Answer: When many different plants fail to thrive, it is often due to some type of soil problem. The problem could be due to pH imbalance, lack of fertility, or poor soil structure, or possibly even something unusual such as a chemical residue from weed killers or a hard pan under old farm land.
I would suggest you begin by running some basic soil tests and see what the results are. Your UMASS extension should be able to help you with that and with interpreting the results. They may also be able to determine the cause of the lack of growth based on either the soil tests or by examining samples from some of the plants.
I would suggest that you review the soil preparation process to determine what was added (or omitted) at planting time and also review how the plants were set out. Sometimes plant roots will fail to extend beyond the original planting hole and this will cause them to fail to thrive much as you are describing.
A common cause of failure is poor soil preparation in that the soil is so inhospitable that the roots can't grow through it. Your soil tests should indicate any amendments that need to be added (such as lime or fertilizer) and the addition of copious amounts of organic matter should also help correct any structural problems. Note that excess fertilizer can actually burn roots, this occasionally happens in a new planting.
Plants may fail to root for a number of reasons, but another common one is that they were potbound (the roots were very crowded in the pot) and have now continued to grow in that limited soil area rather than reach out into the native soil. While it is certainly preferable to purchase plants that are not rootbound, it can sometimes be counteracted by untangling or possibly gently cutting some of the roots and directing them outward into the native soil.
Yet another problem can be watering incorrectly. It is important to water so that the moisture soaks down deep into the soil rather than just sprinkling the surface. Deep watering encourages deep rooting and the plants are healthier due to the more extensive root system and subsequently are less subject to moisture stress. Using several inches of organic mulch will help keep the soil evenly moist. It will also feed the soil as it breaks down, hold down weeds and help moderate the soil temperature.
Make sure your watering is effective by digging down to see how far it has penentrated. It is best to water deeply less often than a daily sprinkling. Most plants prefer a soil that is evenly moist but not sopping wet. One other possible problem is using water that has gone through a salt-based water softener system. This will stunt or kill plants.
I hope this helps you trouble shoot.
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