Why a self-watering pot suits miniature gardens

Miniature gardens are lovely in theory and slightly nerve-racking in practice. A three-centimetre thyme in a shallow pan has almost no compost to draw on, so a hot Saturday afternoon can undo a fortnight of careful watering. Self-watering containers solve the problem by giving the plant a hidden reservoir to drink from as it needs, rather than a drenching followed by a drought.

The principle is old and simple: water travels upward through a wick by capillary action, drawn from a reservoir below into the compost above. The compost stays evenly moist rather than sodden, because the level of the reservoir sits below the roots. You top up once or twice a week instead of peering at tiny pots every morning, and the plants get the steady, gentle moisture that alpines, mosses, small ferns and herbs all prefer.

Best of all, you can build one from things already heading for the recycling bin. A standard build takes about ten minutes and costs nothing.

Your recycled shopping list

For one miniature self-watering planter, roughly the size of a large margarine tub, gather the following.

• Outer container: a clean two-litre plastic bottle with the top cut off, an ice cream tub, or a sturdy yoghurt pot. It must be watertight and about 8–10 cm deep.
• Inner planting basket: a smaller tub, a fruit punnet, or the bottom half of a second bottle. This sits inside the outer one and holds the compost.
• Wick: a 20 cm strip of worn cotton t-shirt, an old cotton shoelace, or a few strands of wool yarn. Cotton and wool wick beautifully; nylon and polyester do not.
• Filler tube: a 10 cm length of plastic bottle neck, a cut-down drinks straw, or a piece of rigid tubing. This lets you add water without disturbing the plants.
• Growing medium: peat-free multipurpose compost mixed with about a quarter volume of perlite or horticultural grit. Avoid garden soil, which compacts and wicks poorly.
• Tools: scissors or a craft knife, a skewer or nail for making holes, and a marker pen.

How to build it in ten minutes

Start with the inner basket. Using a skewer, punch a ring of holes around its base and sides, roughly every 2 cm. These let water and air move, and they prevent the compost sitting in standing water. Next, punch a single overflow hole in the outer container, about 3 cm up from the bottom. This is the most important hole in the whole build: it sets the maximum water level and stops the reservoir ever drowning the roots.

Thread the wick through one of the holes in the base of the inner basket, leaving about 12 cm hanging below and 5 cm coiled inside the basket. Push the filler tube down through the basket as well, so its top sits above the compost surface and its bottom reaches the reservoir. Then sit the basket inside the outer container. It should rest on the base, with a couple of centimetres of clearance around the sides.

Check the fit by pouring water into the reservoir until it trickles from the overflow hole. If the basket floats or the compost line dips below the overflow, add a few clean pebbles underneath as a spacer.

The wick: where most homemade builds go wrong

A self-watering pot lives or dies by its wick. Three things matter more than anything else.

• Contact. The wick must touch the bottom of the reservoir and run continuously up into the compost. Any gap breaks the chain and the water stops climbing.
• Material. Natural fibres only. If you use a synthetic shoelace or a strip of old sportswear, the water will simply sit in the reservoir while your plants wilt above it.
• Length and thickness. A wick that is too short dries out; one that is too thin cannot move enough water for a thirsty plant. For a tub this size, a strip about 3 cm wide folded double is about right. Two wicks suit larger planters.

Before planting, soak the wick in water for ten minutes and give it a squeeze. A pre-soaked wick starts working immediately, whereas a dry one can take days to prime.

Planting up and the settling-in fortnight

Fill the basket with your compost and grit mix, firming it gently around the wick so there is good contact but no air pockets. Plant your miniature subjects — a small sedum, a clump of mind-your-own-business, a tiny fern, a few dwarf basil seedlings — and water from above once to settle them in.

Now fill the reservoir through the tube until water appears at the overflow hole, and stop. For the first fortnight, water lightly from the surface as well, because young roots have not yet reached the wick. After that, the reservoir does the work. Top up when the level drops, which in a warm British summer will be every three or four days, and less in spring and autumn.

Looking after it through the British year

In July, a sunny windowsill planter may need filling twice a week; in November, once a fortnight is plenty. Always empty the reservoir if heavy rain has filled it, and never leave the pot standing in a saucer of water. From October, cut back on topping up and let the compost dry slightly between refills, which suits most miniature plants going into dormancy.

Every spring, lift the basket out, rinse the reservoir, and replace the wick. Compost breaks down and wicks clog with algae over time, and a fresh wick is the cheapest way to keep the whole system working for another year. Give the compost a light refresh too, replacing the top centimetre or two with fresh mix and a pinch of slow-release feed.

Once you have made one, you will spot containers everywhere that could be pressed into service. A row of matching self-watering tubs along a windowsill, filled with tiny alpines or herbs, is one of the kindest ways to grow in a small space — and one of the least demanding.