruziana are kept wet.
Photo credit: Andy Winfield

Replicating the natural environment

In its natural environment, the seeds from Victoria cruziana would be buried in the sediments, stirred up perhaps by grazing capybara and swirling river currents. It wouldn’t be until the high water levels following the rainy season had receded that the water temperature and the amount of light penetrating to the sediments beneath would be sufficient to prompt germination.

In the Botanic Garden, botanical horticulturist, Andy Winfield, first primed the seeds by scratching the tough seed coat with secateurs. The seeds were then sown into topsoil and covered with a layer of horticultural grit. The pots with the sown seeds were then placed in a container of water to a level about 10 cm depth above the seed. This replicates the approximate water depth in the natural environment. The water is heated to a temperature of between 30oC and 32oC; this is critical to start the germination process.

Andy scores the seeds with secateurs
before sowing.
Photo credit: Nicola Rathbone

Victoria cruziana grows around the edges of water bodies and in wetland areas where there is no forest canopy. In order to replicate the amount of daily sun it would be receiving in the tropics and sub-tropics, grow lights on a 12h on/12h off cycle were hung above the pots. Then the whole contraption was covered in plastic film to reduce evaporation and maintain humidity.

Andy had read that germination time is generally about 2-3 weeks in this type of scenario, but within a few days he noticed that the seeds were starting to send out roots and when I visited a week after sowing, the hypocotyledonous stems were clearly emerging from the seeds and shooting upwards toward the surface of the water. 

Preparing to plant Victoria out

At the moment, the water temperature in the pool in the tropical glasshouse is only about 14oC, far too chilly for Victoria. In the coming weeks, however, these plants are

likely to grow quite quickly. Andy and the rest of the team at the Garden will pot them on several

times, gradually reducing their water temperature. At the same time, Bristol temperatures will be increasing and the tropical glasshouse will start getting warmer, as will the pool. By the time the Victoria plants have a few decent leaves, the temperatures between the tropical pool and the plants will have become similar enough that Victoria can be put into the planters in the pond.

The annual light intensity here in Bristol is considerably less than Victoria cruziana would receive in South America. However, the long summer days here mean that during those months more solar radiation is received here in a single day than in tropical South America. This helps Victoria cruziana flourish in the Botanic Garden tropical pool over the summer and it will be worth a visit to see it in flower. See the series of photos below taken the day the seeds were sown.

Andy prepares the loamy mix for sowing.
Photo credit: Nicola Rathbone

The seeds are sown into a loamy mix.
Photo credit: Nicola Rathbone

The soil is covered with a horticultural grit.
Photo credit: Nicola Rathbone

The seeds are sown and are ready for immersion
in a nice warm bath.
Photo credit: Andy Winfield
The pots immersed in the warm bath.
Photo credit: Nicola Temple
Only one week after sowing, the embryonic stem
has emerged and is stretching for the surface.
Photo credit: Nicola Temple

Sources:

Seymour, R.S. and Matthews, P.G.D. 2006. The role of thermogenesis in the pollination biology of the Amazon waterlily 
     Victoria amazonica. Annals of Botany 98(6): 1129-35.

Seed sowing at the Botanic Garden

Posted on by andy.winfield

By Helen Roberts

Last month I met up with Penny Harms, Glasshouse Co-ordinator at the University of Bristol Botanic Garden to explore the many different plants they grow from seed. At the end of May the garden staff were nearing the end of a mammoth seed sowing frenzy in preparation for the summer ahead, but Penny explained that seed is continuously sown throughout the year depending on a particular plant’s germination requirements. 
We entered the glasshouses via the potting house, which contains a whole range of pots, growing media and tools for propagation. There are different shaped tampers for firming down compost, and numerous dibbers and widgers to gently prick out seedlings. The growing substrate typically used for most seed germinated at the Botanic Garden is a neutral peat-free compost composed mainly of composted bark, coir and composted green material. 
Penny explained the finer details of the growing media used, “This compost can be altered by sieving out larger chunks for those seeds that require a finer grade tilth, such as poppy species. Fine and medium grades can be produced depending on the particular size of the sieve pores with perlite often added to help with drainage and aeration.”

