Coral bells dieing
Thank you for your help! Our coral bells have struggled in this location. The location faces east and gets a mix of sun and shade (sun in the morning and early afternoon depending on the season). All of the coral bells were very healthy in the spring. The location gets more sun in the summer time, which is when the plants die off. The green coral bells are the only ones that have fared ok with the change in seasons. The purple and gold ones have not done well. I did use miracle grow bloom booster (the hose sprayer certain) towards the end of this spring and one of the plants died off shortly thereafter, so I stopped using it at that time. The other plants died off later in the summer. The location is on a slight incline, so I think the water should drain ok.
Montgomery County Maryland
A mix of sun and shade is good for Coral Bells (Heuchera), particularly when the shade can cool them off during midday, but they are picky about drainage and need well-draining soil to thrive; they do not do well with "wet feet" (roots often damp or poorly oxygenated). Where our locally-native species grows wild in our area includes sites that are boulder-laden with pockets of organic matter and more limited amounts of moisture than garden beds that commonly have more compacted and often clay-based soil.
If you are irrigating this bed at all, make sure the plants truly need it before watering. Feel the soil about four inches down and, if drying out at that time, soak well. Otherwise, moist soil at this depth should be sufficient for the plants to do well and watering should be skipped until later. Fungal crown rots can afflict sensitive plants which stay too damp, and mulch being used up to the crown can promote conditions which encourage infection. Instead, it's best to have the base of all plants (perennial, shrub, tree) where the growth emerges out of the ground free of mulch by several inches around to promote good air circulation. This mulch looks like one of the recycled-rubber types, which may trap too much moisture by slowing evaporation and similarly trap and radiate heat in the surface soil, inhibiting the cooling-off overnight periods which these and other plants would prefer.
As with the other plants you have inquired about, the fertilizer use sounds a bit excessive and/or unnecessary. With heavy "feeders" like roses such supplementation can be useful, but Heuchera and a number of other perennials and shrubs do not need regular nutrient inputs to thrive. Inorganic mulch like this type and stone have the advantage of not needing much replacement due to degradation, but this lack of degradation also means that they do not add the benefit of organic matter and nutrients to the soil over time. Adding compost to planting holes can help compensate for this, but eventually light doses of fertilizer may help circumvent this lack of replenishment. (For "light feeders" like Heuchera, this would be years in the future, not within a few years after planting.)
For replanting this area, you might consider alternatives that grow in sun/shade mix and what may be damper soil. Astilbe is one option; other perennial possibilities include Ligularia, River Oats grass (Chasmanthium), Sweetflag (Acorus), Solomon's-Seal (Polygonatum), and Turtlehead (Chelone); shrubs like dwarf versions of Summersweet (Clethra alnifolia) can also work well.
I feel really fortunate to be able to seek your advice on what to do with the plants struggling in our garden. Everyone's responses have been so helpful! One follow up question that I have for you is what would be the best way to amend the soil for the plants that need well drained soil? I planted them in a mix of existing soil, commercial top soil, and LeafGro.
Amendments can only help so much with drainage, so in difficult spots, sometimes building a raised bed is the easier solution. That said, organic matter is best to use (compost/LeafGro being one good option; topsoil organic content may vary but is also fine), as mixing it in helps to keep the finer clay particles from compressing as closely together, which would slow water movement and impede oxygen diffusion into the soil. This may be a helpful way of picturing it, though perhaps not the best example: imagine a jar full of particles like peppercorns, versus a jar of larger particles like marbles. Because they don't fit together as tightly, the air spaces are larger between the marbles, and water poured through (and out the bottom of the jar in this scenario) will drain away faster than it will in the jar of peppercorns. Similarly, the larger spaces between marbles holds more air than between the peppercorns. In soils, clay particles are the finest/smallest type and so allow for only tiny gaps between them; they're more easily compacted because they fit together so easily. Compost helps to bind some of these fine clay particles together, increasing the effective size of each of those little clumps, like taking the peppercorns and gluing them together so they're now the size of the marbles. Thus, they can't compress as badly and allow for better movement of air and water through the layers. When plants need good drainage, this amendment improves conditions around the roots, and also allows for easier root growth to a certain extent as the soil is less dense.
Over-use of amendment works against a gardener, though, as changing a soil to a texture that's too light and airy only serves to discourage roots from growing into the un-amended surrounding soil that they will eventually reach. It also might backfire in times of drought, making the soil too well-drained and not able to hold enough moisture. When two different soil types are placed on top of or next to each other, where they meet there can be water absorption issues - one will tend to sponge water out of the other. If this happens around a root ball, where that soil type is very different than the untouched surrounding soil, this will either create a bathtub in wet conditions (the root ball soil sucks up water from the surrounding soil) or the opposite, where the surrounding soil robs the root ball of moisture.
Therefore, it's best to only use just as much amendment as is needed to create a slightly more crumbly (rather than pottery-clay clumpy) texture to your soil, and gradually letting more organic matter work its way into the surrounding soil over time by means of mulch that breaks down or by using an annual top-dressing (before mulch is put down) of about an inch of compost. This also mimics what would be happening in nature - organic matter gets gradually but continually added to the soil with fallen leaves, twigs, and so on that decay.
It's good that you mixed soils together when planting. For the future, only compost or topsoil should be needed, though using both is probably ok if you use proportionately less of each in order to not over-apply amendment.
This is so informative. I understand soil drainage so much better now. Thank you very much!