Adrian wrote:

The gloriosa, in particular, is a very strange-looking beast - being a
long tuber, a bit like a very long, fat potato chip...

Adrian this is far easier to do than explain, so don't hesitate to ask
for clarification if necessary. Anyway here goes...

Every year Gloriosa produces a new set of tubers, the old ones
shrivelling away as they nourish the growing shoots. Unlike many
tubers, Gloriosa has its growth buds concentrated at the tip and not
scattered along the length. Normally, they are paired rather like a
two-pronged fork and joined at the base of the previous year's shoot.
If these the apical buds (those at the tip of the tuber) are damaged
then the entire tuber usually dies.

It's not a good idea to cut the tuber into sections since latent buds
rarely form along its length and you are effectively reducing the
amount of nourishment that will be available for the new shoots. In
the Gloriosa world, the longer and fatter the tuber, the stronger the
new growth, which will produce more flowers over a longer period.

As you've found, these tubers are very difficult to accommodate.
Planting them horizontally means that you need a much wider pot than
is necessary. They should be planted vertically at an angle in a deep
pot with the cut or broken end of the tuber uppermost at the edge of
the pot. The 'active' tip should be towards the centre and several
cms. deeper. This reduces the need for such a wide pot.

When I grow Gloriosas, I dont try to bury the entire tuber and allow
the first few cms.of the cut end to remain above compost surface.
Also, if the tuber is relatively short, it can be planted vertically
in a sweet pea tube and once growth is well under way, the lot then
transferred to a bigger container. Larger tubers are probably better
started in home-made (non-plasticised) cardboard tubes about 10cms.in
diameter and 20 - 25cms. deep. These are then easily transferred to
large pots at a later stage. The cardboard degrades naturally and
roots will grow through it..

Gloriosa tubers are very prone to rotting if the compost is kept too
wet, hence the advice to avoid 'overpotting'. It is very important
that the compost is free-draining and the addition of 1 part
horticultural grit/sand to 4 parts compost ( a good quality, not cheap
multi-purpose is OK) is ideal. Water well and then allow the compost
to nearly dry out between waterings. When the new shoot appears, you
can gradually increase the frequency of watering, but never allow the
compost to remain sopping wet. Once the new shoot is about 12 -
15cms high, you can repot into 25-30cm diameter container and it is
best to insert support canes at this stage rather than risk damaging
the roots later.

After a few weeks you should start to apply a general purpose liquid
fertiliser at 10 - 14 day intervals. This will promote strong growth
and encourage the formation of flower buds. When flowering has
finished, continue feeding and watering until the leaves start to
yellow. It is during this time that the new tubers are being formed
to provide next year's plants so they need a bit of encouragement.
Gradually reduce watering until the top growth has completely
yellowed. Overwinter dry in a frost free place and then carefully
remove the new tubers from the compost in early spring. This time you
will have a pair, which you can either split or keep together. Just
make sure you don't damage the ends.