Why long-term patio containers need something sturdier

A pot that gets planted up in May and emptied in October can get away with almost anything. A pot that is expected to hold a shrub, a Japanese maple or a clump of hardy perennials year after year is a very different proposition. Its compost has to cope with freezing, thawing, baking, drying out and the slow collapse of its own structure.

Most multipurpose composts are built for short-term growing. They are light, airy and cheap, but the organic matter they contain breaks down within a season or two. As it decomposes, the compost slumps, the air pockets disappear and the whole root ball becomes a dense, waterlogged block. That is why a two-year-old pot often looks tired no matter how much you feed it. The problem is rarely hunger — it is the structure of the compost itself.

What loam-based compost brings to a pot

Loam-based compost — often sold as soil-based or by the numbered John Innes grades — is made from sterilised loam, peat or a peat alternative, and sharp sand or grit. That loam is the key ingredient. It is a balanced blend of clay, silt and sand, and it behaves in pots the way good garden soil behaves in the ground.

• It holds water and nutrients. Clay particles and organic matter cling on to moisture and plant food, releasing them slowly rather than in one flush.
• It resists collapsing. A loam-based mix keeps its pore structure for two, three or even four years, so roots stay aerated.
• It is heavy. That weight is an advantage on a windy patio — tall shrubs and standard bays are far less likely to blow over.
• It buffers mistakes. Loam is more forgiving of the occasional missed watering or an over-generous feed than a light, bark-heavy mix.

The trade-off is weight and cost, plus the fact that loam-based composts are less freely draining when young. Both are manageable. Put a layer of crocks or gravel in the base of the pot, and unless you are lifting it onto a balcony, the weight will rarely trouble you.

Choosing the right grade for the job

Grades are based on how much fertiliser and grit they contain, and getting this right saves a lot of fiddling later.

• Number 1 is fine and low in nutrients — ideal for sowing and rooting cuttings, too weak for a permanent pot.
• Number 2 suits most shrubs, perennials, herbs and alpines in containers. It is the workhorse grade for patio pots.
• Number 3 is richer and firmer, designed for mature specimens, trees in large tubs and long-term planting where the plant will stay put for years.

If you are planting a small acer in a 30-litre pot, Number 2 is perfect. If you are planting a bay tree or a camellia into a half-barrel that will not be disturbed for five years, use Number 3 and top-dress each spring.

Peat-free choices that still hold up

Peat-free growing is now the norm in the UK, and loam-based peat-free blends have improved enormously. Look for a mix described as soil-based or loam-based rather than a general-purpose peat-free compost, which is usually too fine and too fast to break down for a permanent pot.

Good peat-free long-term mixes combine loam or a loam substitute with bark, coir, wood fibre and grit. Bark and wood fibre provide structure; coir holds water; grit keeps the mix open. Be aware that peat-free composts often need a little more nitrogen in the first season, because the wood-based materials tie up nutrients as they decompose. A liquid feed from midsummer usually sorts this out.

Blending your own long-term mix

If you want full control, a simple homemade blend works beautifully. Mix roughly seven parts loam-based compost, three parts peat-free multipurpose or well-rotted leafmould, and two parts horticultural grit. Add a handful of slow-release fertiliser per 10 litres and a sprinkling of mycorrhizal fungi around the roots at planting time.

This blend suits almost anything woody or long-lived, and it is particularly good for Mediterranean herbs, lavender and rosemary, which resent sitting in soggy compost through a wet British winter.

Keeping pots in good heart for years

Even the best compost will not last forever, but you can stretch it considerably with a steady routine.

• Water thoroughly, then let it drain. Little and often is the enemy of container roots. Soak until water runs from the base, then wait until the top few centimetres are dry.
• Feed from spring onwards. Slow-release granules fade after a few months. Follow with a liquid tomato or seaweed feed every fortnight through the growing season.
• Top-dress every spring. Scrape away the top 5cm of compost and replace it with fresh loam-based mix plus a handful of fertiliser. This refreshes a pot without disturbing the roots.
• Mulch the surface. A layer of gravel or bark reduces evaporation, suppresses weeds and stops rain splashing the leaves.
• Repot every two to four years. If roots circle the inside of the pot, move the plant up a size, teasing out the outer roots and trimming away any that are dead or matted.
• Raise pots off the ground. Pot feet stop the drainage hole silting up and prevent winter waterlogging.

Get the compost right from the start and a patio pot becomes a genuine long-term home rather than an annual expense. Loam-based compost asks a little more of your budget and your back, but it repays you with plants that settle in, bulk up and grow better every year.