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The Philadelphia Inquirer Michael Mills Column: Garden Q&A

July 4, 2008

By Michael Mills, The Philadelphia Inquirer

Jul. 4–Question: What you would suggest to keep the weeds/grass out of my asparagus bed? We have had the bed for a few years and would prefer something other than constant weeding.

– Helen File Answer: What’s the main difference between asparagus and such crops as spinach, green beans, okra, corn and watermelons? Asparagus is a perennial, and the others are annuals. Those with ornamental gardens may be thinking, “Yeah, so what’s the point?”

For vegetable gardeners, the point is that with almost all their other crops the ground can be plowed in at the end of the season, or a cover crop planted and plowed in come spring. This doesn’t eliminate weeds, not by a long shot, but tough perennial ones don’t have a chance. Also, most annual vegetables need to be spaced — which allows deft hoeing to reduce weed growth. Asparagus grows in a tight patch. Which basically means hand-weeding.

But the need to weed can be reduced by a couple of techniques. In the spring, after the ground warms and weeds start sprouting (but before they are abundant), pull out as many as possible. Then use corn gluten, a completely organic pre-emergent. That noun is a technical misnomer; corn gluten and chemical pre-emergents actually kill plants after germination, but when they are so small you practically need a magnifying glass to see them.

Once the harvest is over in June, mulch the asparagus rather heavily with something that contains neither seeds nor wood. (Wood chips or commercial wood-based mulch will remove nitrogen from the soil.) Shredded tree leaves from the previous fall are good, as are grass clippings from lawns not treated with chemicals. Grass clippings actually add nitrogen.

Since grass clippings will matt and stink if too thick, apply them in stages, or allow the clippings to dry fully in the sun before using as mulch. Sprinkle more corn gluten on top of the mulch. The mulch can be left in place through the winter and into the spring — the asparagus spears will get through it.

If you have a particularly entrenched perennial weed and are willing to use chemicals, very carefully use a paintbrush to wet the weed leaves with glyphosate (the active chemical in Roundup). You may wish to skip harvesting right around that area for a year. Or sacrifice some asparagus by digging up that section of the plot to get the weed root and all without chemicals.

Q: Are there problems with using highly chlorinated water on plants?

A: I presume you mean tap water. Do not use swimming pool water on plants.

As for tap water, there is no real problem from the chlorine added by purification plants. Chlorine is very volatile. That’s why you can smell it around a swimming pool — it is volatilizing, or dispersing. Tap water left out in an uncovered bowl or tub will lose most of its chlorine overnight (and untreated swimming pools become green and/or unsafe) due to the volatility. By the time plants are taking up water delivered via hose or sprinkler, there’s not enough chlorine to cause problems.

There are two minor concerns regarding tap water in the garden. Treatment plants typically produce neutral water (7.0 on the pH scale). Over several years, use of tap water may bring soil closer to neutral. In our area, this is a concern mostly with acid-loving plants such as hollies, azaleas and blueberries. Yet another reason to have soil tested every few years.

The other concern is from fluoridation. Bartlett Tree Experts’ research lab reports that too much fluoride could cause marginal leaf scorch, “but generally this only occurs where there is limited soil volume and tap water provides most of the plant’s water needs.” Meaning, rain will compensate.

Send questions to Michael Martin Mills, The Inquirer, Box 41705, Philadelphia, Pa. 19101 or gardenqanda@earthlink.net. Please include locale. Read his recent work at http://go.philly.com/michaelmartinmills.

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