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To prune or not to prune?

Mark Roberts runs through the why and how’s of pruning – and relays some common mistakes

I have a friend and colleague at Christchurch City Council who asked me to try and put a stop to the practice of pruning at the time of planting. When he told me the practice of pruning at the time of planting was being undertaken and recommended by those who should know better, I was more than a bit surprised.

As a best practice, pruning at the time of planting was proven to be needless and detrimental at least 15 or 20 years ago, as it slows root development and reduces the chances of new plant survival. Not only is the practice pointless, but it requires additional time and effort; you actually have to do more work to harm the plant! If you are doing it, stop it – if you require your staff to do it, stop it. You’ll save time and plants, which is a win-win.

The only time you should prune at planting is to cut out broken bits. However, there really shouldn’t be any broken bits on a new plant. If there is, ask yourself why? It can only be as a result of poor plant care or poor plant selection; and there are no excuses for either and both can be easily addressed.

An old theory debunked

The old-school and incorrect theory behind pruning at the time of planting is that planting damages roots, so the plant needs to be pruned to compensate for the root loss.

As theories go, there is some truth in it. Yes, planting can damage roots, but to prune back the canopy to compensate for the root loss would be like throwing a drowning person into a swimming pool to try to save them!

All plant growth – including root growth – is fuelled by photosynthesis. Roots supply the raw materials and the leaves (photosynthetic tissue) turn those materials into fuel. If you want your plant to grow roots, it needs leaves; if you want your plant to grow leaves, it needs roots. Therefore, removing roots (ie, damaging roots at the time of planting) will interrupt fuel supply, so if you then cut off the leaves you risk stopping fuel production.

Or, to put if differently, plant growth requires water and nutrients, which mostly come via the roots. If the roots are damaged through the process of planting, the plant’s ability to photosynthesize will be reduced, so to then reduce its ability to photosynthesize by removing photosynthetic tissue (such as leaves) is counterproductive.

Top tips from an expert

As we’re entering the autumn planting season here’s a summary of current best planting practices.

The do’s when it comes to planting are:

  1. Dig a hole wider than deep (roots grow out, not down). Check that the hole is at the correct depth – ie, once the hole is back-filled the root flair will end up at, or just above, the finished soil level.

  2. With the plant in the hole, gently remove the plant from the planter bag (or pot) and tease the roots. Don’t be brutal, but don’t be afraid to cut or bend them, separate them and/or move them about so none are circling (dizzy roots kill trees).

  3. Put the original soil back in the hole and firm it down.

  4. Check to make sure that the root-flair is at or just above the finished soil level – if the plant is sitting too high then replant it. If it is sitting too deep, then replant it.

  5. If you have to stake your plant, then do it to keep the root ball in place. Stake it low and only use natural-fibre webbing ties (they’ll rot off when you forget to remove them).

  6. Mulch it to beyond the edges of the planting hole – even further if it is not in a garden bed.

  7. Water it – even if it’s winter, even it’s raining. Water your newly planted plant.

The don’ts when it comes to planting are:

  1. Don’t stomp or ram soil into the planting hole and don’t mix in new or introduced soil (your plant has to survive in the soil that is around it – don’t give it false hope).

  2. Don’t add fertiliser into the planting hole (you’ll probably kill the roots).

  3. Don’t prune pack the canopy/remove foliage to compensate for the root loss (you’ll only hurt the plant some more).

  4. Don’t use synthetic webbing or tree ties. and…

  5. Don’t continue to do things just because we used to do them, or because it’s been done that way for 100 years. Maybe there is a better way?

Poor tree and shrub planting continues to be one of the most common yet easily fixed mistakes that there is. The best practice listed above makes for easier planting.

Doing less and getting better results has to be a win for all!

Mark Roberts is a qualified arborist and tree risk assessor with 25 years’ experience. Mark is also former President of the International Society of Arboriculture, and a past President of the NZ Arboricultural Association. Mark now heads an arboricultural collective based in Dunedin:


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