Forget the movie. This is the secret.
For some reason, discussions of soil type are often missing from water conservation discussions. That’s a pity, because it really makes a difference.
I started out by trying to grow in containers that were filled with my local clay soil. When I got nothing but wizened and dying plants, I realized that didn’t work. So over the years, I’ve evolved a dirt system that does; I use it, with variations, for pretty much all the containers that I water year-round.
Basic soil: I don’t recommend just dumping some dirt into a container. I do use small amounts of local soil in the containers I plant: this adds some local mycorrhizae, minerals, and other things that plants like (mycorrhizae are kind of like yogurt culture; a little will spread. If you want to know more about mycorrhizae, click the link).
The rest of the soil is your basic potting soil. If I’m feeling lazy and rich, I buy the kinds that have amendments already added. If I’m feeling thrifty and self-sufficient, I get the basic kind and add amendments. The important thing to remember is that soil with lots of humus holds water. Soil with clay holds water, too, but it has no air in it and will turn into an impenetrable block if it dries. Sandy soil will let water run through before plant roots get a chance to drink it. A dose of compost helps everything; it’s a kind of soil-builder and amendment in one.
The usual amendments: Because commercial potting soils tend to lack minerals, I add a handful of azomite (a kind of super-rich-in-minerals rock dust from ancient sea beds) to each large container. Lately, I’ve become enamored of calcium, so now I throw in a small handful of dolomite or oyster shell lime. Acid-loving plants need either a commerical amendment or lots of used tea-leaves. (That’s black tea; others aren’t usually acidic enough.)
In containers where I’m growing plants for flowers and fruit, I put in fertilizers that encourage bloom, usually a pre-mixed organic fertilizer with mycorrhizae, which definitely up the fluffy lushness of my container plants. I sometimes vary these fertilizers depending on what I’m planting, but honestly I often just wind up using the same phosphorus-intensive fertilizer for everything. (Phosphorus is the middle number on fertilizers, and it makes roots and fruit – which also means flowers.) For heavy feeders, like roses, foxgloves, and lilies, I may add some extra nitrogen fertilizer, such as alfalfa meal, but a lot of my plants get their nitrogen from the humus and foliar feeds alone.
The unusual amendments: Besides the more-or-less ordinary amendments, I also add earthworms to every pot I own. Earthworms are little fertilizer-factories, taking minerals in and excreting them in a rich, nutritious form the plants can take up easily. They also aerate the soil – important in containers, where dirt tends to pack down hard. I rarely transplant my large containers, and I think the reason is that by adding some fresh soil on top every year, and keeping earthworms in it, the soil gets refreshed without the work of a big upheaval.
The big water-saving secret: The final soil secret is water-conserving polymers, such as TerraSorb. These come in the form of little crystals; you can hardly believe you paid so much for a tub of them, until you try the experiment: put a tablespoon of the crystals in a big mixing bowl and pour on the recommended amount of water. Then watch them swell and fill the bowl. (The picture at the top of the post shows them lying on a plate.)
This is what the crystals do when they’re mixed in with your soil, gradually releasing water they’ve saved up. You mix them in the soil dry; when you water, they swell up. It’s sort of an interior version of bottom-watering, keeping soil moisture levels even, and it’s especially good for water-hungry plants. I tend not to use the crystals for plants that like good drainage. On the other hand, if I have plants that like their feet in water, I create a layer entirely made of the crystals on the bottom, and then cover it with crystal-enriched soil.
Eventually the crystals disintegrate into a potassium-based substance that acts as a fertilizer. You can renew them by injecting them into the soil with something resembling a caulk gun (a pro gardener told me about this; I haven’t seen them. But you might be able to improv with a caulk gun). Or you can just dabble little holes with your hands and sprinkle more on top when you renew the soil; that’s the method I use.
Another version of this is mats, made of essentially the same material, that you put on the bottom of smaller pots and planters. I tried this in a seed flat once, but wasn’t as successful as I’d imagined.
Important point: This last water-saving secret won’t work unless you have the good, humusy soil. So all of you who thought you’d make a shortcut and just get the crystals: it won’t work. Sorry. The plants need the whole package.
Next post: keep water from evaporating with mulch – and a bonus idea.
JUNE: A MONTH TO HONOR WATER
In a way, my whole blog is about low-water gardening; that’s the reason I got involved with tulips, and I already loved natives and Mediterranean herbs. During June, my posts will all be about conserving water in the garden. This gives me scope to cover everything from containers to cityscapes, soil to site to sprays, and of course portraits of more of those stellar plants that spread their glories with little or no watering. (Hint: the “Wild Plants” category will give you quite a few more; so will the “Bulbs” category.)