Q. Please tell me if this is not the correct way to send you a question, your blog is great.  We are still trying to move out from New York, the house we own out there has a good size back yard, and with tenants in the house until we can move, the weeds are not usually pulled, so although I hate to use pesticides I think we need to.             When should we use pre-emergent and post emergent pesticides for the greatest effect?  We want to keep the weeds down, but don’t want to poison the trees or shrubs (or the ground) any more than absolutely necessary.  thanks very much. A. Yes, you can reach me at [email protected] or through my blog or newsletter. Herbicide damage to a lawn. Herbicides were applied to the shrub area and water moved the weed killer into the grass where it was not supposed to go. Pre emergent herbicides are applied to the surface of the soil and watered in lightly. They kill germinating weed seeds by putting down a barrier poisonous to many different weeds that germinate from seeds.             The timing of this is usually in the spring, with applications scheduled about the first of February for many weeds. The barrier must be in place and active when the seeds germinate. The soil is cleaned up of any existing weeds and the pre emergent weed killer is applied. Once the barrier is put down the soil should not be disturbed or you will disturb this barrier.             There are several weed killers to pick from in the nursery or garden store. Read the label to find out how to use it best and any potential dangers. Wood mulch applied to fruit trees to reduce the weed problem             Post emergent herbicides are applied to weeds when they are present. These are chemicals like Roundup that are nonselective and so will kill many different types of weeds but not all weeds. Round up will kill or damage anything that is green and sprayed. Direct it on to the weeds, not plants you value.             Timing is critical and these weed killers work best when weeds are actively growing and not during times of stress like from heat or drought.             One very effective method of weed control are mulches. This is a thick layer of wood mulch or rock mulch that covers the soil surface and helps prevent weed seeds from germinating. Post emergent weed killers are then used to kill weeds that emerge from the mulches.