Diagnosing Top-Down Dieback in Euphorbia (Not Cactus)
A 7-ft "cactus" with upward branches, slender leaves at tips, and top-down browning after 10+ years indoors is almost certainly a Euphorbia (African succulent), not a true cactus. The symptoms point to underwatering or root damage.
Euphorbia vs. Cactus: Key Differences
Sap: Euphorbia = white, toxic latex; Cactus = clear
Leaves: Euphorbia often has small leaves; Cactus = spines only
Flowers: Euphorbia = cyathia (tiny); Cactus = showy
Care: Euphorbia needs more water, less sun
Top-Down Dieback Causes
Cause
Mechanism
Solution
Underwatering
Roots die back; can't supply apex
Deep soak; then dry completely
Root rot (past overwater)
Dead roots can't support top
Unpot; inspect; repot if viable
Pot bound
No soil to hold water
Repot larger; fresh mix
Frost (if near window)
Cold damage to growing tips
Move away from glass
Correct Watering Method
Water thoroughly until drains freely
Wait until soil dry 2-3" deep (not just surface)
In winter: every 3-4 weeks
In summer: every 10-14 days
Immediate Steps
Unpot; check root health (white/firm = good)
If roots healthy: deep soak; increase frequency slightly
If roots rotted: cut above rot; callus; reroot
Move to brightest non-direct light
Euphorbias are more forgiving of underwatering than cacti but less tolerant of wet feet. The "teaspoon a month" advice is dangerously wrong for both.
