Horticultural/Alpine Grit

[LindseyEd]

Hi, newbie here!

I am mostly looking to grow Primula, Dianthus and Campanula, but also a few other species. I am an experienced general gardener but it is a while since I dabbled in alpines and I'm rapidly discovering I've forgotten most of what I once knew and that was probably never enough in the first place!

My latest problem is the standard crushed flint 'horticultural grit' that I remember using no longer seems available in garden centres. So any pointers on what to use would be very welcome.

I'm looking for a source of grit in small quantities (my ambition doesn't yet stretch to using a 1 ton bag!) and after searching online I am now thoroughly bamboozled by the choice of types, frustrated by the lack of small sized bags, and also uncertain as to exactly what features are desirable.

Should I expect to buy different grits for the potting mix from those used for top-dressing?
What particle size should I be looking for?
Is it best to have a mix of particle sizes or all approx the same size?
How importance is sharpness? (many gravels look very rounded)
How do things like crushed Cornish granite compare to flint?
Should I be considering eco-agregates like crushed terracotta?
For lime-lovers, should I be looking for a limestone, or a neutral product and then add garden lime?
And can anyone recommend a supplier for the south of England who will deliver in just small bags not by the ton?

Any advice would be greatly appreciated.

[Darren]

Hi LinseyEd and welcome to the forum. You will get a lot of valuable advice from more experienced alpine growers than myself but I can give you my opinions on some of your questions:

Should I expect to buy different grits for the potting mix from those used for top-dressing?

No - I don't personally think so. Some growers of cushion plants advise using a coarser grade as topdressing to allow better air flow around the neck of the plant. I have found that this often creates nice snug hidey-holes for pests such as woodlice and cutworms.


What particle size should I be looking for?

Around 5mm is good but you have a lot of leeway provided it isn't too small (sand). I'd avoid anything much bigger than 10mm too.

Is it best to have a mix of particle sizes or all approx the same size?

Best to have it fairly uniform, or at least without too much fine material. Fine particles pack into the spaces between the bigger ones and reduce the air-filled porosity (AFP). Good AFP is vital for the health of plant roots and especially alpines. You will hear AFP and 'drainage' used interchangeably though they are not the same thing.

How importance is sharpness? (many gravels look very rounded)

Again - sharper particles pack together better, thus potentially reducing AFP, though in reality provided particle size is fairly even it is OK to use either sharp or rounded as the difference isn't that great. If growing bulbs it is often advised to use rounded gravel to avoid 'damaging the bulbs' but personally I haven't found any damage I can attribute to this.

How do things like crushed Cornish granite compare to flint?

Granite is probably a bit better than flint as when freshly crushed it will provide trace elements from the various minerals as they decompose, whereas flint is largely just silica. Either is really OK though.


Should I be considering eco-agregates like crushed terracotta?

Why not? Even better - try it then tell us how you got on! Many of us are experimenting with fired clay materials such as seramis and even fired clay cat litter.


For lime-lovers, should I be looking for a limestone, or a neutral product and then add garden lime?

Using limestone grit isn't really necessary even for most lime-lovers. Much UK limestone is so hard and impervious that it contributes little lime to the soil unless freshly broken/crushed. I add powdered dolomite lime to substrates for certain lime-lovers such as some Irises. Ordinary garden lime is probably OK if you can't get dolomite.  Crushed tufa is also said to be good but tufa is hard to obtain in the UK now.

[SJW]

I can get Jondo poultry grits locally at a reasonable price. Their range, in increasing particle size, goes from chick, growers, hen, to turkey. I can then select for use - a mix of the first three (depending on species) for use in composts and I top dress with either growers or hens. As Darren says, the Tesco low-dust cat litter makes a really good replacement for grit for many species.

[Maggi Young]

Welcome LindseyEd - we are fortunate in Scotland because  in the building trade  granite gravel odf around 6mm size is widely used  in making render for houses - we call it "harling" stone - so it is available in lots of places - you can buy 20kg bags from  B&Q, for instance.

Most of our plants in pots now are bulbs of some sort and we basically use  a mix that is, by volume:  two parts gravel, one part leaf mould , two parts sharp sand. This can be adapted for troughs or other plants on pots with a touch more leaf mould for woodland subjects or  tweaked for alkaline loving plants as desired.  A very versatile mix that works well for us in North East Scotland.

[David Nicholson]

Welcome to The Forum LindseyEd. I've looked at the link you provided for crushed terracotta and that shows a partical size of 5-14 and in my view that would be a bit on the large size for both topping and mixing. I use a grit 0-4 which is Chard flint (widely a available in the South West) or sometimes Kilmington gravel. I shake this through a reasonably fine riddle and what comes through the riddle I use in my potting mix and what stays in the riddle for topping.

I grow a lot of Primulas in pots under glass and upto a couple of years ago always topped them with limestone grit. This became hard to find and I now use the grit I referred to above for every thing. I haven't noticed any difference. I do add a bit of lime to my potting mix,dolomite lime when I can get it or if not pelleted garden lime in small quantities.

If you feel like giving us an idea where you garden I'm sure Forumists in your area will let you know where the good suppliers are.

