Fertilize your lawn on the schedule listed in the lawn care calendar found in the following site. https://extension.umn.edu/planting-and-growing-guides/lawn-care-calendar Your tree roots will  obtain all of the fertilizer that they need from the turf fertilizer. It is best to avoid using fertilizer stakes. Spikes are a little more expensive than granular or liquid fertilizer. Sideways fertilizer movement is limited in clay soil so stakes only distribute nutrients to a small section of the root zone, causing the root system to concentrate root growth near the spike more so than in other areas. To optimize root production, water uptake, and establishment of newly planted trees. Eliminate turf and weeds from the base of the tree out to several feet beyond the plant canopy. Leave the top of the root ball bare and start the mulch application at the outer edge of the root ball. Apply a 3-inch layer of wood chips mulch around newly planted trees in a  doughnut shaped circle that extends several feet beyond the tree canopy. The mulch will slowly release more nutrients to your tree. Newly planted tre  require more frequent watering than established trees because they do not have the  large root system to absorb water. The following site will tell you how  many years that you should continue watering your new trees. https://extension.umn.edu/planting-and-growing-guides/watering-newly-planted-trees-and-shrubs Hybrid Elms require considerably more pruning and training than other landscape trees during the first 15 years.  In the case of elms, a small investment in maintenance during the "formative years" will have a huge payoff when they are approaching maturity. Knowing when and how much to prune and maintain trees requires experience, so if you are not sure how to work on young elms, contact an experienced tree care professional to get you started right.  Arborists believe that the risk of Dutch elm exists in hybrid elms so it is best to prune when the tree is still dormant. Pruning in late winter, just before spring growth starts, leaves fresh wounds exposed for only a short length of time before new growth begins the wound sealing process.
For a step-by-step guide on how to conduct a soil test, check out this recent video, or visit the UMN Soil Test Laboratory for written instructions. It’s impossible to know how much nitrogen, phosphorus and potassium to add to your soil without first knowing what amounts are there already. Many areas of the state have excess amounts of phosphorus and potassium. The soil test will give you the type of fertilizer that your flowers need so you do not use too much phosphorus or potassium that pollute our lakes and causes algae blooms.Too much fertilizer (applied too often or too much) can burn or desiccate roots, ultimately killing the plant.  The soil test will also give you the pH of your soil.  Many areas of the state with clay have alkaline soil pH >7. There are a few flowers that need acid fertilizer and sulfur to lower the pH of their soil. Water your soil before applying fertilizer. Allow the water to soak into the ground and then water again immediately after fertilizing. Avoid washing granular fertilizer into hard surfaces like sidewalks, driveways, and streets. Slow-release fertilizers dissolve with watering and release nutrients in small doses over time. https://extension.umn.edu/find-plants/flowers#perennials-2391511 https://extension.umn.edu/yard-and-garden-news/hot-weather-fertilizer-tips-your-yard-and-garden
Mulching your flower garden will reduce the amount of weeding  you  need to do. Fallen tree leaves create a natural mulch.  They save you the expense of purchasing mulch,  enrich your soil, retain moisture and protect your plants from winter's fluctuating temperatures. Consider shredding them first with a mulching mower, shredder or leaf blower on the vacuum setting. Adding  shredded tree leaves to your flower garden in fall will slowing  improve your clay soil. Good luck with your yard.