Herbarium Technique

Making a herbarium is exciting and rewarding, as each plant you press provides you with a valuable experience, allowing you to connect more closely with the plant through observing and working with it, and helping you to understand and remember it in a way that is never possible from just reading about it in a book. It consists of a three part process – collecting the plant material, pressing & drying, and mounting. Here are my recommendations learned via experience and years advising students and receiving their helpful feedback.

Niki Lawrence

COLLECTING

Materials list for plant collecting: -

  • Plant press
  • Plastic bags
  • Garden secateurs & trowel
  • Small note book & pencil
  • Jeweller’s tags (optional)
  • Camera (optional)
  • GPS & altimeter (optional)

Plants chosen should be good representatives of the species and should contain all the essential features necessary for identification, i.e. leaves, stems, flowers & seeds (+ roots if suitably small and the plant is common and abundant).

Collecting too many plant specimens during field trips is wasteful; it is recommended that you collect only about three samples of your chosen species to ensure you have adequate space in the plant press and sufficient time and attention to devote to each specimen. Taking 4 – 10 specimens is standard practice for official herbarium collectors as usually one specimen is given as a courtesy to the nearest herbarium in the region or country of origin and another is lodged with the main official herbarium in your own country (e.g. Kew Gardens Herbarium), and another might be sent to a specialist to view.

Sexuality: Remember to check if your plant is bisexual, or if it is monoecious (sex organs on different flowers but on the same plant), or dioecious (plants contain either male or female flower but not both). For example, Urtica dioica (Nettle) is dioecious so you will need examples of both male and female plants in order to represent the species clearly.

If very small plants are being collected then gather enough so that several small specimens can sufficiently fill three A3 mounting sheets – mounting one tiny specimen on a big sheet looks odd.

Preferably collect specimens in dry conditions, a good time being mid-morning, after the dew has dried but before the heat of the day causes plants to wilt. If specimens are at all wet or you need to wash soil off the roots then dry them carefully before pressing.

Field notes must be recorded at the time of collection, noting the following:

Date, collection number, location, habitat, habit, special characteristics.

Use a pencil for these notes rather than a pen because any damp/wetness can cause ink to smudge and be unreadable.

Attaching a numbered jeweller’s tag (sold by most high-street stationers, and sometimes called swing tags or hanging labels) to each specimen can be a very useful option as the tag number will link to the number in your note book, avoiding any later confusion. This is especially important if you are not certain of identification or are collecting several plants species.

GPS & altimeter (optional) – these are very useful for pinpointing your exact location, especially relevant if you aim to give a sample to an official herbarium or if you are in the middle of a poorly mapped jungle!

Camera: (optional) – taking photos to support your herbarium can be very helpful for showing the surrounding habitat and to evidence important characteristics such as unusual plant form, exudates such as oozing latex, visiting pollinators, etc.

Rare plants – of course permission is needed from the landowner before collecting any plants, but you also need to check if a plant is rare and if so, then just photograph the area and take a close-up picture of the plant. To find out if a plant is rare you can check it against the ‘Vascular Plant Red Data List for Great Britain’; <http://www.jncc.gov.uk/page-3354>; although this list is not particularly easy to work with. The botanist/herbalist Robyn Klein kindly referred me to a very relevant website called the ‘United Plant Savers ‘’At-Risk’’ & ‘’To Watch’’ list, <http://www.unitedplantsavers.org/UpS_At_Risk_List.html>. This list is well worth checking out as it has been compiled by a range of experts, including US and American herbalists, botanists and wild crafters who through personal experience in the field have noted medicinal plants which they feel are now at risk or need to be observed for possible future concern.

Bringing plants home from overseas – this can also be problematic and to make sure you are not breaking any laws you should first consult the CITES (Convention on International Trade in Endangered Species) web site: <http://www.cites.org/eng/com/PC/index.shtml>. You may also need to check out the CBD (Convention on Biological Diversity) web site: <http://www.cbd.int/>.

Transporting your plants home can be difficult and whilst plastic bags are good for collecting robust plants, delicate structures such as flowers can be damaged. To help prevent this you can blow air into the bag and seal it – this will help to cushion the plants and also provide some moisture. If the journey is lengthy you could also add a tiny amount of water to the bag or wrap the roots in wet tissues or cloths. If the specimens are slightly wilted when you get home you could place the roots/stems in water to revive them first before pressing.

If you own a light weight slatted wooden press (available from the US via the web, but they are expensive), it is useful to take it with you to the field so that delicate plant material can be placed in this protected environment for the trip home. Alternatively you can just take newspapers, corrugated card and string with you – your plants can be placed between sheets of newspaper and an outer layer of card, and tied securely for protection on the journey. Finer plant position adjustments can be done later at home.

