Garden Docs: Don’t hurt tree roots when you plant at the base

Lori C. of Santa Rosa asks: I would like to plant a ground cover under a few of my trees for added color. Is it OK to rototill the soil around the trees to loosen up the soil to make it easier for me to plant? One of the trees has large roots that are growing on top of the ground. I know not to rototill over those, but what about the roots I can't see?

You need to be careful of cutting into tree roots when rototilling under any tree, as most tree roots are located within the top 18 inches of the soil. Several tree species have surface roots that you can see, like you mentioned, growing along the surface of the soil. Perhaps soil got washed, moved away, or that is the nature of that trees' root system, exposing large roots, thus making them vulnerable to being damaged by a rototiller or even a shovel of some sort.

It would be better if you were to start with 4-inch potted plants, or smaller, and with a trowel, dig the appropriate sized hole for each plant, rather than rototilling up a large area, disturbing the soil, and cutting into the tree's roots. If necessary, you could mix in a little compost with the soil for better drainage and aeration for the plants. Cover the bare soil with mulch and water accordingly. Keep a few feet away from the trunks of the trees as that's where you will find the largest part of the trees' root system, and it will be hard to find a spot to dig and plant the plants.

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Gary B. of Santa Rosa asks: I have some bean/pea inoculant that I used last year. Is it still good to use on the beans and peas this year? I heard it doesn't last that long.

An inoculant is usually a powdered form of bacteria or fungus that you can buy at your favorite nursery or garden center. It's added to the soil when you shake the powder on the beans or peas, coating the seed prior to planting. You would put the seeds in a jar, sprinkle the directed amount of inoculant in the jar, cover, and shake until the seeds are coated. There are some liquid inoculants available, but the majority that are used and sold are powders.

The bacteria most commonly used are Rhizobium bacteria for inoculating legumes such as beans and peas for the home garden. Another nitrogen-fixing bacterium is Acetobacter, but not as widely used. There are many others, like Bradyrhizobium japonicum used to inoculate soybeans for nitrogen fixing.

What the Rhizobium bacteria does is, it stimulates the legume roots to grow nodules that “fix” nitrogen. (Nitrogen is the “N” in N-P-K fertilizers.) What “fixing” means is that the nodules absorb nitrogen from the air and convert it into ammonia in a useable form (NH3) that the plants can use and readily take up through the roots for growth.

Since inoculants are inexpensive, and most importantly, they don't have a long shelf life, especially if accidentally left in a warmer-than-warm area, it's best to toss them out and start with fresh inoculant. Don't feel that you need to inoculate your seeds. They will produce just fine if you sow them in a well-composted, good draining soil.

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Debra M. of Santa Rosa asks: Can I add green potato skins, rhubarb leaves and tomato leaves in my compost pile even though they are poisonous parts of the plants?

Even though green potato skins, rhubarb leaves, and tomato leaves are known and said to be toxic, it IS safe to add them to your compost pile.

Remember, if your compost pile is working correctly, and the plant material you've been adding is breaking down, these leaves will also break down and become part of the compost material. These leaves may be toxic to humans, but they are not to plants when added to the garden. Even if you dig a hole and bury the leaves right in the ground, they will simply decompose and become organic matter with no toxic residue.

Dana Lozano and Gwen Kilchherr are garden consultants. Send your gardening questions to The Garden Doctors, at pdgardendoctor@gmail.com. The Garden Doctors can answer questions only through their column, which appears twice a month in the newspaper and online at pressdemocrat.com.