Digging the planting hole - wait!
Customers ordering trees sometimes ask us how big they need to dig their planting hole, as they are being super prepared and getting it ready in advance of receiving their order. HOLD YOUR HORSES!
All very commendable, getting your spade out so you can get that tree straight in the ground when it arrives, but we recommend waiting until you have your tree before digging the hole, or 'planting pit' as it's often called.
Firstly, it's much easier to see how big a hole you need once you've got your tree. Be it bare root or in a container, the hole shouldn't be much larger than the roots. About a third bigger is plenty.
The more soil you loosen around the planting pit, the less stability there is likely to be once it is filled back in. Meanwhile, if you've dug an over-large hole and compensate by squashing that soil down too firmly, you may do more harm than good. Over-compaction removes the tiny pores in soil that hold air and water, essential for plant roots. Good soil structure is a precious thing to preserve.
In addition, digging can damage the helpful microbes down there, which ultimately help trees to grow. We're mostly talking about the underground threads of fungi that have a symbiotic, two-way, relationship with tree roots. They take quite a while to grow.
The rule of thumb is to disturb the soil as little as possible, except for removing grass and weeds at the surface. There's a concept in ecology whereby the more disturbance there is in an environment (like soil), the less biodiversity it supports. It's a bit like 'a rolling stone gathers no moss'. A soil teeming with a network of tiny organisms, like earthworms, is a good environment for a tree to grow in.
Weather and the planting pit
The other key reason to wait to dig is that soil exposed to the elements changes. This is quite intuitive: dry weather = dry walls on your hole; wet weather = hole may fill up with water; freezing weather = sides of hole may harden and change.
That hole you backfill during planting, wants to be a nice fresh hole unsullied by the above-ground atmosphere.
When the tree roots meet the walls of the planting pit, they don't want to hit unfriendly territory that's dry, hard and uninviting. They may well turn around and stay in the looser medium within the small hole you have dug. In this case, they will not thrive as the soil/compost and moisture in that hole will not support ongoing growth.
Equally, you mustn't drown the roots in a swimming pool. It's a risk whenever you slice into soil that it becomes less permeable on that surface, especially in clay soil that contains a lot of fine silt particles that stick together. Although it's unlikely a pre-dug planting hole will act exactly like a bucket, you can minimise the risk of making it too sodden down there by just waiting to dig until you are ready to plant.
We don't generally recommend using mechanical diggers for planting our trees. They're a sledgehammer to crack a nut: they will usually dig a too-big hole. Plus they cause compaction and can 'polish' the sides of the hole as they slice into soil, creating that unfriendly barrier to roots.
In brief, you should wait till you have your tree before digging the hole:
- to see how big a hole you need
- to minimise the soil disturbed
- so the exposed hole doesn't dry out or fill with water