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Will tree roots harm a house's foundation?

"Fielding Questions" columnist Don Kinzler also answers questions about geranium cuttings and planting onions.

tree close to house.jpg
A reader wonders if this linden tree about 12 feet away from their house will cause problems with their foundation.
Contributed / Special to The Forum
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Q: We have a Greenspire linden that’s 12 feet from our home’s foundation. The tree is about 20 feet high. Will the roots cause problems with our foundation? Should the tree be cut down before it causes any issues? — Derek G.

A: Can tree roots harm a foundation? Purdue University summarizes the answer well:

"Tree roots are very opportunistic and will only grow and penetrate structures that already have problems. Typically, when roots encounter solid, impervious surfaces such as pipes and foundations, they are redirected laterally or up and over. However, if there is a breach or a crack nearby, they can exploit those voids in search of moisture. As an example, tree roots don’t damage sewer pipes, instead, the roots are just very capable of finding existing leaks and joints and moving into the moist pipes.”

About 90% of a tree's roots are in the upper 24 inches of soil, spreading outward, and a tree's root system extends out from the trunk two to four times the height of a tree. Your 20-foot-high linden has roots that will reach the foundation even if it were planted much farther away than it is now.

The tree was planted too close to the house, of course, but the tree is more likely to damage the shingles as branches grow and contact the roof, than it is to damage the foundation. Trees planted too closely to structures often begin to lean away, as this linden is doing, usually in search of space and better light.

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Q: What am I doing wrong? I tried starting geranium cuttings in water, but the cuttings turned black, and there’s nothing happening after three weeks. The cuttings are about 5 inches long, and I trimmed all leaves except the top few. Any ideas how I can do it correctly? — Rick H.

A: Although rooting geraniums in water can be successful, the cuttings easily rot. I've found it's much easier and more successful to root cuttings in perlite, vermiculite, well-drained potting mix or a mixture of sand and peat moss.

The ideal geranium cutting is about 3 inches, taken from the tips of geranium branches. Remove blossoms and buds and snap off leaves from the lower part, leaving only the top two or three leaves.

Sterilized cell packs or small pots make nice containers. Fill with the pre-moistened mix you’ve selected, and insert the cuttings about 1 inch, and firm the mix around the cutting’s base so it doesn’t wiggle, and then water gently.

Geranium cuttings will rot if kept too wet. When the surface of the media begins to dry, sprinkle lightly, but don’t saturate the mix. Keep in a sunny window or under fluorescent or LED lights. Rooting takes two to three weeks.

Although late August is my favorite time to take cuttings from our outdoor geraniums, they can be successfully rooted any time of year.

Q: I’ve been reading articles about the possibility of growing onions by planting seed directly into the garden. Everything I read says they need to be started indoors, or by other methods. Is there a reason they can’t be planted from seed? — Anne G.

A: Onions will grow largest if given a head start. Some gardeners plant onion “sets,” which are small, dry bulblets sold at garden centers, and they produce nice-sized onions that store well.

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The very largest onions, though, are grown from onion plants transplanted into the garden, either from seeds started early indoors in February, or from plants sold packaged in bunches from garden centers.

Onions can be seeded directly into the garden, but the bulbs won’t get as large by the season’s end. They do make great “green” onions, though, throughout the growing season, and they’ll produce small- to medium-size bulbs by fall.

If you have a gardening or lawn care question, email Don Kinzler, NDSU Extension-Cass County, at donald.kinzler@ndsu.edu. Questions with broad appeal may be published, so please include your name, city and state for appropriate advice.

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