Barberry

Barberry is a deciduous shrub that is ideal for bonsai—its small leaves, beautiful bark, thorns, and bright berries make it very expressive.
Bonsai made from it is outdoor bonsai and can only be kept indoors occasionally.

Lighting
Full sun or light partial shade.
Minimum 5–6 hours of sun per day.
In the shade, it loses its foliage color and compactness.
The more sun it gets, the brighter the color of the leaves.

Temperature
Summer: comfortable: +18...+28 °C, critical: +38 °C
Winter: comfortable: 0...+5 °C, upper limit +8...+10 °C
Can tolerate -7...-10 °C for short periods.

Soil
Optimal composition:
40% akadama
30% pumice
20% lava (drainage)
10%–15% organic matter (see here)

Transplanting
Early spring, before bud break
Young plants — once every 1–2 years
Mature plants — once every 3–4 years
Root pruning.
Remove up to 30% of the roots.
Leave radial roots. 
Shorten thick roots, keep thin ones.

Watering
Water when the top layer is dry.
In summer — often, sometimes daily.
In winter — rarely, but do not overdry.
Do not water according to a schedule.
Control — according to the watering indicator (see here)

Shaping and pruning
Spring — main shaping.
Summer — pinch back new shoots.
Fall — sanitary. 
Shoots are shortened to 2–3 buds.
Wire is applied in spring or fall.
Check every 2–3 weeks.
Barberry bark is easily damaged — do not over-tighten.

Fertilization:
From spring to fall: organic fertilizers for bonsai (see here)
In fall: minimum nitrogen, potassium + phosphorus (see here)
In winter — do not fertilize.

Wintering
Cold wintering is required from –5 to +5 °C.
Cold greenhouse, balcony, basement with a window.
In sub-zero temperatures, make sure the roots do not freeze.
It is strictly forbidden to winter in a warm room.

Flowering and berries
Blooms in spring.
Berries set in good sunlight.
Light pruning is required after fruiting.

Pests and diseases
Aphids.
Spider mites.
Powdery mildew.

Prevention:
Ventilation.
Do not overwater.
Inspect leaves once a week.
Neem oil preparations (see here)

Common mistakes made by beginners
Pot too deep.
Warm wintering.
Overwatering.
Very heavy pruning.
Lack of sunlight.