Pumice soil – What can I grow?

The following was a question from a reader of the Chiloquin News

What kind of shrubs or flowering shrubs or climbing vines can I plant in pumy dirt? I’ve tried amending the soil for proper ph and still can’t grow anything.

Brown thumb

Pumice soil is made up of lava rocks which have large air spaces in the rock; enough air that you’ll find the rocks floating down rivers. I was pretty intrigued the first time I came up on floating rocks when paddling the Wood River. This means that when the soil dries out, there’s a lot of air in it, which plant roots do not like. When it’s wet and the air spaces fill up, it holds a lot of water, but that means that the nutrients in the soil get very diluted, and are easily leached from the soil.

So, assuming that you will be watering and the soil will not dry out too much, the main problem in growing anything in pumice soil, is the lack of nutrients. pH is not much of a problem unless it’s extremely acid or alkaline, and anything you would use to amend the pH except compost, would be quickly leached from the soil.

 The easiest way to grow flowers and vegetables is to get in a load of good topsoil and plant them in raised beds. Their roots don’t go very deep, and they can grow quite happily in a foot of topsoil. It’s a different story for trees, shrubs and vines that have deep roots.

One method that sometimes works is to dig a BIG backbreaking hole – as big as the area the mature roots will cover, and fill that with topsoil. If the roots are able to penetrate out into the pumice soil, then the plants are able to grow quite well by relying on the nutrients in the topsoil area, though it will be necessary to fertilize that soil over time. Most often, though, what happens is that you have made the equivalent of a large pot plant in the ground, and the roots cannot get out of the hole you’ve dug. Then your plant ends up stunted and not very happy.

A second method would be to add nutrients to all of the soil in your garden area. Adding synthetic fertilizer will do you no good at all. It will be leached out of the soil when you water as fast as you add it, and just end up contaminating the underlying water table. The only option is to add organic matter in the form of compost. Compost is amazing stuff. It makes clay soils more porous, sandy soils less porous, and it will enable your pumice soil to hang on to nutrients, by slowing down the leaching. It will take a lot of compost, and you will have to add it every year, but if you do that, you will end up with good soil and healthy plants. The first year I would try to put on about a 6” layer and dig it in a bit. After that, you can just add it on top each year. It’s not always so easy to make compost here because our summers are so short, so I have resorted to other methods as well. I do make compost, and add it when I have enough, but in addition, I also add shredded leaves, llama poop (from a friend) and finely chipped garden waste, without composting them first. Fertilizer from llama poop is the most wonderful of soil conditioners and fertilizers. By the time the llamas have processed the food through their 3 stomachs it is very well broken down. If you have horse manure it’s better to compost it, and never add fresh chicken manure. Chicken manure is very potent and will ‘burn’ your plants unless it is composted or is very old. Lawn clippings are also better composted. Then when you plant, try to choose plants that do not require very rich soil. This will still give you lots of options though – lilac, honeysuckle, potentilla, flowering currents, ornamental quince, elderberry, spirea, hardy shrub and rambling roses, to name a few.

Plants to try in ammended pumice soil

Finally, you can just go with the native plants that have adapted to grow on pumice soil, though you will still have to give them some water while they are getting established and every year if they are normally found in wetter areas. Tall deciduous shrubs include Rocky Mountain maple (Acer glabrum), serviceberry (Amelanchier alnifolia), oceanspray (Holodiscus discolor), mock orange (Philadelphus lewisii), Pacific ninebark (Physocarpus capitatus), Tall Oregon grape (Mahonia aquifolium) and Scouler’s willow (Salix scouleriana). Mid-height deciduous shrubs include baldhip rose (Rosa gymnocarpa), russet buffaloberry (Shepherdia canadensis), shiny-leaf spirea (Spiraea betulifolia), snowberry (Symphoricarpos albus and S. mollis), manzanita (Arctostaphylos patula), and wax current (Ribes cereum).

Native plants for pumice soil

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