Bottom-watering is one of the best tricks I know for saving water in container gardens. But you can make containers even more water-efficient by working from the top down with that old water-saving standby, mulch. Like bottom-watering, mulch keeps moisture levels in the soil more even, protecting plants from the shocks they get when water supplies fluctuate.
And here’s the bonus: mulch will keep your weeding chores to a minimum, too.
The broadest definition of mulch is something that provides a barrier between the soil and air, so moisture stays in longer. That means pebbles can be a mulch. So can cardboard. So can plastic. Some commercial self-watering containers come with a plastic cover that acts as a mulch for the container.
You can also mulch containers in more traditional ways – you just have to think tinier. Instead of big wood chips, use shavings. Instead of hay, use grass clippings, or alfalfa meal, if you’ve got roses or other more nitrogen-hungry potted plants. The woods mulches my plants for free with oak leaves and pine needles; I only brush these away if they’re getting in the way of plant growth. Otherwise, those leaves and needles are providing nutrients and moisture-protection for my plants. (The photo at the top of the page shows some of this natural leaf mulch.)
I’ll confess that I mostly use the most traditional mulch of all: tight planting. If you plant your containers like little landscapes, with groundcovers, tall plants, and seasonal appearances, you’ll find that the plants themselves cover the ground thoroughly.
Before you get to the point of self-mulching plants, you’ll still want to mulch for maximum water savings. The easiest time to mulch is when you’ve just finished planting, when there’s the most visible space between the plants where you can stuff as much mulch as possible. And do stuff. Mulch isn’t effective unless it’s at least two or three inches deep, packed down. That means it has to be even higher and fluffier than that going on.
Of course, that may be difficult to achieve in the confines of a container. Don’t worry about it; if you’ve already got your bottom-watering system going, you’re miles ahead of other container gardeners. Just put on as much mulch as you can, and, when there’s a chance, slip in a little more. If you have tender plants, you can lay on a complete new coat of rottable mulch at the end of the season; some of it will rot and enrich your soil before you plant again, the rest of it will act as a packed-down mulch base for next season.
Less-traditional mulches, such as plastic and rocks, are a little trickier. Rocks obviously need to go on after planting, but plastic goes on after you’ve got soil prepared and before plants go in. Then you cut an X in the plastic, and slip the plant in (you can also plant seeds this way, but I usually start mine in a seed plug flat before I put them in the bigger containers).
If you live in a hot climate, be careful about the color of plastic mulch you use. Dark plastics heat soils, which is why some people use them, but if you have very hot summers they may heat the soils enough to kill plants, especially in a small container. Look into more reflective plastic mulches, or consider using traditional mulches that rot. While rotting mulches need to be replaced, they also act as part of your fertilizing system. Plastic mulches need replacement, too – good ones last about two years, in my experience – and they tend to shred and chip and leave little bits of plastic about if you don’t catch them in time.
The same principle applies to rocks: dark rocks attract heat, white rocks reflect it. While rocks allow a bit more moisture to escape, that’s still a consideration when you’re mulching with them. One of the prettiest (and nicely reflective) mulches of this type that I’ve seen is Conscious Gardener’s recycled glass mulch.
Whichever kind of mulch you use, take a look at the top of your container, and think of the best way you can keep moisture from escaping.
Bonus idea: Another way to conserve water from the top is to use foliar sprays made to conserve moisture (they’re sometimes used to provide “shine” for tropical houseplants). The sprays coat the leaves, slowing transpiration (plant breathing, in which moisture is “exhaled”). The coating also makes the leaves slightly shiny, reflecting the sun. I have used these sprays and I think they made a difference, but I wasn’t very scientific about how I used them. I think basically my system works so well without it that I don’t bother, but it might be helpful in some cases.
Next post: putting it all together for a beautiful container garden
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.)