Botanical art tends to be time consuming and it can be difficult to make your plant specimen last as long as it takes to complete your painting.
Ideally work from a growing specimen. A plant growing in a pot that can be taken to your workspace is the best option.
Try to have several hours and several days to work solidly on a painting until it is complete.
More often than not I have to paint from a cutting of a plant and I will need to keep that piece of plant in good condition for as long as possible.
If the plant specimen is particularly large put the end of the stem in a water filled plastic bottle and then enclose the entire stem in a large clear plastic bag. When you return to your workspace keep it outside in the coolest shadiest place possible.
Some plants still wilt even when treated in this way, especially if it is a large stem or something very delicate. If this happens cut the specimen into smaller sections and put each piece separately into vials of water and then put them in your fridge. I highly recommend cutting one or two flowers and putting them separately into vials and containers too. Flowers at the end of a long stem or a tree branch often wilt quickly.
An emergency treatment worth trying for a wilting flower is to float the individual flower in a suitably sized bowl of water. This will often re-inflate a floppy flower.
Drying plant specimens is another good way to preserve your plant material until your painting is finished.
To do this I press the specimens between blotting paper and boards, then label with the plant name and the date; this is still worth doing even if your plant material is no longer fresh. Then, if needed, botanical details can be clarified whilst you are still working on your painting.
Re-hydrating dried plant parts
It is possible to rehydrate dried pieces of plant if they are needed for further dissections. Gently bring to boil the necessary plant part in a small pan for about 30 seconds and then let it cool. The plant part will then be pliable enough to cut with a scalpel and examine.
Alternatively put the plant part in a little tub of water with a tiny bit of washing powder/soap flakes and leave it overnight and then the plant will be suitably soft.
One method I would recommend is to use a large airtight container with lots of cotton wool or similar light soft material and florist vials of water with caps. Take these with you to the location where you aim to collect the plant specimens.
Cut your plant part and immediately put the cut end of the stem into the water filled vial with the cap on. Place the plant specimens on the cotton wool carefully, put the lid on and put it into your fridge or somewhere else cold. This can extend the life of most plants considerably.