Pot-in-pot nursery yards: gel rates that survive overhead irrigation

The irrigation clock in a pot-in-pot yard usually runs a short cycle, four to six minutes, two or three times a day through summer. That is 3 to 4 mm per pass over the canopy, most of which never reaches the root zone because it lands on foliage and the gap between the inner and outer pot. Any gel rate calculated for a field planting will be wrong in this system, and it will be wrong in the direction that costs the most: too much water held, not too little.

Pot-in-pot is a specific hydraulic environment. The inner pot sits inside a socket pot buried in the ground or in a gravel bed, and the space between them drains freely. Roots that escape the drainage holes air-prune in that gap. The media in the inner pot is usually bark-based, coarse, and already holds a lot of water per litre.

Why the field rate over-hydrates here

A bark-based nursery media at 60 percent bark, 30 percent coir and 10 percent perlite holds roughly 55 percent of its volume as water after drainage. Add gel at a field rate of 3 kg per cubic metre and you can push the water-holding capacity past 65 percent. In a container, that means the air-filled porosity at field capacity drops below the 15 percent that most woody species need. Roots still grow, but slowly, and the root ball stays wet for hours longer than it should after each irrigation pass.

The visible result is a plant that looks slightly stunted with dark, healthy-looking foliage and a root system that has not filled the pot. Growers often read this as under-watering, because the surface looks dry between cycles, and increase the irrigation. That makes it worse.

Rates that hold up under overhead irrigation

Three variables set the rate: pot volume, media composition, and how the yard is irrigated.

• Small pots, 3 to 10 litres, overhead irrigation: 0.5 to 1.0 kg per cubic metre. The pot is small enough that the media alone does most of the buffering, and the margins are tight.
• Medium pots, 15 to 45 litres: 1.0 to 1.5 kg per cubic metre, and only where the media is bark-heavy and free-draining.
• Large pots, 100 litres and up: 1.5 to 2.0 kg per cubic metre is usually enough, because the surface-to-volume ratio is lower and the pot loses water slowly anyway.

These are starting points, not gospel. A yard in a hot, windy location with a coarse media will sit at the top of the range. A yard under shade cloth with a coir-heavy mix should be at the bottom, or using none at all.

The result that backfired: heavier pots, more blow-overs

Two seasons ago a 12,000-pot callistemon yard increased its gel rate from 0.8 to 2.0 kg per cubic metre to cut irrigation frequency during a water restriction. Irrigation frequency did fall by about a third, which was the aim. The unintended result was a row of blow-overs in the first spring wind event: the root balls were 15 to 20 percent heavier than in previous years, the pots were taller than they were wide, and the extra weight sat above the socket, not in it.

They also lost 400 pots to root rot in a section where the drainage gravel had silted up. The gel did not cause the silt, but it held the water that the silt could no longer drain. In a pot-in-pot system, drainage is the assumption everything else rests on, and gel raises the cost of a drainage failure.

How to check a yard without digging up pots

Weigh ten pots of each size at field capacity, then weigh them again 24 hours later. The loss is the evapotranspiration plus drainage for that period. Compare a gelled row against an ungelled row of the same size. If the gelled pots lose less than 60 percent of what the ungelled pots lose, you are probably at a rate that is doing something. If they lose almost nothing, the media is too wet and you should cut the rate or improve drainage.

Then pull three root balls at the end of the season and look at the bottom third. Firm, white roots reaching the drainage holes mean the air-water balance is acceptable. Brown tips and a sour smell mean it is not.

Set the rate on one bench before you treat the yard. The application guide covers container and media dosing alongside field rates, and copying one from the other is how a whole season goes wrong. Send the pot size, media recipe and irrigation cycle for a trial layout.