[ChrisB]

Hi LindseyEd,  welcome to our forum, we love it when new people find us!

As you can see, it depends a lot on where you are what sorts of grit are available.  I'm very envious of people in Scotland who can get the Harling that Maggi can get.  If you could add your location to your sig here we might be able to help you more.  Good luck with your plans if we can help any more just ask us!

[Graeme]

I can get Jondo poultry grits locally at a reasonable price. Their range, in increasing particle size, goes from chick, growers, hen, to turkey. I can then select for use - a mix of the first three (depending on species) for use in composts and I top dress with either growers or hens. As Darren says, the Tesco low-dust cat litter makes a really good replacement for grit for many species.

SJW - sadly I can only get the mixed Jondo grit near me.  However it is good for Primula Allionii as it has crushed oyster shells in it and small limestone chips as well as the normal grit.  Dionysia's also seem to like it.  Struggling to get good grit at the moment as our local builders merchant has stopped trading and he used to get me crushed 6mm from the local quarry in bulk bags.  They use it for putting under the vehicles in the quarry when they get stuck.............  They won't sell it to me directly........ I used to pay around £40 a bag from the builders merchant as ballast - the other quarry nearby want £120 a bag for 6mm as they are classing it as decorative gravel - so a bit stuck at the moment and I am down to the last bulk bag 

[LindseyEd]

Thanks for the welcome everyone and for the excellent advice!

Darren, thank you in particular for the time taken to give such detailed advice! I feel I should print out this thread and stick it in the potting shed.

I've added my location to my sig as ChrisB suggested. Unfortunately down here in Hampshire we are a very long way indeed from 'harling', and it sounds as if the crushed terracotta is indeed too big. I think I can get Cornish granite in small bags so I will check out what particle sizes they are offering and use that if possible, otherwise I will have to speak to the local chicken keepers about their suppliers.

David Nicholson, I do know where to get dolomite limestone in small quantities: The Organic Gardening Catalogue sell 3kg or 12kg bags. It isn't cheap (but there is a small discount if you are a Garden Organic member) but fortunately I don't yet require a lot so I have ordered some. I will report back on quality if you are interested.

[Matt T]

The English equivalent of harling is probably "pebble dash"? At least, it seems to me the bags of angular gritty material about 6mm in size are the same. Being decorative stone, it's not cheap, but from your builders' merchant it should costs less than the bags of "horticultural grit" that most GCs are selling. Ask the supplier which is the best value as it does vary dependent on transport miles - the cheapest we can get is white marble chips because it is quarried locally.

[David Nicholson]

David Nicholson, I do know where to get dolomite limestone in small quantities: The Organic Gardening Catalogue sell 3kg or 12kg bags. It isn't cheap (but there is a small discount if you are a Garden Organic member) but fortunately I don't yet require a lot so I have ordered some. I will report back on quality if you are interested.

Thanks for that Lindsey. You might get some useful information on local suppliers for grit etc from the Hampshire AGS Group:-

http://hampshirealpinegardeners.org.uk/page13.htm

[Maggi Young]

Good suggestion, David - we've let Paddy Hinton know that Lindsey may be getting in touch. 

The Hamps. AGS group are a friendly bunch, Lindsey - hope they can help you.

[Maggi Young]

Reply via twitter from Hampshire AGS -  "Grit gravel ... http://www.jcphillipsandsonltd.co.uk/Gravels  great selection well worth a visit more there than on Website "

Very helpful  of them 

[jomowi]

For anyone living in the Stirling area, Jondo poultry grits can be bought from Carrs Billington (Tel: 01786 474826) who are based at the Stirling Agricultural Centre just off the A84 roundabout near Dobbies GC.  I would suggest phoning first because they did not have the growers grade today but can get it in from another branch if asked for.

[Lawrence]

I appreciate this subject has been discussed in some detail already, but I still haven't reached a happy conclusion.
The compost mix I have started to use is one recommended by Eric Jarrett in AGS journal, June 2011: equal parts by volume... Grit, perlite and very sharp sand, three parts of this to one part JI2. The grit I have been using is the same one as I use for top dressing and is universally sold as " horticultural grit", however on speaking to Eric and Brian Burrows at AGS Loughborough show, both growers, said the horticultural grit was to large to use in the compost. They both did say however, how difficult it is to obtain a suitable sized grit for compost use.
The compost mix does have a " nice feel" especially using Keith Singleton John Innes but was very surprised by the poor AFP ( air filled porosity) of the mix, which both growers attributed to the use of horticultural grit, rather than something finer
I believe Jondo products do a flint growers grit, chick Flint grit and a hen flint grit, has anyone had experience of using these products and which grade you would recommend
Thanks

[Matt T]

My understanding is that to increase the AFP you probably want to increase the proportion of grit in the mix, otherwise the other components just fill the spaces between the grit particles. Smaller sized grit will just result in small spaces between the particles. I've just started adding perlite to my mix and (without any empirical evidence) feel that this makes the mix lighter and airier, which is better in my damp climate. As well as the discussion on this forum, there's a lot of useful info on the Pacofic Bulb Society website about materials for compost mixes...will find a link and post it here in 5 mins.