PRESSING & DRYING

Materials list for initial pressing: -

  • Plant press
  • Newspaper
  • Greaseproof or flimsy paper for delicate structures
  • Blotting paper
  • Corrugated card

As noted above, official plant presses made of slatted wood allow good air circulation and are light weight, but solid wood is adequate for plants in the UK. A home made press is easy to construct from two sturdy pieces of A3 sized plywood and either two lengths of strap with buckles that can be cinched tight, or else four wing nuts & bolts at each corner that can be screwed tight.

Place your plant between folded-out sheets of newspaper, although flimsy or greaseproof paper is preferable for delicate material. Blotting paper can be used, being placed on the outside of the newspaper sheets – this is expensive, although it may be worth the cost, especially if you are pressing very succulent herbs, and the blotting paper can be dried and reused again.

Arrange the plant carefully, trying to avoid overlapping. If too crowded, either fold or trim some shoots or leaves. If trimmed, it is important that you leave a stub to show what has been removed. Large plants can have their stems folded to accommodate them within the confines of the page or you can have several sheets for one plant showing the top, middle and bottom of a plant.

Specimens can be adjusted once they are partially pressed as this makes fine adjustments easier. This first adjustment could be after just half an hour of pressing with delicate plants, or perhaps the next morning with more robust plants.

At least one leaf and one flower should be turn to expose the back surface.

Important note: If the specimen has thick or lumpy parts, use folds of newspaper to add padding over less bulky structures to help distribute the pressure evenly. If this is not done, delicate leaves and flowers may receive insufficient pressure and end up wrinkled and discolored.

Extra separate parts such as flowers, leaves and seeds are useful. A small strip of bark saved from a tree or shrub can also be very useful.

Delicate flowers should be spread out and dried in folds of greaseproof or tissue paper. Tubular flowers may be cut lengthwise and opened up before drying.

Succulent or fleshy plants may be cut longitudinally and/or transversely and it may be necessary to scoop out the inner tissue. Succulent plants can be killed by placing in boiling water for a few minutes or they may continue to grow and have even been known to send out a flower suddenly after years sitting in a herbarium!

Bulbs and thick roots may be cut in half lengthwise (off-centre to prevent the plant from falling apart).

When you have finished arranging the specimens within the newspaper sheets (or whatever combination of papers you have chosen), you then need to intersperse them between corrugated card sheets to aid ventilation. Finally place everything in your press and tighten well.

For the first two to four days you will need to check daily and change the blotting paper and/or other surrounding papers, and retighten the press, but as the plants dry these checks can become less frequent.

The rate of drying is dependant on the plant material being pressed and the temperature, pressure and the paper used. Drying may occur within two to four days but often takes somewhat longer. Quick drying is preferred as it helps retain the natural colours and prevents decay and mould formation.

Any bulky fruit such as large Hawthorn berries or nuts should be removed from the plant and dried separately over a heater or in an oven. These can then be stuck back into place when mounting and/or placed in an attached packet.

If drying thick or fleshy material, change the sheets of drying paper more often but if you are not experienced then it is best to avoid very succulent material, e.g. Bluebells. They are very difficult to dry completely and if mounted moist are liable to become discoloured and smelly.

Note: if your specimens are not completely dry, mould may develop on them and this can be very dangerous if you breathe in the spoors.

Warmth may be used to improve the drying rate, for example over a radiator or in an airing cupboard (in a humid climate, heat is essential). Placing your herbarium press sideways over a heater can be useful so that the hot air rises up through it, drawing out the moisture, but make sure that all you newspaper sheets lie in the same direction, so that the folded end is placed at the lowest point, preventing plant parts from falling out!

An oven set at 50°C may be used but the heat must be no higher, otherwise the specimen will become very brittle and damaged.

MOUNTING

Materials list for mounting plants: -

  • Cartridge Paper
  • Flimsy paper or fine grade tracing paper
  • Double sided acid free tape
  • Water soluble glue
  • Computer-generated labels
  • Folded paper for making loose specimen pockets
  • Hobby knife & ruler for any edging needed

Always prepare your working environment so that you have plenty of space to work in and make sure it is clean to avoid plant particles, cats’ hairs, dust etc. sticking to the tape before mounting.

Cartridge paper for mounting your specimens should preferably be A3 size and acid-free; the weight should be a minimum 180g/m2, and ideally with a rough textured surface. Heavy grade papers (e.g. Daler Rowney 300g/m Acid Free Aquafine) are a more expensive but excellent option, as the firmer cartridge looks nicer and will support a specimen better than the lighter grades. If you buy paper that is too big, cut it down to A3 size (the size required by official herbariums). Jagged edges look very messy and unprofessional – careful use a ruler and a hobby knife with a fresh blade, as this will produce clean, straight edges.

