Why seasonal climate forecasts aren’t always accurate

Do you use predictions of seasonal climate to plan your garden work? Or are you frustrated because they don’t seem to be very useful? I’ve been getting a lot of complaints this year about how bad the climate forecast for winter was because what we have seen so far has not matched the predictions in many parts of the country. Let me take a few minutes to explain how they are made and what you can learn from them.

First, let me specify that I am not talking about long-range climate forecasts for 50 years down the road. Nor am I talking about weather forecasts for the next week. I am talking about the forecasts that cover the period from about 15 days to 3 months, which climatologists call the “seasonal to sub-seasonal forecasts”. These are the kinds of forecasts that say “Winter is likely to be warmer and drier than normal” or “Get ready for a big warm-up in the next month.” They can be useful in planning garden work a few weeks ahead, but they come with caveats.

“Glory of the Snow” in the snow. Taken by User:Ruhrfisch April 2006, Commons Wikimedia.

Unlike weather forecasts, there are only a few models that predict climate in the monthly to seasonal time period. That is because we can’t just run the weather models out four to twelve weeks and expect to get anything like real weather. The weather models are built to handle short time steps and detailed information about temperatures, rainfall, and all the other factors that make up your daily weather, and to do it fast enough that you can actually use the forecast to decide when to wear your raincoat. They are useful out to about a week, but then their accuracy starts to break down because there are too many things going on around the globe to capture accurately over time, and so the short-term models tend to drift away from reality the farther from “now” you get. Models for monthly to seasonal climate tend to be based not on dynamical atmospheres like weather models but on statistics.

La Nina causes the jet stream to move northward and to weaken over the eastern Pacific. During La Nina winters, the South sees warmer and drier conditions than usual. The North and Canada tend to be wetter and colder. Source: https://oceanservice.noaa.gov/facts/ninonina.html)

NOAA’s Climate Prediction Center (https://www.cpc.ncep.noaa.gov/) is the biggest provider of seasonal forecasts, although there are a few others out there. This year we are in a La Niña, and so most of the seasonal forecasts have been based on that affecting our climate this winter. I won’t discuss La Niña here today (that is a topic for a future post, perhaps) but you can read a good general description at https://oceanservice.noaa.gov/facts/ninonina.html. The basic patterns of La Niña affect the temperature and precipitation across the United States in fairly predictable ways, and you can use statistics to show these patterns. You can see some examples of how La Niña has affected past winters at https://www.weather.gov/mhx/ensoninaanomalies. This year, the primary predictors of the winter climate have been the La Niña and the persistent trend that we are seeing towards warming temperatures due to greenhouse warming. From a statistical standpoint, it made great sense to predict that this winter would be warmer and drier than normal in the southern US and colder and wetter than normal in the north, because that is statistically the most likely pattern to expect in a La Niña winter, even when the climate is trending warmer over time.

So why did it not work this year? Because statistics can’t account for rare events that don’t follow the expected patterns. At the end of 2020 the atmosphere over the North Pole experienced a Sudden Stratospheric Warming (SSW), which means that the atmosphere about 10 miles above the North Pole suddenly got much warmer than usual. That messed up the usual distribution of temperatures in the Northern Hemisphere and helped push the really cold air to the south. It also pushed the winter storm track far south of where it usually occurs, making this a very wet winter in the Southeast, which is not what we expected! My farmers are not happy, but at least it means less likelihood of drought this summer. You can read more about the SSW at https://climate.gov/news-features/blogs/enso/sudden-stratospheric-warming-and-polar-vortex-early-2021. It might happen only once every ten years, or the cold air might just get pushed in a different direction next time, missing you and your winter garden altogether. Since the models are based on statistics, they will always show the most likely pattern, and instead we might experience winter that happens just once in ten years. Not so different that being the lucky person who gets rained on when the National Weather Service predicts just a 10 percent chance of precipitation!

90-day temperature departure from normal. Source: https://hprcc.unl.edu/maps.php?map=ACISClimateMaps

The good news is that we are getting better at these sub-seasonal to seasonal predictions, and we can expect to see improvements in the future as computers become more powerful and we have more experience looking at these periods. But for now, statistical models will continue to control the predictions at these intermediate periods, and we will continue to see the occasional miss when an unusual weather event occurs.

The dirt on rock dust

One of the newer “miracle products” targeted to gardeners is rock dust. Rock dust (also called rock flour or rock mineral powder)  is exactly what it sounds like. It is a byproduct of quarry work and is generally a finely pulverized material that resembles silt. It’s heavily promoted as a way to provide macro- and micronutrients to your soils and plants. Is it worth adding to your gardens?

