I’ve been thinking about how to tackle the problem of planting into grille-covered tree pits, especially the very small ones that Hackney Council has created lately. The basic method needs a bit of adapting to cope with this but is essentially the same, except that one needs smaller tools and dainty fingers to cope with working through the grille.
A blunt old eating knife is good for weeding, since a trowel won’t fit into the gaps between spokes – but a hand fork can be used, sideways on, to dig out deep-rooted weeds from as far down in the pit as possible and dislodge more superficial ones.
Take care to keep a temporary lid on the watering pipe throughout, so as not to foul that up with decomposing compost etc. (Find whichever plastic tub lid in your kitchen or bathroom cupboard fits most neatly over the pipe and tape it down, if necessary, to keep it in place while you mulch and plant.) Mulch carefully over the grille, in layers, watering down the compost each time until the grille spokes etc reappear, approximately level with the wetted-down compost.
Then drop small bulbs between the spokes of the grille and push down as far as possible; insert whatever small plants you can where the gaps in the grille are widest; and sow seeds (mixed with fine sand, to keep them from being too closely sown) into the remaining small gaps. Add more compost on top of the bulbs and around the plants – but finely sift only enough compost over the seeds to cover them to their own depth. Then water yet again.
Don’t forget to take the cap off the pipe when the mulch has settled around the plants; then water through the pipe for the tree’s benefit, as well as between the plants.