When can I start putting a pre-emergent herbicide such as Preen on my garden beds?

There are two types of chemical weed controls: post-emergent and pre-emergent. Post-emergent herbicide kills weeds that are actively growing. A pre-emergent (Preen being one of these) prevents weed seeds from germinating, but does not kill existing weeds.

Pre-emergents work in the top 1 to 2 inches of the soil by creating a chemical barrier that prevents cell division in developing weed seeds. Different weeds germinate at different times of the year, and pre-emergents last anywhere from 3 to 6 months, depending on the brand you use and local conditions. The rate at which pre-emergents biodegrade in the soil and lose their effectiveness will depend on temperature, soil type, and moisture conditions of the soil. They will need to be reapplied more often in hot, humid conditions.

Depending on the brand, some pre-emergents are applied over the top of mulch, and some are applied to the soil before mulch is applied. To be effective for the spring season, apply pre-emergent in late winter or early spring, before the soil warms up.

While pre-emergents will not harm your plants if they come in contact with them for a short period of time, always wash or brush off stray granules from plant foliage to minimize damage or discoloration that could be caused by long-term direct contact with the chemical. Never use a pre-emergent herbicide in your vegetable garden, and be careful around areas where you intend to sow grass seed. If you spill a little in an area where you intend to plant grass, it will not grow.

The October 2006 “Tree Tips” column about oak tree problems discussed anthracnose and that a spring spray would help control that disease. What kind of spray would be best for my 2 very tall trees?

Since your trees are “very tall,” it is likely you cannot reach them while standing on the ground to prune or to spray. As such you need to call a professional. Also, the equipment used to help prevent anthracnose diseases in oaks requires the power of commercial grade hydraulic sprayers to propel the protectant.

Call a certified arborist who can examine your oak trees for problems, make an accurate diagnosis, and determine if they are in need of a preventive spring spray. A reputable arborist will be licensed by the state not only in tree knowledge but also in chemical application. They have the proper equipment and protective gear that are necessary when dealing with these products. They know the correct mixture and application regime to accomplish what is needed.

Is there an herb, or another low-growing plant, that I can plant to choke out mint and another unidentified spreading weed?

Before you can plant another low-growing plant to do battle with your mint and the ubiquitous “USW” (unidentified spreading weed), you need to weed out, with a spading fork and cultivator, the existing mint and weed root systems. Mint not only spreads through seed dispersal but also by sending out underground stems into the surrounding area, popping up a new plant where you least expect it. You simply can’t send in a new contender to the garden bed and hope it wins if the mint and USW are that well-established.

Your mission this season, if you decide to accept it, is to clean the invaded area of the noxious mint and USW, finding and removing as much of the roots as possible. If you had not done so last year, you might want to shear the mint carefully and dispose of spent flower heads. This will help prevent latent seeds from accidentally landing in the cleaned area as you remove the bulk of the plant.

A vigorous herb and low-growing plant to fill in the area would be any of the thyme varieties (Thymus). There are some wonderful lemon-scented and walkable cultivars now available. Depending on the size of the area, you could plant several varieties together, enjoying what each has to offer.

Since the mint is growing too well, the site is probably full sun and well-drained. Thyme loves those conditions as well. It will rot quickly in wet areas and suffer meltdown in hot, humid summers.

The next phase of your mission is vigilance. If you want the new pockets of thyme to thrive, you need to be on the lookout for mint and USW seedlings, pulling or cultivating them out before they can take hold and put down a root network. Look around your property, as well as your neighbors’ borders, for breeder plants that could be supplying seed for wind and animal dispersal. Taking a zero-tolerance approach this season to the former mint and USW area will give the thyme varieties a chance to establish themselves. They will cover the ground thoroughly, providing fragrance and flavor for your kitchen as well.