Rock crushing at a quarry

First, it’s worth acknowledging that repurposing an industry byproduct is always preferable to throwing it away. Fortunately, the last few years have yielded some peer-reviewed research that we can use to make informed recommendations.

What’s in rock dust?

Obviously, the mineral content of rock dust is dependent on the rocks used to make it. This means the mineral content varies considerably, but in general rock dusts contain:

  • Large amounts of silicon, aluminum, and sometimes iron
  • Lesser amounts of calcium, copper, magnesium, manganese, potassium, sulfur, and zinc.
  • Potentially toxic levels of aluminum, arsenic, cadmium, chromium, copper, lead, nickel, and sodium.

I’ve added some tables from a few research articles that analyzed their rock dust mineral content below. Note the high silcon, aluminum, and iron content. (LOI = loss on ignition, meaning some materials were burned off during analysis.)

How is rock dust used as a mineral source?

Rock dusts must be solubilized to release minerals. There are some criteria that can speed mineral release:

  • Decreasing the particle size of rock dust.
  • Blending the rock dust with nutrient-rich organic matter like manure. This provides an acidified environment for mineral solubilization.

When is it beneficial to use rock dust?

There are documented benefits to using rock dusts – but only in agricultural production systems:

  • Rock dusts can contribute minerals to nutrient depleted soils, such as agricultural soils that have been overworked for decades.
  • Organic farmers can use specific rock dusts to supply micronutrients, rather than commercial fertilizers which are not certified for organic crop production.
  • Cereal crops – members of the grass family – require silica as a micronutrient (though silica is rarely if ever deficient in field conditions).

What’s the bottom line for gardeners?

As one article states, “…there is a potential for using [rock flour]…where there is a lack of these nutrients and where conventional chemical fertilizers are either not available or not desired.”

And how do you know if you have a lack of a certain nutrient? Why, by having your soil tested, of course! There is no point in adding anything to your soil unless something is missing. It is MUCH harder to treat a nutrient toxicity than to add a deficient nutrient. Iif  a soil test reveals a lack of a particular nutrient,  a carefully chosen product could supply this mineral. But you would have to know what else was being supplied and possibly creating a mineral toxicity.

At this point, there is no evidence to suggest that rock dusts are of any value to a home garden or landscape.  And adding these products can easily contribute to aluminum and heavy metal toxicities. I would never add it to this soil, for instance, as it already has excessively high aluminum levels.

Aluminum is already at potentially toxic levels in this soil. No need to add more.

This blog is full of great ideas on how to manage your soil naturally, sustainably, and safely. Rock dusts are just the latest garden product with lots of marketing but little benefit.

Houseplant Hubub: The rage about variegation

It is no secret that houseplants are hot right now.  Interest was growing before the pandemic, especially with millennials and younger folks.  Then the pandemic hit.  Houseplant interest skyrocketed since people were stuck at home and wanted to bring a little bit of nature indoors to make their spaces a little more cozy for 24/7 habitation. 

This has caused the demand, and price, of many houseplants to increase, especially if they are on the rarer side.  One thing that increases the price of many plants is when a variegated version of a standard plant has been developed. 

My reading nook/houseplant oasis

Just as an example, after posting a photo of my “reading nook/houseplant oasis” in my home office I was informed that variegated form of a Monstera deliciosa vine that I had was the highly sought M. deliciosa “Albo-Variegata” cultivar, usually referred to as a Monstera albo, or just Albo.  Folks were reaching out to buy cuttings right and left.  I ended up selling 5 single leaf/node cuttings over one weekend and made $675 in the process.  That’s right, $675!  The most variegated of the leaves sold for $200, and that was actually a bargain price.  The garden writer for the local paper, the Omaha World Herald, even picked up the story and shared it as a focus on the four new houseplant shops that have popped up in the city over the last few months.

Had my plant not had the variegation that made it an albo, each of those cutting would have been worth a few dollars apiece.  So what makes some plants variegated and others not?  Sometimes the variegation is the standard form found “in nature” and sometimes it is a cultivar or variety that has been bred or discovered by chance.  Let’s take a look at all the ways that a plant can get that variegation, whether it is standard or rare. 

Chimeric variegation

My Monstera albo that caused the hubub

This is a common form of variegation and the one responsible for the variegation of my Monstera.  In this form, a genetic mutation in some cells changes that cell’s ability to produce chlorophyll.  It may reduce chlorophyll production, resulting in yellowish or silver coloration, or eliminate chlorophyll altogether, resulting in white coloration. 

