Horticultural Grit, Sand and Other Drainage Materials Compared

What improves drainage is particle size and shape, not the name on the bag. Angular grains of a consistent size wedge apart and leave air gaps; fine, rounded or mixed-size particles pack down and seal the gaps. Grit and washed sharp sand suit pots, seed mixes and raised bed fill; perlite is for aeration in containers; vermiculite holds water; and bulky organic matter is what changes clay over the seasons. Tipping grit onto heavy clay on its own will not fix it.

Is perlite the same as horticultural grit?

No, though they often do the same job in a pot. Grit is crushed rock, angular and heavy, and it doesn't break down. Perlite is volcanic glass heated until it pops into light white granules full of air. Both open up a compost so water drains through, and for herbs in pots either will do at up to a quarter of the mix by volume. Choose grit where weight and a permanent, rock-hard texture help, such as top-dressingAdding fresh compost, feed or grit to the soil surface around an established plant, rather than digging it in. Full definition → alpines or adding body to a seed mix, and perlite where you want a light, airy container mix.

Vermiculite is the odd one out. It is sold alongside perlite but does the opposite, soaking up water, which is why it belongs in seed-starting mixes and not where you want drainage. Sharp sand sits between grit and soil: washed, angular sand for potting mixes and raised bed fill, and not the same thing as builders' sand.

The mechanism, before the products

Drainage is a question of pore space, the gaps between solid particles where water moves and air gets in. Clay particles are flat, microscopic platelets that stack tightly and leave almost no room for water. Something coarse and angular between them props them apart and creates channels. Something fine, rounded or that breaks down under pressure fills the gaps and can pack into something closer to concrete than soil.

Lightweight porous particles such as perlite and vermiculite work differently: they hold their own internal air or water rather than propping the soil apart, which is why they behave differently in a pot than dug into a border. Bulky organic matter (composted bark, well-rotted manure) works a third way, by feeding soil biology and binding clay particles into stable crumbs.

What each material is

Horticultural grit

Crushed rock, screened to a consistent size, with sharp angular faces rather than the rounded edges of naturally weathered stone. It is commonly sold in roughly the 1 to 6mm range, but grades vary by supplier, so check the bag: finer "potting grit" is sold for seed and cutting mixes, and coarser "horticultural" or "landscape" grit for mulchingA layer of material spread over the soil around plants to suppress weeds, retain moisture, and improve soil structure as it breaks down. Full definition → and borders. Reputable horticultural grit is sold at a neutral pH, which matters near lime-sensitive plants such as rhododendrons, blueberries and heathers, which need acidic soil.

Grit also does a second job in a wet climate. A top-dressing of grit around the crownThe base of a perennial plant where roots and shoots meet, often sold and planted as a ready-made dormant plant rather than grown from seed. Full definition → of alpines, sedums and other plants prone to rotting off keeps foliage and stems away from standing surface moisture over winter, which is often what kills a borderline-hardy plant rather than cold itself. Raising a container onto pot feet or bricks does the same for the drainage holes, so they do not sit in a puddle through a wet winter.

Sharp sand (horticultural, washed)

Coarse, angular sand washed to remove silt and clay fines, which clog pore space rather than opening it, and sold for potting and soil-improvement use. It is washed, gritty sand for potting mixes, and it is not the same as builders' sharp sand, even though the two share a name.

Perlite

A volcanic glass that is crushed and heated until trapped water vaporises and the particles pop into lightweight, porous white granules. It holds very little water or nutrient, dries quickly, and its job is aeration and fast drainage inside a growing medium rather than drainage in open ground. Its manufacture and transport carry an environmental cost, so where perlite was traditionally used, grit or gravel is a lower-impact alternative. Crushed granite and other rock grit are quarried in Scotland, so local supplies are usually easy to find. Perlite and vermiculite both turn up in bagged peat-freeCompost made without peat, which is harvested from a slow-forming, carbon-storing wetland habitat that peat extraction damages. Full definition → compost as the ingredient doing the aeration or moisture-holding job peat used to do, which can explain a mix's drainage behaviour.

Vermiculite

Heat-expanded hydrated mica, in soft spongy flakes. Unlike perlite it holds water readily and holds more nutrient, so it acts as a moisture reservoir rather than a drainage channel. That is why it is the standard addition to seed-starting and cutting mixes: it keeps germinatingA seed sprouting: breaking dormancy and putting out its first root and shoot, given the right warmth and moisture. Full definition → seed from drying out while roots push through easily. It is the wrong material if you want water to move away faster.