Some seeds need constant warmer temperatures

Seed sowing occurs in a number of places within the gardens. Some seed needs a dose of warmth to get germination underway and are sown in propagators in the glasshouses to ensure a stable warm temperature of about 22°C. Others can be directly sown into prepared soil and include many of the ballast seed garden species like Calendula officinalis and Amaranthus caudatus, which form a display in the gardens. Those species destined for the grain barge are grown under glass and have recently been ferried across to the barge and planted. I spied seedlings destined to make the watery journey including Avena sativa (oats) and Eruca sativa (rocket). 
Some of the Amaranthus caudatus Helen has grown from seed
at home. Photo credit: Helen Roberts.
Back in the warmth of the propagation glasshouses, Penny pointed to a number of seedlings planted at the start of April including the beautiful but very poisonous half-hardy annual Ricinus communis var. gibsonii, otherwise known as the castor oil plant. This species is grown for its dark red metallic foliage and is planted out in the hot border once all risk of frost is past. A dark purple bronze variety of this species, equally as lovely, is ‘New Zealand Purple’. 
Penny explained how to grow Ricinus communis, “The species is easy to germinate from seed, but does require a temperature of 20-25°C, so it is best grown in a propagator case in a cool greenhouse where the temperature can be kept stable. Once big enough, it is carefully pricked out and hardened off to then be planted out in June.”
Growing steadily under cover of glass are a number of seedlings destined for the hot border that act as effective border fillers. They include the lovely canary creeper, Tropaeolum peregrinum, a half-hardy annual climber with pale green stems, leaves and yellow flowers, and Tropaeolum majus ‘Black Velvet’, another half-hardy annual with beautiful almost black flowers. Other climbers sown in the glasshouses are the common but wonderfully scented varieties of Lathyrus odoratus (sweet peas). The sunny yellow flowers of Tithonia rotundifolia ‘Yellow Torch’ and Helianthus ‘Mongolian Giant’ currently growing in the glasshouses are also useful border fillers. The latter species is a giant that grows up to four metres high, making it a favourite amongst children. More delicate looking blooms also need the warmth of the glasshouses for germination, such as Digitalis lanata (Woolley Foxglove), which has woolly spikes of fawn coloured flowers with a pearlized lower lip. 

Some seed is worth the wait

Some seed germinates very quickly if conditions are right – sometimes within a week – but other seed can be extremely difficult and requires a great deal of molly-coddling in order to get germination success. Penny carefully pointed to a seedling of Cardiocrinum giganteum var. yunnanense, the Giant Yunnan Lily, which as an adult is a beast of a plant and can grow up to 2.5 metres with huge fragrant nodding creamy inflorescences. This species is normally propagated from either seed or by bulbs and can take as long as 4 to 5 years before it flowers. It is a monocarpic species that will die after it flowers leaving offsets, which will then become subsequent plants. This is certainly a species for a patient gardener; it takes a long time to flower but it also takes a considerable time to germinate as Penny emphasised.
Blooms of the Cardiocrinum giganteum var yunnanense
– the Giant Yunnan Lily.
Photo credit: Col Ford and Natasha de Vere
[via Flicr CC licence 2.0]
“This species can be tricky to germinate,” explained Penny, “This one has taken over a year to germinate so it is quite special.”
This is a species that appeals partly due to the fantastic inflorescences, but it has freaky (and rather scary) looking seedpods that resemble vegetative heads with fangs. In my opinion, the lengthy germination and time to flowering is worth the wait. 
Tropical and subtropical plants that only survive as mature specimens in the glasshouses can be even trickier to propagate by seed. The seed from Passiflora, a large genus of mostly vines needs to be sown quickly when fresh as dried seed takes much longer to germinate. The subtropical vines of the Aristolochia species, aptly named Dutchman’s pipe, require similar treatment. Species grown at the gardens are A. labiata and A. trilobata, and have beautiful ornate blooms of about 15cm. A. labiata flowers resemble the mottling and coloration of a rooster’s comb. Although tropical and sub tropical species can be a bit trickier to grow up from seed, most species can be sown throughout the year. 
Some species do not need the cosseting of warmth and will happily germinate outside although some seedlings, like borecole (kale), are protected with wire mesh to prevent bird damage particularly from pigeons. Species that have germinated and are growing happily outside at the Garden include the mixed colours of Salvia viridus, more commonly known as the Clary Sage. This produces small spires of lovely flowering bracts loved by pollinators. These are intended for the Mediterranean beds along with the tall spires of Echium italicum, the Pale Bugloss, a beautiful pyramidal plant belonging to the Borage family and Viola arborescens, a pretty violet with large lavender coloured flowers. 
Other plants need to be sown at different times of the year and some species have enough flexibility in this that you can sow depending on when you want a plant to flower in the subsequent year. For most gardeners, autumn and spring sowing are the busy sowing months. At the Botanic Garden, for example the open faced flowers of Papaver somniferum, (Opium Poppy), are sown in the autumn.
“These poppies are treated differently to other poppy species,” said Penny, “in that they have been sown in September and will be good strong plants by the time they are planted out the following year, producing flowers earlier than if seed had been sown in the spring.”

