Re-using potting compost

[MrsCordial]

For some reason, my potting-on technique (which I'm very happy to accept is probably quite shockingly poor) usually leaves behind most of the compost a seedling had been sitting in. Sometimes I manage to scoop out the whole plug, but often not.

Given the sheer quantity of seedlings I have on the go, that leaves me with an awful lot of part-filled cell tray cells. My question is, if I scoop it all out and mix it with fresh potting compost, can I use it again for the next batch of seedlings or pottings-on? I can't bring myself to just stick it in our compost bin, and I'm horrified to think how much I could end up spending on bags of potting compost.

[rustylady]

Yes you can re-use it.

If you are potting on from cell trays I think you are doing it too early. You need to wait until the seedling roots fill the cells then the whole lot (roots and compost) should come out in one go.

If using plastic celltrays a push from the bottom (underneath the cell) should pop the entire cell out.

[MrsCordial]

Ah, thank you rustylady

[Kristen]

I have a "tub" (actually its a "huge tub" ) into which I chuck all the spent potting compost. Let over bits, the rubbish I get after sieving MPC for potting on, compost from pots of plants that "didn't make it" (although not from anything diseased), and from pots of plants/crops that have "finished".

I use this for potting on "Big things". For example, potting on a tomato into a 9" pot or bigger, or cuttings that I have taken which have now grown up a fair bit and are going from 2L pots into 5L ones ... and so on. Also for earthing up Spuds in containers, and mixed 50:50 with sharp sand for growing Carrots in containers.

However ... if you are leaving behind a fair bit when potting-on seedlings from modules then that might be worth addressing

I've used those really flimsy module trays for pricking out individual seedlings into, and then potting-on into, say, 9cm pots later on. They are a nightmare! You have to push and squeeze and deform the module trays to get the plants out or push a plant-label in on all four sides to "release" the plant. I think all of that must cause significant disturbance to the roots of the little plant. Also, if the compost falls-to-bits then perhaps you are potting on too early, and there is not yet enough root to hold the rootball together?

I was also unhappy that the module trays were pretty much wrecked in the process, which made reuse impossible. They are cheap-as-chips, but nonetheless I hate that sort of waste.

I now use more rigid modules, which have a nice large hole in the bottom, and I can much more easily push the "plug" out with the blunt end of a pencil.

Something like these:



However, I have toyed with the idea of using Blue MDPE water pipe, cut into 1" or 2" lengths and pricking out into those. They would sit nicely on capillary matting, I can do exactly the number I want (whereas with modules I have to mix plants, or have gaps, in a tray - plus they don't all mature at the same time). The pipe would not be cone-shaped though, and I have no idea how much disturbance there might be when I pushed them out to pot them on ... I need to make an experiment though

Anyone got any thoughts on that?

[MrsCordial]

Hmm. I think there are a few problems coming together here. I'm potting on because things are getting leggy in the cell trays, because I have only a few small spaces where I can put things indoors so nothing really gets enough light. What you both say about roots makes complete sense. Perhaps I need to start later, sow less and make better use of the space I have.

Wholly agree about crappy seed trays Kristen. I can't stand waste either so will re-use and re-use.

[Kristen]

Leggy is a not-enough-light issue as you say. They'll be much happier in a greenhouse with good light (if not heated them you might have to carry them indoors on cold nights).

If no greenhouse then sow-later is a sensible option. or buy some supplemental lighting. My Home Office is lit up like a Christmas tree from ... ermmm ... "Christmas" until around the end of April. No problem providing heat in my nice, insulated, home so I figure that adding Light is cheaper than trying to heat a greenhouse (and in a Spring like last year the light is rubbish anyway - we were 25% down on light in Spring 2013, and everything outdoors / in greenhouse suffered - my stuff under lights was fine )

[rustylady]

Hmm. I think there are a few problems coming together here. I'm potting on because things are getting leggy in the cell trays, because I have only a few small spaces where I can put things indoors so nothing really gets enough light. What you both say about roots makes complete sense. Perhaps I need to start later, sow less and make better use of the space I have.

Wholly agree about crappy seed trays Kristen. I can't stand waste either so will re-use and re-use.

You shouldn't pot on because things are leggy - you need to leave them in their cells and give them more light. Most things can be stood outside during the day at this time of year, and just brought back in at night.

