Do watering spikes and globes actually work?

These devices are bought by the million every summer and are the subject of a lot of confident nonsense in both directions. The honest answer is that the category contains one mechanism that works well and several that do not, and the difference is entirely about what controls the flow rate.

Step by step

1. Understand what controls the flow.The useful question about any of these is: what decides how fast the water comes out? If the answer is the dryness of the surrounding soil, it is a good device. If the answer is gravity and an air lock, it is unpredictable, because the same globe empties in an hour in loose dry compost and in a fortnight in dense wet compost.
2. Treat unglazed terracotta as the one that works.A porous terracotta spike or a buried olla releases water through the clay wall only when the soil outside it is drier than the water inside. As the soil wets, the flow slows and effectively stops; as it dries, it resumes. That is self-regulating irrigation, which is exactly what you want, and it delivers water at root depth rather than to the surface.
3. Be realistic about glass globes.A globe with a narrow stem pushed into compost releases water when an air bubble can get in, which depends on the compost structure, the size of the hole you made and whether the stem has silted up. They are decorative, they are cheap, and their delivery rate is close to unpredictable. Useful as a top-up for a few days on a houseplant; not something to leave a garden on.
4. Know that the plastic bottle version is the same device.An upended plastic bottle with holes in the cap is a homemade globe and behaves the same way — fast at first, then stalling. Burying a bottle with holes in the sides, filled and capped, works considerably better because it delivers at depth and the cap controls the air path.
5. Size it against the actual demand.A large tomato plant in full sun can transpire more in a day than most spikes hold. Work out roughly what the plant drinks — the volume you normally give it, times how often — before deciding how many days a half-litre reservoir will cover. This arithmetic is what stops people being surprised.
6. Prime the compost before fitting one.None of these devices can rescue dry compost. Bone-dry compost is water-repellent and the device will simply sit in a dry pocket while water runs around it. Soak the pot thoroughly first, then insert the device to maintain what is already there.
7. Make the hole properly for a terracotta spike.Water the pot, then make a hole with a dibber or a piece of dowel rather than forcing the spike in, which cracks them. The clay must be in firm contact with damp compost all round, or it has nothing to wick into.
8. Use them for what they are good at.Evening out watering for a plant that hates drying and rewetting, keeping a houseplant going over a long weekend, and maintaining moisture at root depth in a hot spell for a container you are also watering normally. They are supplements, not substitutes.

Tips

• Buy one and test it on a pot you can watch for a week before buying a dozen. The delivery rate in your compost is the only number that matters and no packet can tell you it.
• Terracotta spikes need cleaning if the clay clogs with mineral deposits. A soak in a weak vinegar solution and a scrub restores the porosity.
• In hard water areas the clay furs up faster. It still works; it just slows down over a season or two.
• None of these help a pot that is too small for the plant in it. If a container needs water twice a day in summer, the plant has outgrown it and the answer is a bigger pot, not a gadget.

Common mistakes

• Fitting a spike or globe into dry compost — Dry compost repels water and the device sits in a dry pocket doing nothing. The pot has to be soaked thoroughly first.
• Relying on globes for a two-week holiday — Their delivery rate is unpredictable and usually far too fast at the start. People come home to a drowned pot with a dry rootball or an empty globe and a dead plant.
• Forcing a terracotta spike into compacted compost — It cracks the clay, which then leaks at the crack and empties in hours. Make a hole first with a dibber.

Alternatives

• Unglazed terracotta spike: Self-regulating, delivers at root depth, works with a bottle or a reservoir. The best of the category. Needs a properly made hole and firm contact with damp compost.
• Buried olla pot: The same principle at larger scale — a porous clay vessel sunk in a bed and topped up. Excellent for a vegetable bed or a large container, and it lasts years.
• Glass globe: Cheap and decorative, delivery rate essentially unpredictable, prone to emptying quickly or silting up and stopping. A few days at most.
• Buried plastic bottle: Free, works reasonably well when buried with holes in the sides rather than upended in the surface. Ugly and effective.
• Capillary matting with a reservoir: For trays of small pots this beats every spike, keeping a whole tray evenly moist for well over a week for very little money.
• Battery timer and drip line: The only genuinely reliable option beyond a week. Inexpensive, needs an outside tap, and must be tested before you rely on it.

Questions people ask