Thank you. I really don't understand exactly how they work, and searching online has only been providing the "they release faster at higher temperatures" type of vague information.

Moisture does play a role but really only in softening the prilled surface. Almost all are released by temperature factors, with increasingly high temps (soil and air) triggering faster release. Living in a very mild, cool summer climate, the CRF's I use can last as long as 6-8 months. In a warmer summer climate - one that routinely sees days in the 90's - the same CRF will only last 3-4 months.

These are actually intended to mimic Mother Nature to some degree. When soil temperatures cool in fall, plant growth and the need for nutrients also declines. When the soil heats up in the spring and summer, plants put on growth rapidly and have much higher nutrient demands. The CRF's work in a very similar fashion...a high rate of release when soils are warm/hot and a much slower release rate when they are not.

It doesn't change the equation when the fertilizer is on top of the container media instead of incorporated into it? I thought osmosis was essentially how the fertilizer is released from the prills - something that would happen when in the media solution (i.e. incorporated into the media) or when in the irrigation stream (which made me wonder about cold irrigation water and release rates).

You have to incorporate the prills into the top layer. You don't want to just have them laying on the surface. They'll dry out too fast and not really release the ferts.

water a few pots.. and then just experiment to find out how fast the chilled soil warms ...

i have been to many a large nursery ... and have never found it incorporated ... with long term potted plants.. like trees ... and yes.. i have seen it incorporated into the media mix with annuals and perennials ... apples and oranges with the strategy with different plants ....

do be aware.. that if comes in various length time release .. in my cold winter clime.. i would want a 4 month release ... so i know the fert is dumped.. and washing out.. by the time its fall ... so the plants will slow down and go into winter dormancy ...

it would be a potentially big problem if i used a 9 month release ...

black media.. in black pits.. will probably be much warmer than you might think ... and if takes a bit longer in spring to warm.. and release.. so be it.. you will be on mother earths warmth schedule ...

based on killing lots of pots ... go light with application ... you can always add more water sol later ,....

ken

Nil, how do you define the "top layer" when using a fast draining mix like the 511? The top inch or so dries out quickly in the sun even with frequent irrigation.

I have seen multiple sources that recommend mixing it in to the top layer, but that mention it will be fine if you do not.

And while I know many nurseries use liquid fertilizers, I would be extremely surprised if those that use a topdressing of CRF as some part of their fertilization program take the time ( = labor cost) to mix it in (though - anyone with knowledge of the nursery industry feel free to correct me on that..)

With more than 20 years experience in the industry, I can confirm that nurseries routinely use a commercial CRF similar to Osmocote to fertilizer container plantings. And it is only applied to the surface as a topdressing, seldom if ever mixed in. This is for retail nurseries, not wholesale growing operations, which could take a different approach.

Exposure to sunlight does not dry out the product....or certainly not to the point of the product becoming ineffective. Routine irrigation will allow water to be absorbed into the prill, dissolving the nutrients it contains (that's the where the osmosis comes into play). Once the CRF prill has absorbed water, soil temperature determines the release rate of nutrients. If anything, surface applied CRF's might have a faster release rate just by virtue of being exposed to hot sunlight.

I use this method - topdressing - as well, for all my permanent containers of woody plants. At the start of each growing season, each container gets a liberal sprinkling of the CRF on the top of the soil surface. By the end of the season there is no sign that this product had been applied other than the healthy growth of the plant - no residual prills or empty resin shells.

And while some suggest adding the CRF to the planting mix prior to potting up, the CRF's will start working almost immediately when they come into contact with even a slghtly moist soil mix so you will need to pot up promptly to maximize the benefits of the fertilizer. I do add CRF's when potting up but to each individual container when it gets planted, not to the mix as a whole.

Well I suppose things are a bit different down here in SoCal than the PNW. I have never in 20 years and tens of thousands of plants seen a grower lay crf just on the surface. I only deal with wholesale growers and they always mix the crf into the mix before potting so that it is thoroughly incorporated. The ones I deal with repot every season so there would never be a need to topdress. My experience with topdressing down here is that it just sits there and nothing happens but we are pretty arid. A mulch layer on top can solve that.

The top layer would be the top inch of media.

All the growers I deal with use both CRF and water soluble. The CRF is used to reduce the concentration of the WS and thus limit nutrient leaching. There are plenty of studies on that.

I wouldn't touch woody perennials from a retail nursery down here if my life depended on it. I have seen too many root horror shows for that.

I'm not sure why things should be any different based on location :-) A CRF is a CRF regardless of where on the planet it is being used and the mode of action is essentially the same. It can be used in either manner and is just as effective both ways. Arid climate or not, the containerized plants still need to get irrigated (typically even more frequently when hot and dry) and that is more than sufficient to trigger the osmosis and the release mechanism.

Having been active in this industry for more than 20 years as well as a professional gardener/consulting horticulturist, I shy away from any absolutes when it comes to growing plants. They never are absolute - there are always exceptions :-) I have yet to see many containerized root issues that couldn't be corrected. And you get to know the growers that maintain the best sizing and conditions to minimize this. I oversee the planting of hundreds of woody plants - trees and shrubs - each year in my business, most of which are grown in containers, and I have had relatively few failures - almost none - from root issues if appropriate care is taken at planting time.

CRFs are routinely applied in a number of ways. Sure, some are applied using top dressing, but I see roughly just as many sub-dressed (that's evident when you take a plant out of its pot and see a line of CRF partway down the soil). Sometimes they are mixed into the medium and sometimes poked in using a tool. The biggest issues with top dressing are the fertilizer falling out if the plant tips over (pretty easy to avoid) and the fertilizer not getting enough water if the pots are irrigated by drip.

Thank you all for the great responses!

Why would things be different? IDK, because it gets to 120f with single digit humidity? Lol But sure tell me how they grow down here.

"Lol But sure tell me how they grow down here."

Really? That's a bit of a snarky response.....I don't recall telling you anything of the sort! I merely questoned why the actions of a CRF should be any different in different areas, other than they may not last as long in hotter climates.

And I do have some familiarity with SoCal wholesale growers as a buyer for a local retail nursery - many of our plants came from them. And some did arrive with CRF's visible on the container soil surface so the incorporation you state is routine is obviously approached with some flexibility.