The Challenge of Planting Under Large Gum Trees in Sydney

If you live in Hornsby or Ku-ring-gai LGA, there's a good chance you've got a big gum, turpentine or angophora somewhere on your block. They're beautiful, and they're a big part of what makes the area feel the way it does. They're also incredibly hard to garden under. That bare, patchy spot beneath a mature tree is one of the things I get asked about most.

Why it's so tricky

Three things work against you. First, the soil is dry and short on nutrients, because the tree gets first pick of both. Second, the dense canopy means low light all year round. Third, mature Eucalyptus (gum trees), Syncarpia (turpentine) and Angophora (apple myrtle) have large, shallow root plates, so there's very little soil depth left for anything new.

Turpentines deserve a special mention. In our part of Sydney they're part of the Sydney Turpentine-Ironbark Forest, which is listed as a critically endangered ecological community. Hornsby Shire alone holds 323 hectares of it, which Council says is about 27% of everything that exists in the world. So looking after these trees matters well beyond your own backyard.

The one thing not to do

Please don't build up soil against the trunk. I see this all the time, usually with good intentions, because someone wants a nice level garden bed. But extra soil piled against the trunk cuts off oxygen and traps moisture. That leads to trunk rot and disease, and eventually a sick, structurally compromised tree that can drop large limbs. Keep the soil level where it is and work around it.

What to plant under large trees

Go small. Tube stock style plants is the way to go, because young plants can get their roots established in the gaps between the tree's surface roots. These are the ones I recommend:

Dianella caerulea is a tough strappy native grass. Hardenbergia violacea is a hardy native climber and groundcover. Westringia fruticosa is a tall native shrub that's great for screening. Agapanthus orientalis and Clivia miniata are hardy exotic flowering options.

A quick note on agapanthus.

Landcare NSW says it isn't officially designated a weed, but it can spread and has escaped from gardens into bushland and roadsides. Cut off the seed heads, bag them up, and never dump garden waste in bushland.

Mulch and water

Put down a layer of mulch 5 to 7 cm deep, but keep it well clear of the main trunk. Give new plants some extra water while they're establishing, then ease off. Once they've settled in, they need to rely on their own hardiness.

Check before you touch the tree

If you're thinking about pruning or removing a tree, check with your council first. Ku-ring-gai Council requires permission through its Tree and Vegetation DCP unless an exemption applies, and its guidelines say a tree not suiting your landscaping isn't a reason for removal. So the best approach is usually to design around the tree rather than fight it.

Quick FAQ about planting under large trees

Can I add soil to level out the garden bed under my tree?

No. Adding soil against the trunk can cause rot and disease, and it can leave you with an unstable, sick tree. Keep the soil level as it is and plant into the gaps between its buttressing roots.

Why use tube stock rather than bigger plants?

Young plants from small tube stock can establish their roots safely in the gaps between the tree's surface roots, and that's exactly what you want here.

Will I need to water these plants forever?

No. Water only while the plants are establishing. After that they should manage on their own.

Can I remove the tree if it's too hard to garden under?

Usually not. In Ku-ring-gai you need Council approval to prune or remove a tree, and "doesn't suit my landscaping" isn't accepted as a reason. Contact your council before doing any work.

Need a hand with your tricky spot?

Every tree and every block is different. If you'd like an on-site consultation, I can walk your garden with you and give you design and plant ideas that suit your conditions. Phone Susan Read Landscapes on 0418 635 781, or get in touch through www.susanreadlandscapes.com.au to book a time.