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The coco peat I have is a mix of dried and thoroughly washed fine coconut pith, and some coconut husk fibers. I have been using the coco peat as a germination medium for cress (Lepidium sativum) and dill (Anethum graveolens) in a 10 cm length, 15 cm width, and 6 cm depth (4 x 6 x 2.5 in) disposable polypropylene food container. After the seeds have germinated, and the stems are about 5 cm (2 inches), I gently uproot the seedlings and replant the seedlings in topsoil.

I am concerned that I may be uprooting the seedlings too early. From my understanding, coco peat contains no nutrients for plants and if seeds are germinated in coco peat, all nutrients used for germination come from nutrients that are stored in the seeds. I therefore feel under pressure to transplant them to topsoil so that the seedlings can get nutrients.

The problem: during such transplanting attempts, the tiny seedlings and their roots are still fragile and I have accidentally severed many stems from their roots when trying to untangle the seedlings' roots.

Should I add some kind of quick release fertilizer to delay the transplanting until the roots and stems are sufficiently strong? Or perhaps I should be using a seed tray that has one cell for each seed?

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Don't uproot the seedling. Take a plug of soil around the seedling with the roots in it, and transplant that entire plug without disturbing the roots any more than you need to. It is much less stressful for the plant than uprooting. This is the idea with those one cell per seed trays that you mentioned - each cell can be popped out as a single plug.

If the roots from neighboring plants are tangling, you will have to make tough decisions. Cut decisively between the two and let the roots be damaged where you cut them. As you've seen, there is no way to separate them without damage.

Two soil plugs with small seedlings, each with a pair of seed leaves and a first true leaf.

I wouldn't try fertilizing the coir if it's working as is, the seedlings will need transplanted out of that 2.5in. tray soon anyway.

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Seedling size 2"/5cm?

I'd say you're waiting much too long to transplant, which is why the roots are so tangled.

Or, if you are going to wait that long, you need to space the seeds far apart for sprouting. Implying the need for a much bigger container, or more containers with only a few seeds in them.

The ideal time to separate crowded seedlings is when they are tiny and their roots are not yet complex. While there is risk of damage, it's possible to extract and transplant a seedling with only the cotyledons and a simple root, while waiting for it to grow bigger ensures entanglement with neighboring seedling's roots.

With adequate care, you can sprout on a paper towel or in a clear gel and transplant with only the root, before the shoot and cotyledons even come up.

One-seedling-per-cell trays are a different approach to avoiding entanglement, if you want to wait for the seedlings to be larger.

Neither approach is "more right" they are simply different, and indeed you might transplant into "One-seedling-per-cell trays" when transplanting tiny seedlings.

Another approach, when seed is cheap and plentiful is to plant many seeds per cell and kill off all but one after sprouting to get one seedling per cell, with lower odds of having empty cells. Fussing with transplanting tiny seedlings is labor-intensive, and more easily justified for things with rare or expensive seeds that also have low germination rates. Rare or expensive seeds that have reasonable germination more easily justify just planting one per cell, as there won't be too many empty cells; and it's rare or expensive so less risk in transplanting is a good thing.

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  • "Seedling size 2"/5cm?" — Yes, I transplant when the height from the soil line to the tip of leaves is around 5 cm.
    – Flux
    Commented May 28 at 11:12
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+1 on MackM's answer, it should be the accepted one. This here is several comments on it too lengthy to insist them being incorporated.

One bit learned from a local gardening newspaper: water the plant both before and after transplantation. Before: keeps the rootball intact. After: "fuses" the roots with the dirt around.

Another trick for when transporting a larger plant, say a young tree: destroy it's furthest roots. Calculate what amount of soil are you going to be able to move with the roots, get a spade and cut a circle at the edge. This will sever the plant's furthest reaching surface roots and force it to grow new ones inside the compact semisphere of soil designated. Do this several weeks prior to transportation. Prune hard after replanting.

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