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Monday, 25 January 2016

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

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.