The name chimeric or chimeral is based on the fact that the plant displays two (or more) chromosomal patterns on one plant.  In Greek mythology, a Chimera is a frightening fire-breathing female monster with the head of a lion, body of a goat, and the tail of a serpent. 

Image result for chimera
An ancient chimera statue

This variegation can be stable, where the pattern persists throughout the plant.  Or it may be unstable, where it is random on certain leaves and parts of the plant can revert back to the standard green form.  These plants can also produce leaves that are almost totally white, which usually results in a leaf that will die since it can’t photosynthesize. 

This type of variegation also means that cutting or propagations may or may not be “true” to the pattern.  It can be random.  For my Monstera, the presence of white striping in or around the node that will become the new plant is the important marker for whether the new plant will be variegated or not. 

One common chimeric houseplant is the plant formerly known as Sanseveria, now a Dracena (Snake plant or mother-in-law’s tongue). Many of the different color patterns on some of the cultivars are due to cuttings taken from different parts of the “original” natural type that display different colors on them.

Viral Variegation

Image result for tulip mania
Viral variegation that was all the rage in Tulip Mania

While beautiful, this variegation will often reduce the productivity of plants if not kill them outright.  There aren’t a lot of houseplants that have this variegation, but some Hosta cultivars do.  Probably the most famous case of viral variegation is the Tulip Mania during the Dutch Golden Age (in the 1600s).  Prices of tulips skyrocketed and people were buying them as investments (maybe like the current houseplant craze, or GameStop stocks, or bitcoins).  Unfortunately, as the virus reproduced plants kept getting weaker and weaker.  Eventually the tulip market collapsed and lots of people went broke.  Let’s hope that doesn’t happen with the houseplant market….at least with my fancy Monstera. 

Natural Variegation

Natural variegation on Tradescantia

This type of variation occurs when the patterns or colors of the variegation are written into the DNA of the whole plant.  It will occur regularly throughout the entire plant, not randomly on some parts as in chimeric or viral variegation.  This variegation is passed through cuttings and usually through sexual reproduction from seeds as well, though different variations may pop up that cause a more desirable or rare cultivar. 

Common houseplants such as Tradescantia, Maranta (prayer plant), and many more common plants have this type of variegation. 

Blister, bubble, or Reflective Variegation

Reflective variegation on Phildodendron ‘Birkin’ following veins in the leaf

This type of variegation occurs when there is an air pocket or bubble between the lower layer of tissue and epidermis, or skin, of the leaf.  The lower level typically has green pigmentation from chlorophyll and the epidermis does not, resulting in a pattern that is usually white, silver, or yellowish though other colors could appear.  This pattern can be blotchy or splotchy like in some types of Pothos and Pepperomia.  It can also occur along the veins of some plants, resulting in white or silver veins on green leaves, as in some Alocacia, Anthurium, and Philodendron varieties. 

In conclusion…..

Even if you don’t have an expensive plant hiding in the corner, houseplants can add lots of fun and color to your living spaces.  And sometimes, your houseplant obsession can even pay for itself.  Online swap and sale groups have houseplant afficionados swapping and selling cuttings and plants all over the place.  So enjoy your plants….and maybe you’ll find a cash cow hiding in the corner.  Don’t mind me….I’m just over here propagating more Albos to fill up my “mad money” jar. 


Variegation mutants and mechanisms of chloroplast biogenesis

Variegated Indoor Plants: The Science Behind The Latest Houseplant Trend

Chimeras and Variegation: Patterns of Deceit

Why Fresh is Best—when it comes to mulch?

Fresh wood chips!

One of the most misunderstood gardening practices is mulching. There is much mulch misinformation in horticulture books, web pages and even extension leaflets. First,what is Mulch? Mulch is any substance the covers the soil surface. Mulch can be inorganic (rock), hydrocarbon (plastic) or carbon based (chips, bark etc.) While any material applied to the soil surface could be considered mulch, the benefits of mulching especially to woody plants are greatest from fresh woody chippings of tree trimmings–so called “arborist chips” applied fresh—not composted. Annual plants such as vegetable plants are often mulched as well but usually with materials that rapidly break down such as straw or some mixtures of shavings and manures. These materials are easily incorporated later when the next crop is planted. For woody plants such as trees and shrubs, mulches that persist for a longer time are desirable. Plastic mulches used in agriculture are not suited to shade trees or other landscape uses nor are landscape fabrics. Each of these deteriorate into landscape trash rapidly and do not benefit soils under the mulch layer. Stone mulches while used extensively in the South west US are not as beneficial to soils as arborist chips.

