Container mix with larger bark chips and dealing with voids/air pockets

[rossn]

Given our variable spring weather, I’ve taken to putting about 1/3 of my trees in containers long-term, to ensure some seasons I will get fruit when we get late cold snaps.

For those using a mix with larger bark chips, how do you avoid air pockets or when watering, washing out the lighter material and leaving voids, which will expose roots?

I have a few apricot that came from Purvis that I need to pot up tomorrow, so thanks for the help!

Background:

For the last 2 years, I’ve used a variation of drew’s mix, at 3 parts 1/2" bark fines + 3 parts promix + 1-1/2 parts DE + 1/2 part compost. This mix never had any voids, but I learned through discussion that longer term container plants may be better with larger bark chunks, so they don’t break down so fast and create an anaerobic situation.

So, this year, I transitioned the bark fines to ~1-1/2" bark chips. I planted up a bunch of apples I grafted, and was watering from a 2 gallon bucket (have a new water breaker and wand on-order). While of course the water dishes out the soil, in this case it was either creating or exposing voids deep into the container.

[elivings1]

I have found that taking it off an incline does wonders with runoff. Another thing I have found is it kind of fixes itself over time. After a year or 2 your roots will have combined with the soil enough so lesson soil erosion. Think about when soil erosion happens in nature. It generally happens after a wildfire when many trees and bushes have died. Thriving roots help keep the biomass of soil at bay. So add things like ground cover trees, bushes etc to your soil to help prevent it.

[disc4tw]

I just spent the end of the night grafting persimmons in 14" long pots which have been in storage bins for over a year. The roots are coming out the bottom of those pots, exposed to open air and are happy.

All that to say, roots like air pockets, they need to breathe too. I wouldn’t worry too much about it as long as you keep your potted trees hydrated. They’ll work around any pockets too big.

[Drew51]

When you install pavers with sand in between them, you vibrate the pavers with a machine to tighten the sand up. If you did this somehow you could reduce the air space. I have a heating pad that vibrates. That would probably work by placing the pot on it.

[alan]

@rossn

how do you avoid air pockets or when watering, washing out the lighter material and leaving voids, which will expose roots?

Air pockets in the soil do not seem to dry out roots because of the constant humidity. Roots grow right through them. Gardeners often excessively tamp down soil, especially when transplanting. It can deleteriously compact soil, especially when it is wet.

Of course, with typical potting soil mixes, compaction isn’t much of a risk, but I notice that commercial growers of annuals often keep mixes very loose, maybe partially to save on potting mix. I like to tamp potting mix down firmly on the bottom of pots and then more gently further up. As far as your fear of air pockets, I don’t think you should worry much about them. I’ve seen roots grow right through loose wood chips many times. Natural soil is filled with many air pockets as moles and voles do their work. Even worm tunnels are prized by roots as they gather nutrients from the castings.

[FigGuy]

I have the same problem with potted perennials, not all are for fruit.
The big problem is the loose soil washing out the drain holes with the water. So to stop the soil movement, I use one sheet of news paper, I have tried wet cardboard also. the easy & best for one gallon pots(3qt) is coffee filters. CF slow the lost of water a little, so it will be soaked up by the soil in the mix & stops the soil from washing out the drain holes. When the water wash the soil, peat, compost & micro pine bark fines out, it clears channel like rivers for the water to flow even faster the next time you water.
I have used this filter for only about eight weeks, with no problems, because I water only when the pots are dry to the point no soil sticks to my finger when I press it into the soil. I have used the filter on all new fruit plants, Daylilies, Astilbe, Phlox, Crocosmia, Garlic, Mums, Coreopsis, Dianthus, Heuchera & Lantana. Eight weeks is a short time so I have no ideal how long the filters will last. Some of these plants like hardy Mums & Lantana will have to be moved up into 3 gallon pots or planted in the field, they are growing like weeds.
Misting or drip lines every day or every few hours can slow wash out also.

[Drew51]

I like to use coffee filters when I germinate seeds and move to a quart pot I use coffee filters. By the time I transfer them to permanent home, maybe four weeks or less the coffee filter is completely composted. Amazing how fast they compost. In large pots I buy a roll of drywall mesh tape and cut off piece to cover hole. These take decades to compost. Still there years later. Works good at holding soil.

[poncirusguy]

I use old shirts of polyester and cotton combination. The cotton rots and the loose polyester mesh is very water permeable with out letting dirt through. I never thought of drywall mesh.

[Oregon_Fruit_Grow]

@alan

Air pockets in the soil do not seem to dry out roots because of the constant humidity. Roots grow right through them. Gardeners often excessively tamp down soil, especially when transplanting. It can deleteriously compact soil, especially when it is wet.

this is true, until this year I was probably filling my containers double capacity by tamping way too much. My plants grew, but I think they could have done better.

With large bark chunks some mx will settle eventually, probably soaking the bark for few hours before mixing would help the contents to stick to the bark.

[disc4tw]

Sawzall without a blade, or sds max drill would do about the same thing.

[rossn]

Hi all - so many good responses here - thank you!!

I think my understanding of anaerobic soil was a bit off.

Learning a lot today… I had no idea that roots could be exposed to air without prematurely drying out. I guess in my head, with the DE in the mix, I felt I was having a little more wiggle room on the watering frequency, but I guess the overall soil mix will still create the humidity needed. I would think that pockets visible from the surface would also dry the soil faster. I feel like I should be able to add something that will percolate down and fill the holes. Maybe it should be the finer pine bark, or just more Pro-Mix, though it washes out.

On that topic, I reading up on pine bark, as I had read here on a few occasions that the breakdown of the smaller pine bark can lead to the anaerobic situation. This webpage specifically says:

Anaerobic decomposition is rare for pine bark because lignin is the main component rather than cellulose. Pine bark is unique because it can be used without composting as either mulch or soil conditioner.

