Follow these common words of bad advice and watch your landscape take a dive.  Do the opposite, and your yard and garden will thrive.

9 Ways to Ruin Your Garden:

1. You can never use too many wood ashes; after all, they're all-natural.  Au contraire, my wood-burning friend.  Wood ashes are a rich soure of ptash (potassium), an essential ingredient in garden soils.  They also contain some calcium, phosphorus, and magnesium.  Because they are alkaline with a pH of 10.0 to 12.0, they will sweeten acid soil.  But beware their overuse.  Excess potash may make your soil alkaline and burn earthworms and beneficial microorganisms.  It could take years to recover.  Apply only if a soil test shows a pH of less than 6.5. The usual application is to rake ashes in at a rate of 5 to 10 pounds per 100 square feet.  Avoid letting them sit out in the rain, as the good nutrients will leach, leaving behind the salts that increase soil pH,.  Avoid spreading ashes where you've put down seed and don't use them around acid-loving shrubs such as azaleas, rhododendrons, and blueberries.
2. Don't bother paying for a soil test when you can simply peek at your neighbor's results.  How different can two adjacent yards be?  It's risky relying on over-the-fence wisdom for second-guessing your soil needs.  Only a soil lab can tell for sure what your soil pH and nutrient levels are.  Varied soil grading, different cultivation practices, and previous vegetation all affect the soil.
3. The best time to work the soil is when it's nice and soft after a food soaking rain.  Remember wedging clay in art class to make it more pliable and elastic?  That's what happens when you work or till wet clay soil.  Don't do it!  It will take a year or two of freezes and thaws for the soil to return to anything resembling its natural texture.  Sorry, but adidng a little sand to clay soil only makes bricks; what you need to truly loosen it up is lots of compost, dug in when the soil is at the dry, crumbly stage.
4. For a quick effect in your garden, choose plants the nursery ahs labeled "fills in rapidly" or "easy spreader."  Beware of using invasive plants for groundcovers unless they're contained by a barrier such as a sidewalk (though such a border won't stop invasives that spread by seed).  Mint is notorious for spreading mainly by roots.  Put it in a large container, either on your patio or on pot feet--not on bare ground.  Some notorious, but commonly sold, invasive groundcovers and vines include goutweed or bishop's weed, oriental bittersweet, snow on the mountain, English ivy and Japanese honeysuckle.
5. For a neat, tidy look, use a string trimmer to keep unsightly grass from growing up around tree trunks.  Many trees die or are severely stunted from having their bark slashed by string trimmers.  Right underneath that thin layer of protective corky cells is the "cambium," the living part of the trunk through which water and all other nutrients flow.  Cut around the trunk, and you've cut off all circulation.  Garden shears are much more kind.  Or better yet, remove sod in a circle encompassing the root zone and mulch.
6. Rev up the power hedge trimmers to make a nice straight cut across the top of your hedges, and trim the sides so they're wide at the top and narrow at the base.  Don't prune hedges with the sides narrowing to the base; you'll cut off sunlight to the inside and your hedge will end up with naked legs.  Instead, make the hedge slightly wider at the bottom and either squared or rounded at the top.  Don't give forsythia and other flowering shrubs crew cuts; you're just removing the next year's blossoms. To keep their natural graceful form, thin out one-third of the oldest, fattest stems close to the ground level.  Prune spring-flowering shrubs after they've bloomed and fall-blooming shrubs in the spring.
7. Do as those "professional" landscape crews to and pile mulch volcano-style around tree trunks.  Mounding mulch against your tree trunk invites disease and can deprive the roots of water and air.  Mulch is great for keeping down weeds and letting the tree grow without competition, but when it is piled against the trunk, it invites rodents and insects to move in and take a "bark snack."  Think "doughnut," not "volcano," and never pile on mulch more than 3 inches thick.  And keep in mind that evergreen trees are self-mulching.  There's nothing sillier than raking away the needles and replacing them with purchased mulch!
8. To add color under black walnut and butternut trees, plant petunias and azaleas.  Before you plant anything beneath black walnuts or butternuts, be aware that these beautiful shade trees have evolved to produce their own herbicide.  Don't bother trying to plant under one without selecting plants that will tolerate the presence of the juglone toxin emitted by the roots.  Tolerant plants are usually shade lovers--begonias, impatiens, violets, and pansies--plus a nice list of native and garden perennials that includes Jack-in-the-pulpit, bleeding hear, coral flowers, Hosta species and hybrids, mayapple, Jacob's ladders, great Solomon's seal, spiderwort and ferns of all sorts.
9. Always choose plants that are already in bloom so you know what you're getting and can start the show right away.  Do exactly the opposite.  Avoid buying bedding plants that are already blooming.  These plants are busy making flowers and so take longer to establish a good root system.  Pinch off any existing blossoms before transplanting.  The plant can get its roots in and you'll be rewarded with twice as many flowers.  (This does not apply to single-stem plants that grow from bulbs, tubers, and rhizones.)

NOW TAKE CARE OF THAT GARDEN AND YARD!