[AllInContainers]

I still consider myself a noob in mays ways, so what I'm going to say here may be techincally "wrong". However, I frequently pot stuff on before it's on the verge of getting rootbound, particularly with plants that take up quite a bit of space topside. For example my strawberry seedlings got potted on "early" as I was concerned about there not being enough air around the leaves. I tend to make a judgement call more on what's hapenning above the surface unless I notice a lot of root protruging from the bottom of the cells.

As for reusuing compost, I too tend to chuck recycled seed compost into the final pots rather than reusing it for sowing. I do reuse MPC too, but come the second year it tends to have broken down more which means it is better at moisture retauining (less free draining), so you have to box clever as to where you reuse it (depending how wet/dry the plants for that pot like it)

[Alison]

I think you also need to consider when you're sowing and how many. I used to sow far more seeds than I do now and then couldn't look after them properly as I had nowhere to look after them properly and had weak or spindly plants. I then realised that this was ridiculous and expensive too as I used more compost this way. I now look at the number of plants I want and sow based on that. If I want 2 of a particular variety of tomato I will only sow 3 seeds. I'll probably still get 3 plants as toms are so reliable but it's better than the 6 or more I used to have. Some things are more temperamental and I sow a few more but keep it sensible based on yearly records. Less plants means I can look after them better and give them better conditions without resorting to a very early indoor / outdoor Hokey Cokey (I don't have time every morning and often get home very late) or a lighting rig. I mentioned records before, it really does help, I know which of my plants will really benefit from early sowing and know that by far the majority catch up quite happily.


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[MrsCordial]

Thank you everyone. Alison your last post really speaks to me - I've been making just those mistakes. I have no greenhouse of any description (I got a friend's old one two houses ago, had just reconstructed it and begun replacing cracked glass when we had to move from a very happy house at short notice, so I'm now superstitious and won't have one again!), so it would be the in/out hokey cokey just as you describe, which quite honestly just isn't going to happen. I can't protect outdoor plants in the daytime from (wild) rabbits and (tame) hen and I have so many it would just be ridiculous.

My office would be the ideal spot, but it's in the part of the house that gets light late on in the day - I have the heater on in there much of the year as it so rarely has a chance to heat up.

[Kristen]

Alison your last post really speaks to me - I've been making just those mistakes.

You, Alison, Me and every other gardener since the beginning of time!

Fully agree with keeping records too. I have dates Sown, Germinated, Pricked out, Planted out, First and last harvest date and Qty of eventual plants. I use that as a guide next year, and record notes on each to allow for instructions from the galley "Need more/less next year" "This variety tasted foul/fabulous" and so on.

During the winter I make a list of all the seeds I have got (and thus which ones I haven't, and need to buy) and my intended sowing date. I put the earliest date that I want to sow on, as mostly I am busy at that time and miss the date by a week or two - which nopefully means I then sow at "just the right time"

[Two_Sheds]

my potting-on technique ...eaves behind most of the compost a seedling had been sitting in.

1. You're doing it too early then: wait until the roots are well formed in the plug/pot before potting on.

2. And yes, I reuse compost all the time, mixing it back in with fresh

3. make your own leafmould from autumn leaves, and mix that with your bought compost to reduce the cost

[MrsCordial]

Thanks Two_Sheds - lesson firmly learnt. Should the roots be filling up the cell so much it's visible? I know how much I'd hate to be pot-bound and I assumed plants were the same.

[Kristen]

Thanks Two_Sheds - lesson firmly learnt. Should the roots be filling up the cell so much it's visible? I know how much I'd hate to be pot-bound and I assumed plants were the same.

The plants won't mind too much. The longer they are pot bound the more they will struggle, and the more the roots will circle around the pot and then not easily grow to take advantage of the new, bigger, pot. But for modules you will be able to tease the roots out a bit if necessary (i.e. they were left too long).

Pots are much easier: you can up-turn the plant & pot, with the plant between the fingers of one hand. A sharp tap and the rootball drops out of the pot - just the 1" drop into your waiting hand. Lift the pot off (upwards - the plant & pot are still upside down). Inspect the roots. Carefully put the pot back on, and turn the thing the right way up again.

Modules: No chance! Evidence of roots poking out of the bottom through the drainage hole(s) is the only clue you get.

[chris]

Mrs c - you should check out potty's posts about reusing compost - he's pretty much the reuse hero here :-)




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