Archive for April, 2011

A good potting mix is essential if you want to grow plants in pots. But what you must realise is that just chucking soil (assuming you have any) in a pot just isn’t good enough. Any time you put something in a pot/seedling tray or tub, the whole dynamics of the system changes. How well your mix drains and consequently how well aerated it will be in the pot, depends a lot on the shape of the pot, as well as the sort of mix you use and in a container its not the same is it is in the ground. Wide, shallow containers like seedling trays don’t drain as well as tall thin containers. They are most likely to have a saturated layer at the bottom, especially if they are flat. And incidentally when you buy a pot don’t forget to look at the drain holes and if there aren’t enough, drill some more in. Oh and NEVER sit your pots permanently in water!

Potting mixes are dynamic. Unless they are made from completely inert materials like perlite, pumice and sand. Most potting mixes you buy these days have a fair whack of pinebark in them. Which is fine if its:

1) Composted and
2) Graded so all the dust is taken out.

Pine bark that’s not composted can be quite toxic for plants and can kill roots. If it smells piney not earthy then its no good. Other mixes can have:

• biosolids, OK for soil not for pots, Has that certain smell 🙂
• sedge peat dug up from swamps, looks lovely and black but just goes gluggy and silts up pots so they don’t drain well
• sawdust – can be variable in grade and quality and also needs to be composted or it competes with the nitrogen that your plants should be getting
• sand – of various grades
• coir – made from coconut fibre, a good product and more sustainable than
• German/Irish peat
• miscellaneous other materials, often local eg rice hulls in Queensland, pumice in NZ, perlite, vermiculite, polystyrene.

Most mixes have several ingredients in various proportions.

Any mix that purports to be composted should smell earthy. If it doesn’t its not composted. Its important to realise that as part of the composting process, the pH (acidity/alkalinity) of the material changes. Pine bark starts start out acid (say 4.5), then goes alkaline (maybe up to 8.0) and then finally stabilises back at about neutral (6.5) so it is important that the composting process be finished by the time you pot up.

Now if you ever ask anyone if something is composted and they say yes its been heaped up somewhere for 6 weeks (months) and we’ve kept it moist – well I’m sorry that’s not composting! Composting requires aeration ie turning of the heap on a regular basis as well as being kept to a constant moisture content. Piles also need to be of certain dimensions so they don’t heat up too much in the middle, and also so they don’t go anaerobic (run out of oxygen in the middle). Even when the composting process is over, the pile must be kept moist and aerated or all that good work (and the good bugs with it) goes to waste. The more mature the compost is the more good bugs and hence disease suppression, is present.

A good potting mix will be pH balanced and have fertiliser in it. Pine bark mixes also need extra iron because they tie up iron. It will drain well and won’t carry disease, weed seeds or kill plants from being full of toxic things like tannins/phenols (which are in uncomposted materials) . In our climate it helps to have a bit of weight (sand) so the morning easterly doesn’t send everything flying!

I’ve been playing with making my own potting mixes at home. Mostly trying to make a long term mix because most wood based mixes degrade over time and you really have to repot regularly (and I have lots of pots). So I’m using things like perlite, sand, attapulgite (a sort of kitty litter, a clay that holds nutrients) and coir. That way I can adjust my mixes for particular plants. Its like baking a cake really!

There are standards for potting mixes in Australia. Look for the logo on the bag . There is a standard and a premium mix and each has to conform to strict standards. Cheap and nasty mixes are liable to kill your plants and at the very least won’t support good plant growth and good plant heath. Buying cheap mixes is false economy if your plants end up dying as a result.


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A large part of my life is sorting out other people’s problems. Plant ones that is. And I guess I am always surprised at the things that come up as possibilities, that I would never consider. For example nutrient deficiencies. These seldom kill plants. If you don’t feed something it may struggle, it may go strange colours and it may deform but it will seldom die. Unless it’s a pretty severe micronutrient deficiency and its something that has only one growing point – like a palm and that gets killed (incidentally why is it that people seem to think plants don’t need feeding).

So what do I look for and what questions do I ask when I see a problem?

Firstly, the pattern of the problem, its history. Did it come on suddenly or over a period of weeks or months? Is it patchy or all over. Did it follow a particular weather pattern – a hot humid spell, a period of drizzly rain? Or was there a frost or a heatwave?

One of the strangest things I’ve learnt to ask about in my career, is have you been away lately. You’d be surprised how many things run off the rails while the owner is on holiday! It may be as subtle as a change in watering pattern to evenings, or inexpert handwatering. Non-gardeners, in fact, even many gardeners, don’t realise how long you have to stand there with a hose to water something well, throughout its rooting depth. And if it has dried out recently there may be non-wetting patches there that aren’t obvious so always check with a trowel or spade to see if it is actually WET down below the surface. So simple and so seldom done.

Many times I get called out to problems and I find the problem has been and gone and the plants are now recovering. In the case of leaf spots, thrip and mite attacks and even frquently it doesn’t become obvious for a while because the damaged leaves are so small. So as the leaves grow and enlarge the problem appears to be getting worse! But often its not. It can be rather an anticlimax for the owner to be told his problem has been and gone and there’s nothing you can do.

Its often also really hard to give a definitive diagnosis for some insect problems like thrips and mites. I’m not talking the normal large mites you see crawling around your roses, there are a whole bunch of tiny, tiny mites that do their damage right down in the growing point of the plant and are almost always impossible to recover. They are often a nursery problem more than a garden problem so you only see the after effects – distorted growth, strappy leaves. Often a bit like some nutrient deficiencies. But if you know the plant then you suspect eriophyid/tarsonemid mites. These mites can be very hard to control too because they are so well hidden and often a transient problem. Other bigger insects like Rutherglen bug can also do damage early on and you don’t see it until those leaves grow, enlarge and look really distorted and strange. With some pests its their saliva that causes damage, its phytotoxic.

So back to diseases – if the pattern is patchy, one plant or several and spreading out, suspect disease. If its uniform suspect something environmental – fertiliser burn, frost, wind etc.

One of the other things that most people don’t realise when they want a plant problem diagnosed is that a leaf is usually useless. Doesn’t matter how spotty, technicolour or burnt it is, I nearly always ask for a whole plant with roots. So many above ground problems are caused by root problems – if the plant can’t take nutrient or water up due to root damage then it will do all sorts of strange things above ground, including be more susceptible to diseases. So often the above ground problem is secondary to the real problem which is below ground. Its not hard for you to investigate this as a possibility. Simply dig around the roots or pull the plant up and see if you have nice white new roots. Or if they are brown and stripping. Are they abundant or not? Do they have little nodules on them (they can be good or bad depending). Are they in damp soil or is it dry or is it waterlogged. Lots can be gleaned from a little dig. A lot of this is really simple, just systematic and logical.

OK that’s it for this week. Don’t forget to get your hands dirty occasionally, its really useful. As well as being therapeutic, your plants will love you for it.

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