Bark and composted organic matter

Composted bark, well-rotted manure, garden compost and leafmould are not drainage materials in the particle sense. They feed the soil organisms that bind clay into stable crumbs, opening pore space as they break down, and they keep doing it every time more is added. Heavy clay needs plenty of bulky organic matter, ideally composted bark, alongside raised beds and no-digA growing method that avoids turning the soil over, building fertility instead by adding organic matter on the surface. Full definition →, with organic mulches around permanent plants.

Which material for which job

Grit and sharp sand also have a place worked into the top layer of an ordinary heavy loam border in modest quantity, where they can speed how quickly the surface drains after rain. The scale problem below applies with full force to clay; a workable loam that drains slowly needs far less.

Which plants want grit

Grit earns its keep for plants that dislike wet roots, and the usual example in a wet Scottish garden is the Mediterranean herbs: rosemary, thyme, sage, oregano and lavender. For herbs in pots, add up to 25 per cent by volume of coarse grit or perlite to a loam-based compost. Herbs overwinteringKeeping a plant alive and intact through the cold months, whether left outdoors, moved under cover, or stored dormant. Full definition → outdoors need to avoid being frozen, saturated by rain or allowed to dry out. In a raised bed for alpines, bulbs or herbs, double the proportion of sharp sand in the fill mix. Full recipes are in making your own peat-free potting mix, a gritty mound for lavender is in pruning lavender, and a rosemary that has already suffered is covered in reviving woody rosemary.

Horticultural sand against builders' and play sand

The mix-up is common because the builders' merchant is cheaper and closer than the garden centre. Three differences explain why gardeners are steered away from builders' sand:

• Particle uniformity. Horticultural sand is graded to a consistent size. Builders' sand is a mixed range by design so that it compacts well for concrete and mortar, which is the opposite of what drainage needs.
• Fines, silt and lime. Builders' sand is not washed to the same standard and can carry silt, clay fines and traces of lime, which can shift soil pH and encourage the compaction you are trying to avoid.
• Origin. Horticultural sand is washed and screened for growing use; builders' sand is not marketed or tested against gardening standards.

Soft play sand is often very fine and rounded, which packs together rather than propping apart. Builders' sand has its own uses, but for potting mixes, seed compost or anything in direct root contact it is a false economy.

Cost and availability in Scotland

Prices vary by supplier and size, so compare before a large project. For grit or sand by the tonne bag rather than a few 25kg sacks, a local quarry or aggregates supplier is usually cheaper per kilo than garden-centre bags, so it is worth a phone call. Buy perlite and vermiculite in the quantity a season's seed-sowing and potting needs, as they lose usefulness once compressed or repeatedly wet down.

For scale, in a 3:2:7 mix (organic matter : sharp sand : topsoil), sand is a sixth of the fill by volume. Work out the bed's volume (length x width x depth) and take a sixth of that to size the sand order, rather than guessing: a 1.2m by 1.2m bed 30cm deep is about 430 litres, so about 70 litres of sand, a few 25kg bags and not a pallet.

Free drainage in pots and raised beds matters more the wetter the plot. Edinburgh's Royal Botanic Garden averages about 728mm of rain a year and Tiree about 1,275mm, so a container mix that copes in a dry spell can sit waterlogged for days after heavy rain on a west-coast plot. Some gardeners also find a grit mulch is an uncomfortable surface for slugs to cross, though the effect is not firmly established. Machair and other sandy coastal soils in the Hebrides and along stretches of the west and north coasts already drain too freely; there the priority is bulky organic matter to hold moisture, not more grit or sand.

Why grit alone will not fix clay

This is the mistake that wastes the most time and money. For a soil with about 40 per cent clay, halving the clay proportion in the cultivated layer enough to make it easy to work would take roughly 250kg of grit or gravel per square metre. That is several bulk bags per square metre, impractical and expensive beyond a very small bed. Adding materials to clay can also make it less stable, so the soil becomes harder to manage.

What changes clay soil is raised beds (bypassing the problem soil rather than amending it), a no-dig approach, gypsum where the soil is not acidic (test on a small area first), and bulky organic matter such as manure or composted bark, repeated, plus organic mulches around permanent plants. Grit and sand have a real role inside a raised bed fill mix or a pot, not as a stand-alone fix stirred into unimproved ground. The raised bed materials guide covers what to build the bed from.

Common questions

Sources