Watering through a 35 °C summer without losing the collection
Hot-climate summer is not winter with more light. It is a second, less obvious rest period in which the plant's water use collapses while the substrate's evaporation rate triples — and those two curves moving in opposite directions is what kills plants in August.
What actually happens above 30 °C
Most Echeveria, Aeonium, Sedum and Sempervivum close their stomata above roughly 30 °C during the day and switch to CAM-idling: they take up almost no carbon dioxide and transpire at 10–20 % of the spring rate. The roots remain alive but stop drawing water at any useful rate.
Meanwhile the pot is losing 40–80 ml a day to evaporation at 35 °C in moving air, which is three to five times the spring figure. The substrate therefore dries out while the plant is not drinking, and the top 2 cm reading bone-dry tells you nothing about whether the plant needs water. It does not.
The three-week rule and the two exceptions
In the six hottest weeks, water at three-week intervals, in the evening, and only if the plant shows a genuine symptom: the lowest ring of leaves beginning to lose firmness, or leaf margins rolling inward. Firm leaves at 35 °C mean do not water, however dry the surface looks.
The two exceptions are Lithops and any plant in a pot under 8 cm, which dry out faster than the plant can survive on stored water, and Aeonium and other strict summer-dormant species, which are covered by their own rule of a measured 200 ml every 30 days.
How to apply the water
Water at the rim, in the evening after 19:00, and give roughly a third of the volume you would give in spring — 150 ml for a 15 cm pot rather than 450 ml. Do not soak to runoff: runoff in a hot pot wicks back up as the surface cools overnight and leaves the crown wet at dawn.
If the plant is in a tray or saucer, empty it within thirty minutes. Standing water at 28 °C grows Pythium aggressively, and the first sign is a stem that goes soft between two healthy leaf rings.
Shade, air movement, and the afternoon gap
Between 11:00 and 16:00, provide 30–50 % shade, more for plants that spent winter indoors. Sunburn at 85,000 lux and 38 °C happens on the upper leaf faces in a single afternoon and the scarred tissue never recovers its farina.
Air movement matters more than temperature. A 15 cm fan running continuously across a shelf at 35 °C lowers leaf temperature by 4–6 °C through transpiration and cuts the rot rate dramatically. If you can only add one thing to a hot room, add the fan rather than the shade cloth.
| 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 | Growth | Lengthening days, rising heat. Watch the leaf turgor test rather than the calendar. |
| July | Growth | Hottest weeks. Water in the evening, a third of the spring volume, and only on a visible symptom. |
| August | Growth | Heat eases at night. Prepare mix and pots for the autumn repotting window. |
| September | Growth | Autumn window for winter growers. Root-prune and repot while the soil is still warm. |
| 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
- Above 30 °C most succulents stop transpiring but the pot keeps drying — surface readings mislead.
- Water every three weeks in the hottest six weeks, and only on a visible symptom.
- Give a third of the spring volume, at the rim, after 19:00; never soak to runoff.
- Moving air lowers leaf temperature more than shade does.
Informational only — This page is a cultivation record, not horticultural or plant-pathology advice. Any treatment should follow the product label and local regulation.