Propagation & Pot Culture of Boronia Species

An old and still evolving method used by Don Sands for the propagation and pot culture of Boronia species (now including Cyanothamnus spp.)

Notes taken by Maurie Tucker & Glenn Leiper, November 2021. Photos by G. Leiper

Published Online Only: April 2022

Don’s aunt taught him these skills in the 1950s and he’s since adapted and modified as required. Don initially learnt how to grow Boronia serrulata for reintroduction into a bushland reserve at Dee Why, near Sydney. He also learnt how to grow B. floribunda, B. pinnata and several other Boronia species.

Don’s professional entomological interest has transferred since retirement from CSIRO pest management and weed biocontrol to various insect conservation topics such as the Richmond Birdwing Butterfly project, which of course has led him to the plants they have relationships with. The moths that are obligatory pollinators of threatened Boronia species have taken Don’s interest in recent years and are of high conservation significance.

Boronia keysii

Boronia rosmarinifolia

Cuttings

Use small hot houses consisting of a solid base with 3 to 6 holes drilled in grooves for drainage, a hinged transparent cover with adjustable air holes for ventilation and a removable cellular plant tray. (Bunnings sells “Saxon Mini Green House” with seed tray 38cm long x 24cm wide x 18cm high with 2 adjustable air vents on top and a 24 cell removable cutting tray, for $11.98.)

Base of mini green house with holes drilled for better drainage

Don felt the best models also had adjustable air vents at each end of the clear cover but if this type was unavailable, he drills two or three 8mm drain holes in the base of the plastic floor of the mini green house.

When in use, the mini green houses were placed crossways on benches with the back section slightly raised and sloping forward to the front with the drainage holes.

The cutting sand is washed to remove fine material before filling the wells. This is done by pouring some sand into a bucket then adding water and stirring before tipping out the water and fine material in suspension, being repeated three times.

Cuttings of various lengths are trialled with one to each cell. Length of cuttings depends on internode length of each species. For Boronia repanda, he uses 4-5cm, and for B. rivularis 6-7cm. (See photo above)

After one month, take them out and wash separately then rest their bases in purple Clonex Cutting Gel for one hour before placing them back in the cells in the Mini Green Houses. (see photo below)

Do this step every 6 weeks thereafter until they either root or die.

At each 6 week mark, test each cutting to see if roots have appeared – this basically involves soaking them well before using a dibble stick (eg, a small skewer) to gently lift/tease each cutting from the mix.

Don only uses tank water. He has a misting hose nozzle and on hot days would mist once a day by folding back the clear top and quickly misting by hand.

Rooted cuttings are initially potted up separately in their own plastic pot. A clear round plastic container (open at one end) is placed on the pot over the cutting and has 2 air holes in the sides near the bottom and another 2 near the top. (see right hand photos above)

“Potting on” into terracotta pots is done when the plants have reached about 50 to 60 cm tall. Only terracotta pots are used after the basal holes have been enlarged using a hammer and small round chisel: enlarged to minimum 25mm holes for 15 to 17cm pots, 35 to 40mm holes for 19 to 21 cm pots. (see photos below)

Fertiliser is never used except during spring for certain “wet-soil” species such as B. safrolifera. Watering regimes vary with species but always respond when the surface is dry. Misting is much better than using a hose and nozzle.

Pot Culture

Consider whether each species favours

• • A – More wet sites/conditions, or
  • B – More dry, rocky or very well drained situations

Note: Is it possible to categorise all Queensland species – Don’s tried a few –

• • A – Boronia falcifolia, B. rivularis, B. safrolifera, B. microphylla
  • B – Boronia glabra, B. splendida, B. rosmarinifolia, Cyanothamnus quadrangulatus

Don only uses terracotta pots for on-growing all his plants in lieu of black plastic pots to avoid over- heating and as they do not breathe.

He also avoids using squat terracotta pots for larger species – depending on their size, they could be 19 or 21cm pots or even bigger.

NB. Don grows Boronia polygalifolia from Helidon in a 17cm diameter terra-cotta pot with 15cm high sides that makes it look more like a squat pot – though the ultimate height of this species is only 10-30 cm.

For Group A species, enlarge the solitary drainhole in the terracotta pot using the threaded end of a medium size bolt against the edge of the hole and tapping the head with a hammer (like using a chisel) to chip off pieces to enlarge the drainage hole to a diameter of 25mm.

For Group B species, you need a larger hole for very good drainage, so use the same process to enlarge the hole to 35 to 40mm.

For Group A species, place a single piece of broken terracotta pot over the hole and more smaller broken pieces in the bottom of the pot to a depth of at least 1cm. (see first 2 photos below)

For Group B species, use the same approach but increase the depth of the broken pieces to at least 2cm.

Crush up some charcoal (don’t pulverise) into small pieces and add to the pots so it’s 1 to 2 cm deep depending on whether it’s a Group A or Group B plant. (see last photo above)

This will improve drainage and also add some nutrients. Ash with small-grain charcoal has potassium in it and is used for dry country species only. Use nutrients sparingly and only in late spring.

Soil mixture ingredients consist of –

• • Brunnings Propagating Sand that has been washed – see previous comments
  • Peat Moss
  • Vermiculite
  • Tea Tree Mulch that has been sieved (fine parts used, twigs etc removed)
  • Searles Native Plant Potting Mix

As noted previously, Don rarely uses any fertilisers. He admits that he needs to experiment more but is reluctant to experiment with his valued plants.

However, Don said when he does use fertiliser, it’s Osmocote (the 6 month Native Plant one) and that he uses a little on Group A (definitely none in autumn or winter) and as for Group B, ½ teaspoon for large pots only. He said he doesn’t have the answer yet to some species’ yellowing leaves and the required fertilising regime to correct this.

The plants in their pots are kept on benches in a green house (30% shade cloth) with some special initiatives for certain species (see below). Don doesn’t stand the pots directly on metal frame benches.

• • Boronia safrolifera – stand in a shallow bowl of water and top up every 3 days (only do this after drying out in winter).
  • Boronia granitica – sit on an inverted tile to keep roots cooler rather than a weld mesh potting bench. Don said that the roots of the granite-loving species head for the sides of the pots. Don recommends never sitting pots on metal benches as the heat will kill the plants’ roots. He sits them on bits of inverted tiles.
  • Adjust light according to species’ needs by varying overhead shade cloth densities.

Again, Don only uses tank water and the watering regime varies depending on the appearance of the pots’ surface drying out and also the particular species.

Earthworms are a disaster if they get into pots and plants will die rapidly.

Don recommends that the best two species to start with are B. polygalifolia (a moist soil lover, photo below) and B. safrolifera (a wet soil lover) and that all species need good drainage!