Using only one side of your thick A3 cartridge paper, arrange your specimens carefully, making sure that they represent the way the plant grows naturally. For example, a plant with a basal rosette of leaves and a single flowering stem arising from the centre should be represented this way. Don’t try to arrange flowering stems artistically in a separate fashion. Remember you are aiming to show the viewer what the plant looked like in nature.

If jeweller’s labels were used leave them on the plant when you mount it.
The specimens may be mounted directly on the paper with water-soluble glue – white PVA woodworking glue is ideal. Apply as a series of small dots to the back of the plant parts. Water-soluble glue allows for later removal of samples if required. Avoid using spray-on adhesive as it sticks to everything and you end up with a horrid sticky mess. Alternatively, specimens may be secured with thin strips of paper or sown on with linen or cotton thread but these methods are not as popular because specimens can more easily detach and become damaged.

Loose specimens such as seed pods, pieces of bark, flowers and leaves should be placed in a small paper packet that is then fixed to the mounting sheet. These paper packets should only be folded and not glued or stapled together as one has to be able to open them out flat in order to access delicate specimens easily without damaging them. These paper packets can be secured with a paper clip as required.

Specimens must be correctly identified and labelled, see figure 1:

Figure 1.  Example Herbarium Label

Scientific name: Ranunulaceae, Ranunculus ficaria (family, genus and species)

Vernacular name(s):  Lesser Celandine, Pilewort

Collector’s name and specimen number: Lawrence 1

Date of collection:  20th March 2003

Locality:  Orleans House Gardens, Twickenham, England

Habitat: damp, clay soil, 20 yards from riverbank, growing in dappled shade on the edge of deciduous woodland; nearby plant is Dock (Rumex obtusifolius).

Habit: perennial herb, up to 20 cm tall, with stems creeping and rooting

Characteristics: leaves hairless glossy green, flowers bright glossy yellow, turning white with age

Medicinal uses: astringent for haemorrhoids (Hoffman 1992: 223)

It is recommended that you design a label using your computer. This may then be used a template for all other herbarium specimens. Giving clear borders around your label allows for much neater cutting and presentation.

Note that the first species you collect and press should start with No.1 and this number should continue increasing throughout your lifetime with each herbarium specimen you produce. Note that all duplicates of a single species you collect at same time should all be given the same number. If later it is realized that a single species is actually made up of slightly different varieties then you can differentiate them using alphabetical numbering e.g. 1a, 1b, 1c… etc.

With regards to habitat, habit and characteristics, make sure you write only what you see and not what you expect to see. In other words, don’t just write down what your plant flora suggests you will find as this will be misleading the reader.

Don’t forget that characteristics are very important as they relate to all the extra signs that may be missing when a plant has been dried. Examples are:

  • Colour & aroma
  • Surface hairs: describe hairs type, shape and frequency e.g. Mullein (Verbascum thapsus) has stellately branched hairs thickly covering all parts.
  • Stem structure: general shape (e.g. square, ridged, etc), hollow or solid etc.
  • Exudate: describe any notable latex, resins, mucilage, etc (very important defining feature so note its colour, consistency and quantity, e.g. many Euphorbia spp. often produce copious amounts of white latex)
  • Glands: e.g. Hypericum perforatum; contains pellucid (see-through) gland dots on the leaves and flower and when these are rubbed they leave a smear of red colour on your fingers.
  • Pollinators: note insects and animals that you see visiting your plant.

Adding any extra facts such as ‘medicinal uses’ is an optional extra but a one-liner is mostly enough as space is often an issue. If you are going on an ethnobotany research expedition this obvious would entail gathering detailed information with regard to local uses, but this would be extra to the herbarium label.

The mounted herbarium specimen should then be protected from damage by using a covering flap made from fine translucent paper sheet or flimsy. Attach these covering sheets with double-sided acid-free tape. Never use plastic material to cover dried specimens as it encourages mould formation.

Protection from insect damage: if you think you may have an insect problem, seal herbarium specimens in plastic bags and place in a freezer for 2-3 days – it will kill insects such as the herbarium beetle. This is routine practice for all material donated to Kew.

Storage of collection: Storage is in a large A3 paper or card folder, with members of each genus being kept together. These folders are stored flat.

Manual on Herbarium technique: The Herbarium Handbook, by Diane Bridson and Leonard Forman, pub. Kew, ISBN 1 900347 43 1 – available from Kew Enterprises.

I hope this information is helpful – good luck and get collecting!

Best wishes, Niki

About these ads