the times Saturday January 1 2022
Outside 19
Shop-bought potting or
homemade compost?
The expert’s advice
Stephen Anderton explains which type
of compost you should use and when
to splash out at the garden centre
Potting compost is sterile,
so will keep weeds at baycomes from the greater warmth of a
compost bin, garden compost is often
still full of fertile weed seeds raring to go,
even if they’re only tomatoes. Using it
for potting is massively inconvenient.
The old garden writers would tell you to
riddle it to get the lumps out and mix it
with loam and sterilise it, but frankly
that’s a nonsensical performance when
all you want is a modest bagful.
For its own particular jobs, that
homemade compost, despite its faults —
its weed seeds, stalky bits and banana
labels — is brilliant. This is the stuff
to put into planting holes; it will feed
your soil in every way.
The potential for further decay is
almost the point. Whether you use your
compost as a mulch around shrubs and
perennials or have enough only for new
planting holes, regard the compost as an
injection of life itself. Try not to
suffocate the plant — a 10cm mulch of
happily rotted compost is sufficient.
Mulching with bought potting
compost would have little effect. The
fertiliser in it is intended to last
only a couple of months and
would soon wash through,
leaving a skim of thin
fibre relatively
unattractive to worms.
I often use spent
potting compost as a
sort of weed seed-free
cap on newly
disturbed garden soil,
when it becomes
available after I’ve tipped
out pots of bulbs in
summer.
Tiny gardens may not want to
give space to composting, of course, and
for them it’s possible to buy bags of soil
conditioner, which is nearer to garden
compost and meant for the same
purposes, either at planting time or
worked into soil that is poor or difficult
to handle. It’s cleaner than homemade
compost, but free it is not. You can’t beat
the simple economy of home-brew.My buying tips
Good peat-free compost tends to be
expensive, so you may not find it in little
hardware stores, but most garden centres
will have at least one kind — Westland
New Horizon and Melcourt SylvaGrow
are among the better makes.
The fact is, we all know it’s better to
buy peat-free in order to preserve the
peat bogs’ flora and carbon reserves.
What matters is to do it now, to increase
the market for peat-free and encourage
yet more research into improving it still
further. Soon it’ll be just as good as peat.C
ompost — it’s such a solid,
dependable thing. But it’s
actually two very different
things, which do their jobs
fantastically well. One is
free and comes in a heap;
the other is expensive and
comes in a bag. It’s hard to differentiate
between them, so here’s how to tell
them apart and determine their uses.
Shop-bought potting
compost
To me, it is a waste to use potting
compost to improve the soil in a planting
hole. It’s like oiling secateurs with extra
virgin olive oil. Bagged potting compost
is sophisticated: think of it as
energy-enriched, water-controlled
scaffolding for roots. Yet it has next to
no significant fibre in it, or rottable
material; it’s not going to contribute to
the life of the soil or do much to open
up its texture or attract worms.
Most importantly, it’s sterile. For
potting, sterility is a great thing. It
means your patio containers
and house plants won’t
sprout weeds or latent
pests. Nor will bought
compost produce a
dark river of silt from
the bottom of the pot
when you water it, as
happens when you
pot in garden soil or
garden compost. It’s
clean on paving and on
the hands.
Compost made in
a heap or bin
Back in the day, garden writers used to
insist that homemade compost should
have the colour and texture of Christmas
cake. I know what they meant — after
a couple of years there’s always a clump
like that at the bottom of my heap.
Real life is different. Most of us don’t
make it with the obsessive attention to
detail of an Escoffier, maintaining just
the right balance of carbon and nitrogen,
moisture and alkalinity to ensure that
fast, incredibly hot decay that kills every
weed seed in the heap. Even when it
Garden compost is the
stuff to put in planting
holes— it’ll feed the soil
ALAMY