Helen Roberts is a trained landscape architect with a background in plant sciences. She is a probationary member of the Garden Media Guild and a regular contributor to the University of Bristol Botanic Garden blog.

To grow or not to grow: plant propagation at the Botanic Garden

Posted on by andy.winfield

By Helen Roberts

At the start of December, I met up with Penny Harms, Glasshouse Co-ordinator at the University of Bristol Botanic Garden, to discuss the plants that are propagated at the Garden and how this valuable work is carried out. Over the course of the year, I will be investigating the different forms of propagation techniques used in the Garden to maintain and enhance their existing stock of plants. I will cover briefly how these techniques are carried out (bearing in mind that there are a plethora of books available on plant propagation), but I’ll also examine what is happening at the cellular level and examine the ‘why’ behind certain propagating techniques.

As Penny and I examined some seedling plants, she explained to me why propagation is so important at the Botanic Garden. “If we lose some plants outdoors in a cold wet winter, we have a back up of new plants. Some are not simply insurance plants, but are taken as cuttings as a necessity every year as they survive in our climate as annuals, particularly those plants from the South African collection. Others, such as the Mediterranean plants, do not survive as long here in Bristol as it’s generally much wetter and therefore they need to be replaced fairly frequently. Most plants we take from cuttings are mainly tender perennials and frost tender plants.”

Propagation in the Garden won’t likely restart until the spring depending on weather conditions.

In the glasshouses, Penny showed me many of the plants that have been propagated from cuttings, including some beautiful decorative Aeonium species (commonly known as tree houseleek), as well as Pelargonium (geranium), Clematis, Salvia and Passiflora (passion vines) species. Some plants raised from cuttings  are placed in a unit that is misted with water regularly and the bottom is heated to a temperature of 25°C in order to encourage roots to form. The plants all looked wonderfully healthy, not at all like my puny looking specimens that I had taken cuttings of back in September at home. However, the plants that really caught my eye were some small fern plants potted up, which Penny called “fernlets”.

Ferntastic ferns

Ferns belong to the plant division of pteridophytes (spore-producing vascular plants) and are extremely diverse in habitat, form and reproductive methods. Most ferns grow in moist warm conditions and very few tolerate dry cold places. Although they aren’t flowering plants, the frond shapes and colours can be exquisite. Closer inspection of the undersides of the leaves reveal beautiful patterns of sporangia – the vessels containing the spores.

Fern reproduction 101

Fern lifecycle
Image credit: Carl Axel Magnus Lindman
[CC BY-SA 3.0], via Wikimedia Commons

Like other plants, ferns have alternating haploid (single set of chromosomes) and diploid (two sets of chromosomes – one from each parent) generations; the haploid gametophyte produces the cells for sexual reproduction while the diploid sporophyte produces spores that produce the gametophyte. Unlike flowering plants where the gametophyte is reduced to the pollen and embryo sac, fern gametophytes are free-living. (Although they are admittedly less conspicuous than the sporophyte we generally identify as ferns.)

In brief, the sporophyte produces spores, which are shed and grow into gametophytes (also often called the prothallium). In some species, individual gametophytes will be either male or female, while in others an individual gametophyte will function as both sexes. When the conditions are right, the gametophyte releases mature sperm from the antheridium, which swim to the egg-producing part called the archegonia under the gametophyte’s underside. Fertilisation produces a zygote, which develops into an embryo and eventually outgrows the gametophyte to become the sporophyte.

The plantlet sailboats on the fronds of Woodwardia prolifera.
Photo credit: Andy Winfield.

Like many other plants, ferns can also reproduce asexually through branching of the underground root stem or rhizome. Some species will even produce leaf proliferations known as plantlets or offsets, such as the beautiful Woodwardia prolifera, which comes from Asia and grows in coastal regions. It’s small plantlets (or “sailboats” as Penny calls them) drop off the plant and fall to the ground, securing themselves quickly with their roots.

Fern propagation at the Botanic Garden

Fern spores are carefully collected when the ferns are sporolating by cutting fronds and letting spores fall into paper bags. Spores are only collected when they are ripe; usually the sporangia will swell and will turn brown, black, blue or orange depending on the species.