Can anyone refute or support this? If it is accurate, maybe using the smaller bark isn’t so much of an issue for long-term containers?

@Drew51 - like the idea on vibration. I did try shaking, but you are right on some sustained vibration. I also need to find a way to keep the pro mix from washing out at the surface. Maybe larger bark chips or even the small ones for the top inch or so. Is there a reason you have chosen use bark fines instead of smaller nuggets? Love the drywall mesh tape and coffee filter ideas!

@disc4tw - I guess I was thinking about air not only from a drying perspective, but from a root growth perspective, such as air pruning approaches. Perhaps that makes the voids actually a good thing. I like your tool ideas for the vibration… have sawzalls and sds-plus, but not max. At least with the ads-plus, you have to apply pressure to get the hammering, so maybe I need a base the plant sits on that I can lean into. (I’m in fabric containers, so that make make it a little more challenging)

@Alan - thanks so much for this - very educational!

@FigGuy - since I’m primarily in fig pots I haven’t had this problem very often, but see the challenge. Drew’s comment got me thinking about alternate materials. I built a French drain here, and for it, used a filter fabric… basically 1/8" thick felt-like woven geo textile material… very durable, as it has to last a long time. A swatch of this across the bottom of the pot would turn it into a filtered bottom, preventing material from escaping. Actually has me wondering about just lining larger containers with it to make a rigid sided fabric container.

I think this is what I used. In quantity, this is about $0.30/sf.

https://www.dewittcompany.com/product/filter-fabric-6-oz/

@poncirusguy - that is a new one for me to learn about!

@Oregon_Fruit_Grow - thanks for clarifying on the settling. I suppose I just keep adding some finer material on-top until it trickles down. Will experiment with the soaking.

[Oregon_Fruit_Grow]

If you are looking for long term container soil, I’d say go with pure peat moss and mineral based aeration (sand, pumice, granite grit etc.)

[rossn]

Thanks. These are large containers (up to 30 gallons currently) and I am trying to minimize peat moss use, to the extent practical. That would also be pretty pricey for the larger containers. So I’d like to try some blends first.

What does the granite grit do? Sounds heavy!

[Oregon_Fruit_Grow]

You can pick up a 3 cuft bale of peat moss for $25 some place locally, the bale further expands another 2 cuft. Granite grit is decomposed granite used for poultry, helps in aeration. I have planted blueberries in pathway bark (fir), peat moss and pumice. Haven’t seen any drastic decomposing yet.

[fruitnut]

I use large bark chips as the base for my potting mixes. Then add enough fines to fill in most of the voids. I’d says it’s about 50:50 maybe 60:40, coarse: fine. The fines would be similar to Promix HP.

The fines stay in place. They don’t move down when watered. At least not very far. After watering the surface has a thin mulch of the coarse.

This mix holds up well. Not much settling which to me means not much decomposition. In a few years there are so many roots it can’t settle much. That’s good. After 4-5 years in fabric pots there are no circling roots when fabric is removed. It appears ready for another several years.

In fact, my big banana was planted into a 30 gallon plastic pot that was a fig for 3-4 years that I didn’t want any more. So, I cut out the fig plant while keeping 90% of the old mix in the pot and planted a finger sized banana in it’s place. The banana grew so much I had to up pot it because it was splitting the pot open. After a year and 2 months this is the result now moved to about 90 gal adding the same mix.

This is what the coarse looks like. Add enough fines to fill in most voids but keep it coarse.

[alan]

@rossn

I am trying to minimize peat moss use, to the extent practical.

I take fruit trees grown in 18" diameter fruit bags in real soil and stick them into 25 gallon pots when they are 2.25- 2.50" diameter trunk trees and usually bearing fruit and surround them with a mix of equal peat moss, perlite and compost from a yard that makes it out of wood chips and yard waste acquired locally.

The point of the peat moss and perlite is to keep the pots reasonably light, given it is half soil. The mix completely surrounds the soil and, miracle of miracles, allows good drainage.

I could make the mix out of 60-40 compost to sand (volume) if weight wasn’t an issue and I think I’d still get adequate drainage. I feel certain it would work if I wasn’t also adding a lot of real soil in the middle of the pot-18 of the 21".

Fruit nut is as experienced as anyone on this forum in sustaining fruit trees in pots without any access to real soil. In my nursery I allow a certain amount of escape root into my soil to reduce the need of water for my potted trees.

[elivings1]

You can buy 3 cu feet of peat moss at Home Depot for 24 dollars. It used to be 10 when I did most of my pots and I think it went up to 15 or 17 dollars so it is certainly going up in price and quickly but so has any gardening the last few years. It would be tough to get a mix cheaper than peat moss, perlite and basic fertilizer that last 6 months like Osmocote. I have heard you have to replace it once every few years but our family has been reusing the same soil for years. Our perennial pots are packed with roots so if we had to replace the plant we would have to get new peat moss or soil. Our annuals leave enough space to yank them out without too much soil being lost.

[disc4tw]

I found miracle grow moisture control for $2 a bag last year. It’s mostly sphagnum moss and a little bit of perlite. It was cheaper than buying baled peat and I didn’t have to mix it! Hoping Walmart puts promix on sale at the end of the season again. I almost bought them out but didn’t have the motivation or space.

[elivings1]

I think those are for the small bags though. Costco and Home Depot sell the 1.5 or 2 foot bags. I think the Kellogs ended up being cheaper than the Miracle grow at Home Depot earlier this spring.

[disc4tw]

$11/ ft^3 at Walmart currently for what I grabbed last year. There’s a local chain I bought it from for that price.