Why Grass Doesn’t Thrive Near Trees

    Listen 00:52:57

    When grass fails to thrive near trees, shade gets all the blame. Mike McGrath, host of You Bet Your Garden, will reveal that aqua is often the actual antagonist. Plus your fabulous phone calls!

    Photo: Dave Meier via pictography.co

    Question of the Week:

    “My lawn was once immaculate, but then my wife and I planted several trees without understanding the damage the shade would do to the lawn. I now have more dirt and weeds than grass. And the grass keeps dying. I’m in the process of getting dirt delivered and having it spread, leveled and tapered for the natural flow of rain fall. My worry is how to keep the grass healthy with limited sun that after all that prep work.”

    — Tim in Hendersonville, Tennessee

    Learn more about growing grass near trees »

    Highlights from show:

    Charcoal and Compost Tea

    Will from Kennedy, NY does a whole bunch of composting and would like to know about infusing charcoal with compost tea, like a Mayan Supersoil. Mike says that this “biochar” isn’t just charcoal, but woody plant material, burned very slowly in a low-oxygen environment, and if done incorrectly, it can cause more problems than good. Furthermore, the Bartlett Tree Company says that the best time to use biochar is when preparing a new planting area. You shouldn’t use it on a pre-existing garden or lawn. Now, if you were installing a new lawn, make sure you have real biochar, then bury it six inches deep through the subsoil, spread more soil on top, and sow the lawn on top, and this will improve the health of the lawn. Bartlett has also had great success with it when planting new groves of trees. They have also found that when this woody material has been burned like this, it becomes very porous, becoming a habitat for beneficial microorganisms. Mike also addresses Will’s original question, saying that he may have better luck if he just adds compost on top of the buried biochar, but he’s welcome to try infusing it with compost tea. But he does give the following warning: “All wood ash is alkaline, so it’s best used in a soil that’s already too acidic where you grow plants that like a more neutral pH.”

    Growing Flowers in Pots

    Rick in Downingtown, PA is growing flowers in pots, and so Mike gives him his standard line: Don’t use any garden soil: fill them with soil-free mix, perlite and compost. And make sure that they have good drainage and you should have just as much luck growing in pots as you’d have growing them in the ground. Rick follows up with a question about the pots themselves: he has a container garden growing on his deck, and each winter, if he doesn’t empty them out, the pots all crack and have to be replaced. He’s been growing mostly in plastic and terra cotta containers. Mike explains that terra cotta comes from parts of the world that don’t freeze, so those have to be emptied every winter. On the other hand, thick, strong, heavy, plastic will be more resistant to cracking. You might have better luck if you cover the pots over in the winter, keeping the soil from getting saturated. Meanwhile, what Mike uses are half-whisky barrels, which he has stuff growing in all winter as well. “They are the bee’s freakin knees!”


    Nancy in Merion, PA wants to know all about Pennellia weed. Now, this is a small cruciferous weed that looks like a miniature Jack-in-the-Pulpit, and it spreads like crazy. Mike tells us all about his experience with it, saying that when he first saw it, he quite enjoyed how it looked and kept them, refusing to heed the warnings of those around him. The next year, he found that they’d taken over his beds! After some time, he’s got it down to about a third of its previous number. Mike recommends that she solarize the bed, and to do it correctly, she must level the ground, saturate it with water, and cover it with clear plastic. However, if there isn’t enough sun, Mike recommends that she dig deep and pull out their bulb, because they have a unique and interesting root system: they have a long, spindly root, connected to a small potato-looking tuber located off to the side so that when you pull the plant, you get the whole root, but none of the tuber, so you’ve got to dig deep and pull the whole thing out. “If something pops up in the garden that looks like what we’re describing, get rid of it as soon as you see it! Screen that soil and get those little potato jobbies outta there, or you will have a new hobby for the next three or four years.”

    Tall Tomato Plants

    Eric in Wayne, PA raises chickens. He likes to mix their waste in with the municipal compost so that everything gets all mixed up and well-composted, and then puts that in his tomato beds. Now, he’s been seeing a lot of growth on these plants, growing up to about twelve feet. Eric has had to do some serious pruning to keep these plants from outgrowing their support system. Mike notes that Eric has been cutting off the new growth, which will give him all of his late-season tomatoes, and that they’re supposed to grow that tall. Mike recommends that Eric ditch his bamboo stakes, and replace them with tomato cages. Now, proper tomato cages are made out of animal fencing, such as rabbit wire, or turkey wire. FIrst, cut this fencing into a six-foot linear length, and then form it into a tube. After that, get the bamboo stakes off of the tomatoes, then center the cage around the tomatoes. Use the bamboo stakes in the sides of the cage, to keep it from wobbling. The vines will grow up toward the sun, but since it’s flimsy like a vine, not solid like a tree, it will lean against the cage, growing up the sides of the cage and filling all of that space. And because it’s growing up a greater surface area, a 12-foot vine won’t exceed a 6-foot tomato cage by too much. Eric finally asks about what he should prune, and Mike has this to say: “Every leaf you remove takes away energy from the plant.”

    Nut Sedge Problem

    Arthur from Ocean County NJ has a nut sedge problem in his over-55 community, and the landscaping committee has recommended treating the problem chemically with several recurring treatments over the course of several years. They live very close to Barnegat Bay, and Art wants to know if this would pose a threat to the environment. “Oh, heck yes!” says Mike. “And it probably wouldn’t knock back the nut sedge.” Nut sedge is a weed of “cultural problems,” meaning that the conditions in this community’s soil is perfect for growing nut sedge. Mike remembers that back in the 1970s, there used to be popular gardening books that would tell you what’s wrong with your lawn, based entirely upon what weeds are growing there. He says that nut sedge is a sign that the soil is too compacted, and that the lawn needs to be core aerated as soon as the summer heat stress is over. Just pull little plugs out of the lawn to relieve soil compaction. The loose soil favors the grass, and the nut sedge doesn’t like that. They’re also feeding the lawn way too much, likely because they’re trying to get more money out of the community. The lawn should only be fed in once in the spring, then again in the fall. And if they’re using any fertilizers containing phosphorus, they can also pose great threat to local waterways. The third thing this nut sedge is telling Mike is that these lawns are getting watered too much. With the wet summer we’ve been having, you shouldn’t be watering your lawn. “If these lawns are aerated, the feeding is cut back, they’re allowed to dry out between waterings, and they’re cut to three inches, as opposed to a way too low cut, right now, I can virtually guarantee that the nut sedge will disappear by itself.”

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