“As far as when to collect the spores,” said Penny, “it is really a case of watching and waiting. The beautiful orange [sporangia] on the Phlebodium aureum var glaucum go a slightly darker brown when they are ready, which makes it easier to know when to collect. And if you lightly tap the frond over some white paper you can watch to see if the spores are being released.”

The underside of a frond from Phelbodium aureum var. glaucum,
showing the sporangia. Photo: Andy Winfield.

Penny added that she often collects additional spores by simply placing a fern frond onto a tray containing already wetted peat-neutral compost with bark mulch to allow spores to drop onto the substrate. Penny had great success growing new plants from spores harvested from a miniature tree fern species called Blechnum gibbum. This plant was looking in a sorry state before the move to The Holmes at Stoke Bishop and so Penny collected spores just in case it didn’t survive the move. However, research revealed that this fern was behaving like a deciduous plant -it had died back, but wasn’t dead. Thanks to Penny’s careful propagation, the glasshouse now holds a number of specimens from this species – all grown from spores of the original plant.

The tree huggers

Some the glasshouse ferns are also epiphytic and will reproduce effectively from spores. One such example is Stenochlaena tenufolia, a South African fern that will grow up trees. Its climbing rhizome can reach up to 20m in length and 15mm in diameter. As young plants, they start off on the ground, but soon start to ascend trees, trading in their connection with the soil for life in the trees. Often plants don’t produce fertile fronds until the rhizome has climbed sufficiently to expose the apical region of the plant to sufficient light. These ferns are grown both from spores and vegetatively at the Botanic Garden.

The runners

Other species require a different approach. Diplazium proliferum, a fern that is widespread in the tropics and subtropics, produces little rooting plantlets along its fronds that can be developed into new plants. The frond is simply cut and laid onto bark mulch, pegged with wire and then half buried with the substrate.

The chain fern, Woodwardia radicans (from the Macaronesian region but also found on other Mediterranean islands) also produces bulbils but these are usually located at the ends of the fronds as a hard nodule. The roots start to develop in the air but when they touch the ground will root into the substrate and form new plants.

Penny explained, “We got these plants from Tresco where they grow as huge sprawling mounds. The small bulbils eventually form quite large plants, but are still connected to the original. This gives this fern its very relevant name. New plants can simply have the connection cut and be dug up and transplanted elsewhere.”

A brief step-by-step lesson on how to propagate ferns

At the Botanic Garden ferns are being propagated very successfully, but there is no reason why horticulturists at home should not be able to have the same degree of success. Penny offers her expert advice in propagating ferns by spores below:

Ferns can be propagated vegetatively, by division, or similar to sowing seed from flowering plants, by spores, which are found on the underside of the fern fronds. Some fern species are very difficult to propagate from spores, however Adiantum, Pteris and many Blechnum species are reliable.

Here are the main points for the propagation of cool glasshouse ferns from spores:

  1. The spores should be collected when ripe. The sporangia found on the underside of the frond, will (in most cases) change in colour from a light to dark brown to indicate the spores are ripe. To check, lightly tap the frond to see whether the tiny brown spore cases (sori) are released. If so, the fronds can be cut and gently placed into paper bags in order to collect the fine sori ready for sowing (see point 2) or the frond can be cut and placed directly onto the surface of a pre-prepared tray of compost, allowing the spores to fall naturally as the frond dies away. 
  2. Sow the fern spores. Collect the spores from the bottom of the paper bag and sow immediately. Fresh spores will germinate far more successfully than ones that have been kept for some time and dried out. Use clean, shallow, pots and/or trays with drainage holes. Place a fine layer of gravel on the bottom. Add a layer of peat-free, fine grade compost and gently firm down. Stand the pots and/or trays in water to allow the compost to absorb the water. When the compost is wet, lightly and evenly sow the spores over the surface of the compost. The spores are very fine and on no account should they be covered with more compost, as this will prevent them from germinating.
  3. Keep moist. The trays and/or pots should be covered either with a propagator lid or glass and stood in a shallow tray of water. It is important that the compost does not dry out. 
  4. Position in a semi shaded spot ideally at temperature of 16 – 20°C.
  5. Once the spores start to germinate, the young fern plants (prothalli) should become visible within a couple of weeks. Allow the prothalli to establish themselves for a little while before moving on to the next stage, that of pricking out the delicate new plants.

 Moisture is the most important element for the successful propagation of ferns.