Beginner - help with compost mix for containers

MeadowPipit · September 2021

Hi everyone. Newbie here - to both the forum and gardening! I collected a bunch of plants during july & aug from garden centres meant for putting in pots (outside), but am only just getting round to potting them now. I wanted to ask others if they have a reliable 'recipe' for what to use that works for perennials that you want to leave in the same pot for a bit? I am an absolute novice so apologies for asking for such basic advice, but I want to get it right rather than winging it and then seeing all the plants wither away and not understand why!

I have tried to read up online about the various options for compost, and what John Innes is and grit, perlite etc, but I am still unsure how to apply that all to my pots! I have John Innes no3, a multipurpose compost ('incredicompost') and horticultural grit. I already potted my lavender as a lavender grower recommended john innes mixed with an equal part of grit. I can't find specific advice on the rest of my plants though. They all appear to require moist soil with good drainage so i presume i can apply the same mix to all. I don't understand properly the difference between john innes and the multipurpose stuff so am wondering if there would be a benefit of mixing in both? If you had the above products how would you all use them for the below type of plants? Should i just use john innes and a little grit, or maybe split an equal ratio of JI, the multipurpose and grit? Or would some prefer no grit at all? I am not potting them for just a short period, so they will be stuck with whatever i put them in for a while, unless they outgrow their containers (or die!)

I have:
Nepeta (neptune/prussian blue/six hills giant
campanula (rapido/spring bell/hirsh blue/po ocean/blue sky)
bacopa abunda blue
blue daisy
carnatrion
phalaris arctic sun
nemesia myrtille
aster
veronica moody blues
dianthus
diascia
pennisetum malopecuroides hameln

Incidentally I realise there is no 'right' answer as such, but very keen to hear opinions.

Fairygirl · September 2021

It's quite a variety of plants, so this is a bit tricky.
The Dianthus are hardy perennials, as is the Carnation [ I presume that's what you meant?] Aster, Veronica, Nepeta, and most Campanulas. If they're staying in pots long term, compost is no use. You need a soil based medium for all of them. Campanulas like damper conditions and more shade than the others too, and the others need a gritty mix. They'll be fine short term though, in a general compost, providing you have them in suitable sites.
Most Diascias, Bacopa and Nemesia are annuals, or short lived perennials, so they may not last long. Compost would be fine for them if they are only for the next month or so. I don't grow any of those, so you may need to research the varieties you have.
The Pennisetum isn't very hardy, so you'd need to overwinter it if you're in a colder area.
Phalaris tend to be pretty tough, and can be invasive, so you may need to contain it well. Not a variety I know though.

I don't know what the blue daisy is. You'd have to know the species

MeadowPipit · September 2021

Thanks for your reply, that is all very useful. I had no idea the Pennisetum was invasive, I will be careful with it. That's the problem when you go and buy plants because you like the look of them rather than with any knowledge

The blue daisy is felicia felicitara blue. And yes I did mean carnation, it was just my butter fingers!

So as regards the majority of the perennials just leave out the multipurpose and use John Innes because it won't rot down over time like compost? When you say gritty mix for the non-campanula plants, what kind of ratio of JI to grit would you recommend?

Dovefromabove · September 2021

For growing a mix of herbacious perennials in containers I would make a mix of 3 parts of John Innes No 3 loam-based potting compost, 1 part multipurpose compost and 1 part horticultural grit.

Fairygirl · September 2021

Not the Pennisetum - the Phalaris @susclarke. Pennisetums are fine.

I'd agree with @Dovefromabove re the mix, although just a general mix of grit and a loam based compost will be fine for most.
I haven't grown Felicias for a very long time, so I can't remember how hardy or not they are, but I think they're generally only half hardy, so would need protection.

