Sustainable Gardening Blog

Jeff Gillman knows all about the hot controversies within the gardening world in this era of eco-consciousness and has this complaint: "Everyone seems to want to pass judgment." As a professor of horticulture at the University of Minnesota, he’s concerned that most gardening information is written by people who follow a particular school of thought and are "unable to see beyond their biases." So he wrote The Truth about Organic Gardening (Timber Press) to help gardeners "see beyond dogma" about organic versus synthetic gardening products, one of the primary areas of disagreement and confusion.  I say amen to that. 

Soil Enrichment and Fertilization

So let’s examine some common myths about fertilizers.  First there’s the notion that nutrients in synthetic fertilizers are different from the ones in organic fertilizers, or that synthetic fertilizers are "chemicals" and organics are not.  Not so - they’re all chemicals.  Organic ones are slower to break down and take effect, so they last longer.  They also require a larger quantity to be used and therefore cost more for the same quantity of nutrients.  Synthetics are quick and cheap and concentrated, which accounts for their popularity. 

Another common belief is that synthetic fertilizers are made from petrochemicals, but Gillman says that’s rarely the case.  Synthetics do use a lot of power in their production, but that’s usually natural gas. 

And here’s a surprise: organic fertilizers can be brought to the market through the decidedly unsustainable practice of mining.  For example, rock phosphate is mined in Florida and North Carolina and those mines do considerable damage to the land there.  (Potassium used in synthetic fertilizers is mined in the Western states, with similar deleterious effects.)  So Gillman recommends using nonmined organic fertilizers that reuse nutrients from other living sources - fertilizers like compost, bonemeal, blood meal, seaweed extracts, alfalfa meal, and fish emulsions.

Another myth about organic fertilizers is that they don’t leach into our groundwater the way synthetics do, and Gillman disagrees, saying they’re just as likely to leach into our groundwater "if they’re used in the over-aggressive way that most people fertilize their lawns." 

In working with homeowners as a gardening coach I’ve noticed the widespread assumption that everything in the garden needs regular applications of fertilizer.  Gillman knows better, though, explaining that we should focus instead on making soils more fertile so they’ll support healthy plants, and that means not just the usual N-P-K in most packaged fertilizers but also the right pH, bacteria, fungi, and organic matter.  And the best source of all of that is good old compost.  So for vegetable gardens he recommends tilling into the soil a half-inch of composted manure; just make sure the compost has been cured long enough, especially if it’s manure (otherwise it can contain high levels of human pathogens). 

But how about the rest of our plants, like shrubs, trees, and perennials?  Gillman told me in an email that they don’t even need compost, just a good organic mulch every year.  That’s been my own practice for decades and my plants seem happy enough.  Even roses will bloom without "rose food" but if you want maximum floral performance he suggests two applications of alfalfa meal per season around your roses.  One clear exception to the mulch-only rule for ornamentals is plants in pots because their nutrients are leached out by the frequent watering they need.  Gillman’s favorite fertilizers for pots are fish emulsion and the ones based on seaweed.  And of course you’re using a good potting soil, right?    

Now the plant that homeowners ask gardening experts about more than all the others is turfgrass, so I nagged Gillman for some advice about lawns.  He couldn’t resist first blaming the corporate members of Overfertilizers Anonymous for creating a national addiction to bright green monocultures of perfect lawns.  (To a real gardener, lawn care isn’t even gardening at all and I’ll be doing my best to eviscerate this national obsession in a column coming soon.)  And the answer is that Gillman’s favorite fertilizer for lawns is - surprise - corn gluten, commonly used to prevent weeds.  It’s a twofer!  And a nice segue to weeds.

Weed Control

As a group, organic herbicides are relatively safe, especially the most common one - corn gluten meal.  It’s simply part of the corn plant and is so safe it’s even found in cat food.  It works by creating a barrier between the soil and weed seeds, and Gillman calls it "extremely effective" at doing that. (Don’t be discouraged by less than perfect results the first year; it’ll be more effective in subsequent years.)  At the same time, it’s a 10-0-0 fertilizer and applied in the recommended quantity, it provides all the nitrogen your lawn needs, especially if you also leave the clippings on the lawn.  But get this - it takes 20 pounds per 1,000 square feet to do the job and that’s a lot.  It also needs to be applied at the right time to prevent the weeds - while the forsythias are blooming. Then wait at least 6 weeks before applying any grass seed, since its germination will be prevented, too, not just the crabgrass. 

Now once weeds have germinated, your options start to dwindle.  Flame-throwing them to a crisp works, though not as well as synthetic herbicides, and the method itself has its obvious dangers. Garlic and clove oils will kill the tops of weeds but not the roots (so why bother?) Undiluted vinegar will burn up the top portions of weeds but again rarely kill the roots.  Frying weeds under black plastic, a processed called solarization, works but also kills all sorts of beneficial microorganisms in your soil, so Gillman’s not a fan.

The bottom line about weed removal is:  Use your hands.  Hand-weeding is not just organic but very effective, and free.  Do it once a week or so and you’ll stay on top of them.  But the key to weed control is in prevention and that means using mulch, "without a doubt the best method of weed control" and the best choice for gardeners - as opposed to farmers trying to make a living. 

For anyone needing a larger-scale solution, Gillman concedes that synthetic weed controls are cheaper and more effective, and for guidance in choosing among them he suggests using the EIQ.  That stands for Environmental Impact Quotient and it’s a good measure of any product’s relative dangers to humans and the environment, from a low of 10 to a high of 100.  Unfortunately EIQs aren’t on pesticide labels but they can be found on the Web via Google, using that term and the name of a product.

Glyphosate (the active ingredient in Roundup) is often used to kill weeds in preparation for planting and Gillman regards it as "relatively safe" for humans and the environment.  You may be surprised, as I was, that its EIQ is only 15.3.  (Compared to, say, organic horticultural oil with its EIQ of 27.5 - because it can hurt plants and beneficial insects.)  While acknowledging that some  regard Glyphosate as too dangerous to recommend, he questions the studies that that judgment is based on.  There’s no doubt that frogs are harmed by it, though, so it can’t be used around water.  (Actually they’re harmed by the inactive ingredients in Roundup -  soaps and oils - but unfortunately, the EPA doesn’t require those inert substances to even be identified on packaging.)

The most common weedkiller for lawn is 2,4-D, with an EIQ of 15 to 20.  Here Gillman takes issue with that relatively low number because it’s been shown in highly credible studies to cause cancers.  Scotts Weed n Seed is the biggest source of 2,4-D and Gillman calls their recommended three to five applications of it each year "absolutely nuts."  In his view, having a dandelion-free lawn is simply not worth the potential danger of using 2,4-D up to five times a year, and I couldn’t agree more.  Besides, dandelions are good for pollinators and where would we be without pollinators?  Let’s hope we don’t find out.

Coming soon - my review of organic insecticides and treatments for plant disease.