Ask the Master Gardener: Err on the side of caution when fertilizing your lawn

The most important time to fertilize is in mid to late September. That feeding replenishes depleted nutrients and gives the grass plants time to take up and store the food.

Stock image of a lawn being fertilized.
Let the grass grow some before fertilizing in late May to early June. Light raking and at least two mowings first is a good rule to follow. The most important time to fertilize is in mid to late September.
Contributed / Shutterstock

Dear Master Gardener: Is it time to fertilize?

Answer: It depends. Since most of us won’t bother to get a soil test that would tell exactly how much fertilizer is needed, it’s best to err on the side of not over-fertilizing. The U of M used to recommend four applications of lawn fertilizer, but that is way too much for most yards. If you are leaving the clippings on the lawn (the best practice) then the nutrients in them are returned to the growing grass plants. The clippings also help shade the roots, which conserves water, too.

So the current recommendation is to do a late spring application of a nitrogen fertilizer in late May to mid-June. Let the grass grow some before feeding — it has to be able to metabolize the nutrients when they show up. Light raking and at least two mowings first is a good rule to follow. Read the package directions carefully about how much to apply and water it in well. Sweep up any that is on hard surfaces so it doesn’t run off into the storm sewer or lake.

The most important time to fertilize is in mid to late September. That feeding replenishes depleted nutrients and gives the grass plants time to take up and store the food so the following spring gets off to a strong start.

One additional note about combination products. Fertilizer is sometimes combined with either a pre-emergent or a weed killer. If your turf is healthy and crabgrass and weeds have not been a major problem, save your money and avoid unnecessary chemicals and don’t use a weed preventer product. Timing is so critical with these — crabgrass doesn’t germinate until the soil is 55 degrees (when the lilacs are in bloom) so applying too soon means the chemical film they create may have time to develop “holes” where weed seeds can then sprout. Applying too late is a complete waste of time — once the weeds are growing pre-emergents have no effect. On the flip side, putting weed killers on the turf before the weeds are actively growing is pointless. These chemicals have to be absorbed by the leaves of the bad guys, so you need to be able to see them in order to treat them effectively. In many cases, spot treatment, either digging by hand or spraying with a broadleaf killer, will get rid of the offenders. Just make sure you don’t grab a product by mistake that kills everything including the grass!


Dear Master Gardener: I’m told I need to thin and pinch my plants. What does that mean and how do I do it?

Answer: When you plant seeds, chances are you planted too many too close together. Read the back of your seed packet where it will tell you how far apart the mature plants need to be so they don’t fight each other for nutrients, water and sunshine. When your sprouts have their second set of leaves, start thinning by plucking some of them out. If the best sprout is surrounded by extras, use a pointed snips or even old manicure scissors to nip off the extra plants so the roots of the good one aren’t disturbed.

Pinching is done to make sturdier, bushier plants. Hormones at the growth tips tell a branch to keep growing. If you pinch off the tip, using your pointed snips or your thumb and forefinger, the plant gets the message to start new growth points which pop out where leaves are attached to the stem. You’ll get more branches which will yield more flowers or tomatoes, peppers, etc. Houseplants usually need pinching, too, to keep them in check, and the pinched off piece can often be rooted to start a new plant.

Dear Master Gardener: I’m starting a new perennial garden. What’s your favorite mulch to put on top of landscape fabric?

Answer: Uh-oh. I always teach that landscape fabric is the second worst thing ever invented for

home gardeners. The rock mulch you put on top is number one!

Time for a little lesson in soil health. People think of dirt as just finely ground rocks, but in reality soil is a vibrant biome of countless living species, all of which need some combination of nutrients, water, and oxygen. Anything that blocks one or more of these requirements is going to change the quality of the environment in which you are trying to grow plants. Not having healthy microorganisms that interact with the soil and roots of your plants will ultimately affect the health and quality of those plants.

Black plastic obviously prevents water and air exchanges. Landscape fabric is marketed as being porous so that rain and oxygen can get through. That may be true on day one, but in very short order the little holes get plugged with dust and debris so that it becomes mostly impermeable. However, weed seeds are more than capable of sprouting in these small amounts of organic material and their hairlike roots can grow through the holes, so eventually you have firmly rooted weeds growing on top. Persistent weeds that already had extensive root systems underneath the fabric layer will work their way toward the edges or the planting holes to find light and water. Your good plants will send roots upward searching for air and water. Now you have a mass of tangled roots underneath the fabric. I dare you to ask the master gardener who spent all of last summer removing landscape fabric from an old garden bed at the arboretum what she thinks of the product! Just don’t use anything between your soil and mulch.


As for mulch, stick to natural products. Rock mulch will not remain weed-free as debris collects and breaks down, plus it gets very hot around your plants on a sunny summer day. The dyed wood mulches, in sometimes odd colors, are often made of ground up shipping pallets so who knows what they may have absorbed. Pine or cedar bark are naturally waxy so they repel water. Cocoa bean hulls smell wonderful, but tend to mold and are big trouble for any dogs that eat them. Wild rice hulls work well, but may form a waterproof mat that needs to be broken up periodically. Pine needles are a great natural mulch which knit together to stay in place. They are not acidic, last several years, allow air and water to penetrate, and are free if you have pine trees nearby. The best mulch is at least four inches of coarse ground fresh arborist wood chips. They allow air and water to penetrate, any leaves and twigs break down quickly supplying nitrogen, and the mulch itself provides nutrients to the soil as it slowly breaks down. They are easily pulled aside to dig planting holes or to remove plants that need to be divided or replaced.

You may get your garden questions answered by calling the new Master Gardener Help Line at 218-824-1068 and leaving a message. A Master Gardener will return your call. Or, emailing me at and I will answer you in the column if space allows.

University of Minnesota Extension Master Gardeners are trained and certified volunteers for the University of Minnesota Extension. Information given in this column is based on university research.

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