I am buying a new house in Salt Lake County, and the entire back yard is weeds. I want to put in a garden towards the back and fill in the rest with grass. What is the best way to kill the weeds completely, without killing the new grass and affecting the garden?
Weeds are never eradicated but managed. One treatment will knock down quite a few but there are weed seeds that can stay in the soil and germinate up to 50 years (field bindweed). There are different types of weeds and have different growth patterns, some are annuals while some are perennials. Probably the least toxic and effective is using a glyphosate based weed killer ( like Roundup). The plants have to be actively growing for glyphosate to work, the plants take up the glyphosate and if they are not growing it does not get into plant tissue – so read instructions thoroughly to ensure best results. Be careful with any chemical herbicide because more is not better, proper application at the correct time is cost and time-effective.
As for putting in a lawn, here is an excerpt on site preparation to establish a good lawn.
Site Preparation Steps
Clear the site of all building materials (wood, cement, bricks, etc.), as well as any buried stumps, rocks , stones or other debris that is larger than 4-5 cm (2-3 inches) in diameter.
Rough grade the entire area to eliminate any drainage problems on the property. This would include sloping the grade away from building foundations, eliminating or reducing severe slopes and filling low-lying areas. A tractor-mounted blade and/or box are most often used for rough grading, but if the area is smaller, it can be done with hand tools. The rough grading will probably uncover more debris that should be removed and not buried.
Initial tilling, to a depth of at least 5 cm (2 inches), should be completed prior to adding any topsoil or soil amendments. This will control most annual weeds, alleviate subsoil compaction and permit a bonding of the topsoil to the subsoil and improve root penetration and water movement.
Add organic matter (compost) to achieve a total depth of 10-15 cm (4-6 inches), after firming. To the extent possible, practical, affordable and available, incorporate humus (fully decomposed organic matter) into the topsoil.
Apply “starter fertilizer”
Finish grade the entire site, maintaining the rough grading contours and slopes, with a tractor-mounted box blade on large areas or heavy-duty rake on smaller sites.
Roll the area with a lawn roller one third full of water to firm and settle the surface and reveal any low spots that should be filled to match the surrounding grade surface. If time permits, allow the area to settle further with rainfall or by applying irrigation water.
Horticulture Agent, Salt Lake County