MeadowPipit · September 2021

Thanks for the info both of you, you've answered some questions which I have struggled to find clear answers for elswhere on the internet. This forum is fab

@Fairygirl yes sorry, I misread which was the invasive one! Oddly the Phalaris was the one plant that I regreted buying - purely in terms of appearance. It doesn't really work in a grouping with the others. So now I know it is a bit of a bully too, I might relegate it to a corner somewhere on its own

Just one final question. Is there any reason to ever use horticultural sand/sharp sand instead of horticultural grit when trying to get the perfect moisture to drainage balance in containers? They look completely different so wondered if they function differently too?

Fire · September 2021

As Dove says John Innes no3 recipe (a rich, loam-based mix) is a good choice for a long term, perennial container planting - with varying degrees of grit for drainage.

There is lots of confusion about the term "compost". It can mean the waste you put into a compost bin, or the biodegraded mix you get when it's broken down. It can mean "brown, crumbly stuff you buy in a bag from a garden centre" or specifically multipurpose MPC (a short term medium, with or without fertiliser added). It can mean a MPC mix for sowing seeds. Or a growing mix made up to John Innes recipe.

When Fairy says "compost is no use" I think she means MPC, a medium that may have fertiliser mixed to it enough for one season. You might use this to grow annuals, tomatoes, comos etc.

@susclark It's very worth looking up each of your plants and find out what it likes to grow in. Your plants have a wide taste in sun, drainage and nutrition. Find out where it likes to grow in its native sites - on the side of a stony, sun-baked mountain or in sandy soil in shade or in rich soil.

Fairygirl · September 2021

Grit is just bigger @susclarke

Grit sand is fine, but just be careful of the descriptions of sand in various places, because the labelling can be misleading, and the wrong type can make things worse re drainage

Yes- compost of any kind on it's own isn't any use long term

Probably wise re the Phalaris - until you see how it grows

Loxley · September 2021

The sharp sand is slower draining and denser than grit, it will hold on to moisture longer than grit. I sometimes use it as a layer at the bottom of deep pots if I happen to have it to hand, as it's cheaper than compost. The downside is ants love nesting in it.

Jac19 · September 2021

I put a thin layer of bark chipping at the bottom.
And then less than an inch thick layer of clay pebbles.

And then I put a John Innes No. 2 or No. 3 in a wheelbarrow or just on the ground. A peat based multipurpose compost. Some grit or sand depending on how much drainage the plant needs.

When I plant an area in the garden, I generally take away several inches of garden soil off the top and put it to pasture. To the above compost mix, I add several shovelfuls of this. If you don't have a garden, you can buy topsoil in bags.

I mix up the soil-compost really well and add the mix into the pot on top of the clay pebbles.

Instead of my mix, sometimes I buy "Westland Rose Planting and Potting Mix" where a soil-compost mix comes pre-made for me and I add grit or sand depending on the drainage the plant needs. There are other potting mixtures with perlite in them I have bought in the past. These mixes are expensive though, but fast and easy to pot up.

At the top I add in a couple of cups of Blood, Fish, and Bone granular fertilizer.

Nowadays, I also add a couple of cups of fish-tank active substrate which seems to work as a slow-release super fertilizer. I bought lots of tubs of active substrate full of nutrition for aquatic plants that go in a fish tank under a pebble layer, providing nutrition for the plants to grow into a thick green layer. I then decided to just use a pebble layer only. So, I have to get rid of these tubs sitting in my garage.

Anyway, I mix the fertilizer into the soil at the top with my fingers.

And then I plant my plants.

I also make sure to buy farmyard manure and mulch in handfuls around the base of the plants.

Jac19 · September 2021

My SUNBELIEVABLE BROWN EYED GIRL dwarf sunflowers doing really well in their pot home. It's an annual and there are 5 plants growing in there. They start flowering in May and go on like this till end of November. I buy them as 2 packs of 6 plug plants - 12 altogether. I bring up 5 each in 2 pots, and I put 2 plants in the ground.

This is staple food for my Bumbles and Honey Bees. Lifesavers for them.

I mostly put only annuals and bulbs or tubers in pots nowadays because I have a garden this time. One exception, I have 8 Lavender pots, big ones. The others are all nursery pots I will put in the ground, waiting for me to dig up and prep patches of land for them.

Beginner - help with compost mix for containers

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