We recently moved into a house with deep beds built into the outside patio wall. I planted healthy bedding plants into these after first adding compost etc, and they all died within a week. When I pulled them up, all the roots are covered in globs of orange-yellow substance (like a hard jelly in texture, or like cavity filler) - any ideas on what this is and how I can remove / treat please? It looks almost like a chewed up toffee; some of it has attached itself to the roots and some is sitting on the surface.
I think I can see the problem looking at the picture, but it may be the way the picture was taken (I could be wrong, just going on what I see)
To me it looks like the soil is not well draining, the location is shady so the soil stays constantly damp. The plant above looks like the roots are in a ball so when you put it in, the plant is not taking in any nutrition or moisture. So it looks like your plant starved to death.
I assume that when you put your plant in with its roots like that you didn't move it around to remove any air pockets. This is exacerbated by having a plant which is slightly root bound.
As the plant dies it provides food for the orange stuff that is living comfortably in the moist soil.
If the above is correct here is what you need to do:
Throw down a tarp and get all of the soil out.
Keep in mind what the soil is like as you will need to diagnose the problem:
Where was the water pooling? In the compost; the whole way through; just the bottom?
You want soil that is a consistent moisture the whole way through.
If the soil under the compost is poor quality then put it in your compost bin and get (buy) some better soil.
With these type of beds often there isn't any drainage. If it was me I would find which corner the water is likely to pool in then drill a whole and add a copper pipe so that large quantities of water can drain eg when it rains heavily.
Next get some larger rocks or crushed tile and place in the bottom of the beds. About 15cm high layer. This will allow your beds to drain freely.
Make sure when adding compost that you mix it in well with the existing soil. Compost can be thick and moist.
Now you should have a good free draining soil in a good free draining bed. You should aim to be able to give it a decent soaking and the soil will hold onto some of the water and the rest will drain out the pipe once it makes it way down.
Now, any of your plants that are still living; take them and tease out the root ball so they will be able to mix in with the soil in the planter. It doesn't matter if most of the existing soil falls off. You are aiming to allow it to feed on the soil in the planter not on the remaining soil it is attached to.
When you plant them into the planter don't dig the holes too deep. Place them into suitable holes and have plenty of dirt (it should mound up slightly at the base of the plant). Push firmly around the edges. Next get a thin stick (plastic chopstick is good) and prod around near the roots to remove any extra air pockets.
The next step; as mentioned you should have some extra dirt at the base of each plant. You should now water the base of the plants with a decent amount of water. As the water washes down the extra piled dirt will fill those last few pockets of air amongst the roots and under the plant.
You should now have a place for healthy happy plants.
I suspect it's slime mold, in which case it's just opportunistically eating the dead plants - it didn't kill them. As for what did kill them - sorry, I've no idea. A picture and/or some details about the soil and moisture conditions might illuminate. Also, what did the plants look like while they were dying? Did the leaves turn color? If so, what color? Etc... A truly uninformed guess is that it's been too wet and/or the compost was too hot.
There's no need to do anything about it - when there's nothing dead left to eat, it will just die off.