Blue Gum and Black Wattle: Should You Burn or Chip Them?

Blue gum and black wattle are two of the trees farmers and landowners in Gauteng most want gone. Old woodlots, windbreaks that have spread, wattle creeping along streams and into grazing land. Once the trees are down, the question is always the same: do we burn the pile or chip it?

For these two species, the answer matters more than most people realise.

Why these two trees are different

Black wattle and blue gum both came from Australia. They grow fast, use a lot of water and spread into veld, riverbanks and farmland.

Under the National Environmental Management: Biodiversity Act (NEMBA), black wattle is a Category 2 invader. You need a permit to grow it, and outside permitted areas it must be controlled. Several gum species are listed as well, and their category depends on the species and where they grow. If it is on your land, controlling it is your responsibility.

So clearing these trees is a legal obligation as well as a practical one. The way you deal with the cut material should help you win that fight, not restart it.

The problem with burning black wattle

Black wattle drops huge amounts of seed, and that seed can survive in the soil for many years. The seed has a hard outer coat, and heat cracks it open.

That means a hot fire on wattle ground can trigger mass germination. A few months after the burn, you can have a carpet of wattle seedlings, often thicker than the stand you just removed.

The problem with burning blue gum

Most blue gums do not die when you cut them. The stump sends up a ring of new shoots, and a fire on top of the brush does not change that.

Gum leaves and bark are also full of oil. Gum piles burn fast and hot, and loose burning bark can be carried by the wind. In a dry Highveld winter, that is a serious risk to your own land and your neighbours'.

On top of this, burning may be restricted on high fire-danger days, may need permission from your municipality or fire protection association, and leaves bare, scorched soil behind.

Why chipping works better

• There is no heat on the ground, so you are not cracking open the wattle seed bank.
• There is no fire risk, no smoke and no pile to watch for days.
• Gum trees produce a lot of material, and chipping cuts that volume down right where it lies.
• Chips can cover bare ground after clearing, which helps hold moisture and slow erosion while grass recovers.
• The material has value. Gum and wattle are dense woods, and once dried, the chips make good boiler fuel. In South Africa, chips from cleared invasive trees are already used by industry as an alternative to coal.

Once chipped, cleared trees become a useful material instead of a waste problem. Read more about how wood chips are used.

A few things to get right

• Watch the seed pods. Where you can, chip wattle before the pods ripen. If trees are carrying ripe pods, keep those chips off clean land so you do not spread seed.
• Plan the follow-up. Chipping removes the trees, not the stumps or the seed in the soil. Gum stumps need to be treated or removed to stop regrowth, and wattle seedlings need follow-up for several seasons.
• Use gum chips in the right places. Fresh gum chips contain oils that can hold back the growth of some plants. They are better suited to paths, bare ground, erosion control or fuel than to mulching around sensitive crops.
• Check access. Make sure a commercial chipper and support vehicles can reach the work area.

The bottom line

With blue gum and black wattle, burning feels quick but works against you. Fire can wake up the wattle seed bank, gum stumps survive it, and in the dry season you carry the risk. Chipping keeps the heat off the soil, leaves you with useful material and gives your follow-up control a head start.

Chipperman clears blue gum, black wattle and other invasive trees for farms, estates and landowners across Pretoria and Gauteng.