Landcare Agony Aunt – Potting Up Perils

Dear Agony Aunt,

I can’t get my native seedlings to thrive once I pot them up. I’ve tried a commercial mix for native plants. Do you think that’s the problem? What do you recommend?

from Frustrated in Fishers Hill

Dear reader,

I had to call up my friend, Agony Uncle for this one. Assuming you have a good watering system in place, sufficient light and suitable climate, he agrees your potting mix may be the problem.  Here’s his advice:

There are a lot of potting mixes on the market and a quick google search will show many people not happy with a number of the commercial mixes.  In my experience they’re right. I’ve used a number of mixes from the “Premium” end of the market and for my purposes many have been inadequate. They may be fine for potting up into larger pots but performance for native tubestock has been less than ideal.

Before going further let’s look at what “Premium“ means. A “Premium” potting mix is one which satisfies Australian Standard 3743 and has the Red Tick logo, the standard essentially says they contain enough fertiliser for 3 months and absorb water at a certain rate as well as a few other criteria.

Characteristics of a good mix

(from: https://www.agric.wa.gov.au/nursery-cutflowers/potting-mixes)

The characteristics of a good potting mix are:

• well drained, which means an air-filled porosity of at least 15%
• re-wets easily – some peat and bark media are difficult to re-wet if they dry out
• does not shrink away from the side of the pot as it dries
• optimum weight – not too heavy to lift, not so light as to blow over easily
• suitable pH, between 5.0 and 6.5 is satisfactory for most plants (all pH values quoted are measured in water)
• free of pests, for example weed seeds, fungal pathogens, or can be sterilised without producing harmful by-products
• can be stored for short periods without significant changes in physical or chemical properties
• readily available
• not expensive.

Generally speaking “Premium” is only referring to fertilisers and other additives in the mix, not on how well your plants will grow. 

In my trials the more expensive potting mix brands work better than the cheap ones (or at least they did under those conditions).

You can make your own potting mix from coir (compressed blocks), perlite and native slow release fertiliser (the perlite is mainly there to let you know when you’ve mixed the fertiliser in).

For our Landcare nurseries I make Paul’s Mystery Mix.  It’s a combination of coir, perlite, crushed charcoal and a few types of slow release fertiliser (Ozmocote and Neutrog Bush Tucker).  It works pretty well and price wise it’s similar to the commercial mixes available, but requires a lot more effort to make up.

Here’s a tip for when you’re mixing it.  Once you have soaked the coir fibre, put the pearlite on top of it, then the slow-release fertilisers on top of that. Then mix them thoroughly.  Mix in some charcoal if you’ve got it, to improve drainage.

Store it in a garbage bin with a lid and keep it relatively dry. When you’re ready to use for potting mix, wet it down first. It’s a good general purpose mix. For some special plants and purposes I do vary the mix.

Happy experimenting and drop me a line via Agony Aunt if you have any other nursery-related questions.