Aeonium arboreum 'Zwartkop': a winter grower that will rot if watered in August
Black rose works on an inverted calendar. It grows, colours and flowers from October to April, and shuts down completely from June to September. Almost every failure with this plant is a summer watering applied on autumn logic.
The inverted calendar, stated plainly
From October to April, at 10–20 °C, the plant is in active growth: it wants water every 8–12 days and light at 30,000–55,000 lux. From roughly 10 June, as daytime temperatures pass 26 °C, growth stops, the rosettes close into tight buds and the lower leaves yellow and drop. This is normal, not a disease.
Through that window the plant wants water every 30–40 days at most, applied to the pot edge, and shade from midday sun. A Zwartkop given normal summer water at 30 °C will lose its root system in two to three weeks and the stem will go soft from the base upward.
Watering through dormancy without killing it
The temptation during summer dormancy is to stop completely. That is worse than a light watering. A plant fully dry for ten weeks at 30 °C aborts fine roots, and when water returns in October there is nothing left to take it up — the rosette opens, then collapses.
The workable middle is a 200 ml watering every 30 days in a 15 cm pot, poured at the rim, at dusk. That is enough to keep the root hairs alive without keeping the mix wet. Then resume properly in the first week of October when night temperatures drop below 18 °C.
Getting the black: light, not water stress
Zwartkop is dark purple-black in full sun and olive-green in shade, and the transition is controlled almost entirely by light, not by drought. The colour saturates at 40,000–55,000 lux with cool nights. Plants kept indoors on a bright windowsill at 15,000 lux stay green with dark leaf margins.
Introduce outdoor sun gradually over two weeks in spring, starting with two hours of morning sun and adding an hour every three days. A plant moved straight from indoors to full sun loses its outer two leaf rings to scorch within five days, and those rings do not regrow until the following season.
Cutting back after flowering
Rosettes that flower die. A flowering Zwartkop sends up a conical yellow inflorescence in March or April, and that head is finished; the plant survives by branching at the base of the spent stem. Cut the flowered stem back to 3 cm below the inflorescence once the flowers fade.
Each cut stem usually produces two to four new heads within eight weeks, which is how a single-stem plant becomes a 1 m wide specimen in five years. The removed head can be rooted: leave it in shade for four days and set it on dry mix in September; roots in 20–30 days.
Parameter table
- Latin name
- Aeonium arboreum 'Zwartkop'
- Common name
- Black rose aeonium
- Rosette form
- spiral
- Light range
- 30,000–55,000 lux
- Water interval
- 10 days at 24–28 °C
- Rest period
- Summer dormant from June to September — the rosettes close and the plant must be kept nearly dry
- Hardiness
- 9b–11 (damaged below 0 °C)
- Offset behaviour
- Branches after flowering or after a cut; 2–4 new heads per cut stem, no stolon offsets
- Soil mix (by volume)
- 2 pumice : 1 akadama : 1 bark fines, by volume
- Mature diameter
- 25 cm
| Month | State | What the plant wants |
|---|---|---|
| January | Growth | Deepest rest. Water only if the lowest leaf ring softens; keep the room as bright as you can. |
| February | Growth | Late rest. Daylight is returning — begin adding an hour of supplementary light every two weeks. |
| March | Growth | Repotting window opens for summer growers. Trim roots, refresh the mineral fraction, dry for a week. |
| April | Growth | Growth restarts. First full watering at the rim, then resume the published interval. |
| May | Growth | Peak growth and the offset season. Feed at quarter strength every third watering. |
| June | Rest | Out of action for this genus — no water unless the plant visibly shrinks, and no repotting. |
| July | Rest | Out of action for this genus — no water unless the plant visibly shrinks, and no repotting. |
| August | Rest | Out of action for this genus — no water unless the plant visibly shrinks, and no repotting. |
| September | Rest | Out of action for this genus — no water unless the plant visibly shrinks, and no repotting. |
| October | Growth | Peak autumn growth. This is the season when most colour work should be done. |
| November | Growth | Daylight falls fast. Taper watering by a quarter and stop feeding entirely. |
| December | Growth | Slow, dry and bright. This is the colour peak for most genera in the collection. |
Tinted rows are the months this genus is genuinely out of action: no repotting, no feeding, and water only if the plant visibly shrinks.
Four things to take away
- Grows October to April; fully dormant June to September — an inverted calendar.
- Give 200 ml every 30 days through dormancy rather than nothing at all.
- Black needs 40,000–55,000 lux; it is a light response, not a drought response.
- Cut spent flower stems back to 3 cm for two to four new heads.
Informational only — This page is a cultivation record, not horticultural or plant-pathology advice. Any treatment should follow the product label and local regulation.