Why use mulches anyway? Mulches support healthy tree and woody plant growth in landscapes around the world. They increase soil organic matter, the diversity and functionality of the soil food web (particularly saprophytic fungi), support mycorrhizal partners of woody plants, supply nutrients and suppress weeds. Thick mulch layers increase root development, and help to suppress soil borne plant pathogens. The breakdown of woody mulches on the soil surface encourages development of soil structure, increased water infiltration, water holding capacity, and nutrient holding capacity of underlying soil layers. Well mulched trees and shrubs grow healthfully without fertilization.

So why not mulch with compost? Compost is not suited for use as a mulch around trees and shrubs. Compost is often screened and is of fine texture. Fine texture presents a few problems. Fine compost will make hydraulic conductivity with soil and allow for water to evaporate through the compost/soil interface. Thus the moisture savings we see under arborist chips will not be the same under compost. Compost is also able to allow weeds to germinate in it so the weed suppression effects of a mulch will also be lost. Composts applied as mulch can make an interface between the soil surface and the mulch layer which should always be avoided as it will impede water movement through the interface.

Another important reason for not mulching with compost is that compost is poor nutritionally for soil microbes. Composts have most of their active or labile carbon burned away during the composting process by the rapid respiration of microbes. The compost is turned aerated and kept moist until the process stops at this point it has some level of maturity. It won’t reheat when turned. The microbes have consumed most of the available carbon for their own growth and respiration in the compost pile, none of this remains for microbes in the landscape. Fresh arborists chips are full of labile carbon. When laid over the soil surface spores of fungi invade and they begin to uses this carbon for their own growth as an energy sources. Placing fresh wood chips on the soil surface is feeding the soil microbiology at the soil-mulch interface. In time (a few years) these processes go deeper in the soil and begin to feed the soil food web beneath the mulch layer. The diversity of fungi increases, mycorrhizae begin to transfer mulch nutrients to their woody hosts and pathogens are destroyed by enzymes that leach from the fungi infested wood chips. While composts supply minerals (all that is left of the feedstock after composting) they can’t supply the labile carbon as a source for microbes. Fresh arborists chips do all this and are thus the best mulch for woody plants.

Fungi eventually invade fresh mulches releasing nutrients and enzymes to underlying soils

There has been some concern lately for using mulches that are recycled as yardwastes. This concerns me as well because gardeners may be disposing of dead plants in their greenwaste cans. In theory, pathogens could be coming through the greenwaste stream to gardeners. Getting tree chips is best because there is little likelihood for soil borne pathogens since the materials are chipped branches. There is some possibility of wilt diseases (Verticillium spp.) surviving in arborists chips but little research has established that the pathogen can infect especially if the chips are stockpiled for a short time. In my own research we showed that pathogens, weeds an insects had very short survival times in stockpiled (not turned) piles of greenwaste. There is very little chance of pathogens coming to your garden from arborist chips and the benefits to your soil and perennial plants are worth the effort to get a “chip drop” from your local tree care company.

Pathogens buried in fresh yardwaste do not survive for very long


Chalker-Scott, L. 2007. Impact of Mulches on Landscape Plants and the Environment — A review. J. Environ. Hort. 25(4) 239-249.

Chalker-Scott, L., and A. J. Downer 2020. Soil Myth Busting for Extension Educators: Reviewing the Literature on Soil Nutrition. J. of the NACAA 13(2): https://www.nacaa.com/journal/index.php?jid=1134&fbclid=IwAR0cPfBl3V-3car-RPeEmlqzwW8bPEOPgND07xMTNgCOa5GkuSWtdD5WzF8

Downer, A.J., and B.A. Faber. 2019. Mulches for Landscapes UCANR publication #8672

Downer, A.J., D. Crohn, B. Faber, O. Daugovish, J.O. Becker, J.A. Menge, and M. J. Mochizuki. 2008. Survival of plant pathogens in static piles of ground green waste. Phytopathology 98: 574-554.

Downer, A.J., J.A. Menge, and E Pond. 2001. Association of cellulytic enzyme activities in eucalyptus mulches with biological control of Phytophthora cinnamomi Rands. Phytopathology: 91 847-855

Downer, J. and D. Hodel. 2001. The effect of mulching and turfgrass on growth and establishment of Syagrus romanzoffiana (Cham.) Becc., Washingtonia robusta H.Wendl. and Archontophoenix cunninhamiana (H.Wendl.)H. Wendl. & Drude in the landscape. Scientia Horticulturae: 87:85-92