Soil recommendations for roof terrace containers

mdproctorxSWx61AG · 27 February

I'm new to container gardening (and gardening in general) and have a number of questions I hope people can help with. I have nine medium-sized containers made from fibreglass on my third-floor roof terrace.
7 x Visio 40 - rectangular (80x30x40)
2 x Cubo 40 - cubed (40x40x40)

I read I should make 1/2-inch holes along each side of the planter and around 5 inches apart. Then cover them with mesh to stop soil coming through. Does that seem reasonable?

I will put olive trees in the two cubes and a mix of tall grasses in the rectangular ones. I hope to keep the olive trees pruned so they never grow too high.

I calculate I'll need around 800 litres of soil. It's going to be great fun carrying up 53 x 15-litre bags

I read about the pros/cons of filling, and in the end, it seems to err towards just filling (unless you want to save costs). Does that seem correct? If I need a filler, I would probably buy a load of cheap plastic plant pots and turn them upside down.

I scanned a few forums and there are many complaints of poor quality soils, but one seemed well recommended, "SYLVAGROW® JOHN INNES NO.3". Do I need anything else, or just that in the pot? Some websites indicated using composite and then top-soil - so I'm wondering what is best.

One thing that worries me after reading several websites is the recommendation to replace the soil every year or two. Some seem to indicate a complete replacement, others just a top layer of soil. This worries me for practical reasons due to 800 litres of soil being three floors up. Are there products I could buy like those slow-release balls to replenish the soil nutrients, that I can just keep adding to avoid replacing soil? Pretty sure I know friends that have planters for years and never replace the soil, just some occasional liquid fertilizer, and the plants seem ok.

GardenerSuze · 27 February

@mdproctorxSWx61AG It is recommended to replace compost every few years as plants growing in containers need extra nutrients and more care than those plants directly grown in the ground. They will also outgrow their pots so potting on needs to be considered too. Pot bound plants won't thrive.
A mix of MPC and John Innes no 3 which is soil based are often used 50/50 for shrubs in pots. Even a small bag of John Innes is heavy perhaps check it out for yourself.
I can recommend Sylvagrow I used it last year for fifty small pots when moving house just oen plant lost, not a compost issue just too wet.
It is important to top dress pots every spring, simply remove the top few inches of compost and replace with fresh.
When planting something new after about six weeks the nutrients in the compost drop and plants will need to be fed depending on what is recommended for each individual plant. Different mediums will be required depending on what you plant and aspect/ wind issues also needs to be considered.
Calamagrostis Karl Foerster or C Overdam would be my go to grasses.They will stand well all winter unlike Molinias and some Miscanthus. Just to add I personally wouldn't feed grasses in a pot.

McRazz · 27 February

your planters arent large enough to warrant anything llightweight so JInr3 will be fine.

I don't know anything about these particular planters but if they don't already have drainage holes then i would aim to drill some 10mm holes to the base (not sides!!!). Furthermore you will want the planters to sit above the floor finish, so if they don't have any feet then i would consider shimming them at least 10-15mm off the ground. Some broadfix plastic shims from screwfix will be a cheap and easy way of getting this done and to a plumb level. If you place these right you'll achieve a nice shadow gap and the shims shouldn't be visible.

Assuming your planters are 400mm deep I would install the following;

-50mm drainage in the form of pea gravel

-300mm topsoil (JInr3)

-30mm fine composted bark mulch

If you can get a layer of woven geotextile in there under and above the gravel then that will help keep any fines from washing through. Regardless your paving/decking will get a little dirty - you just have to accept this as par for the course.

Void fillers such as extruded polystyrene, leca...or upturned pots, wouldn't be a concern of mine until the depth starts to exceed around 600mm, and in that case its usually only to address point loadings so you'll be OK.

If you keep regularly mulching and feeding you should be fine. We maintain roof terraces with planters that are now well into their second decade and we've never replaced any soils.

Butterfly66 · 27 February

Hello @McRazz, from your experience can I ask if there are any particular trees or multi stems shrubs you would recommend for containers? I have some which are roughly 1m cubes, Thank you

Apologies for dipping into this thread off topic

McRazz · 27 February

Butterfly66 said:

Hello @McRazz, from your experience can I ask if there are any particular trees or multi stems shrubs you would recommend for containers? I have some which are roughly 1m cubes, Thank you

Apologies for dipping into this thread off topic

Shrubs/trees regularly specified for planters and containers;

Cornus (mas, kousa), Amelanchier (lamarkii), Malus, Betula (jacquemontii), prunus (serula, plena), Acer (various), Cotinus, Osmanthus (burkwoodii, heterophyllus, aquifolium), Ilex Crenata, Euonymus, Sarcococca, Eriobotrya, Pittosporum, Viburnum, Phillyrea, chimonanthus praecox...

1m3 is a fair size. we have many of the above in smaller containers on our nursery.

amancalledgeorge · 27 February

mdproctorxSWx61AG said:

800 litres of soil being three floors up.

Have you checked for sure that the load bearing capacity of the balcony/terrace you're putting all the extra weight on three floors up? It will be an extra tonne which isn't to be sniffed at considering some flimsy engineering that is allowed in the UK.

Bilje · 27 February

@McRazz…you mentioned shims…I’ve never heard of them so of course had to ask Mr. Google. Well what useful items they look to be. I’ve always had the undergardener make me wedges from scrap wood!

Every day you learn something new.

Waterbut · 27 February

Gardeners World has adverts for small fruit trees to grow in patio pots which might be an option.

Butterfly66 · 1 March

McRazz said:

Butterfly66 said:

Hello @McRazz, from your experience can I ask if there are any particular trees or multi stems shrubs you would recommend for containers? I have some which are roughly 1m cubes, Thank you

Apologies for dipping into this thread off topic

Shrubs/trees regularly specified for planters and containers;

Cornus (mas, kousa), Amelanchier (lamarkii), Malus, Betula (jacquemontii), prunus (serula, plena), Acer (various), Cotinus, Osmanthus (burkwoodii, heterophyllus, aquifolium), Ilex Crenata, Euonymus, Sarcococca, Eriobotrya, Pittosporum, Viburnum, Phillyrea, chimonanthus praecox...

1m3 is a fair size. we have many of the above in smaller containers on our nursery.

Thank you for replying and apologies for the delay in acknowledging your reply. For some reason I’ve stopped getting any notifications. Thats a lovely choice for me to ponder 🙂

Soil recommendations for roof terrace containers

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