The Heuchera, Heucherella and Tiarella are all genuses from the Saxifragaceae family. The Heuchera and Tiarella are native to North America. They all have flowers that attract bees and butterflies and slugs, snails and deer leave them alone.
The flower stalks are 2-3 times as high as the leaves. They are all very hardy and can withstand -31 ºF (-35 ºC). They prefer protection against afternoon sun.
The common name for Heuchera is coral bells because of the small flowers that resemble tiny church bells. Another common name for Heuchera is alumroot because of the alum-like properties of the roots which are used by Native Americans as a wound treatment. Most Heuchera naturally grow in relatively dry, rocky conditions, often in the cracks and crevices of rock faces.
Heucheras are very popular. There are many hybrids and cultivars and the number keeps growing. Their popularity is so high because of their colorful leaves that vary per species, cultivar and hybrid. they also often have beautiful flowers that attract bees and butterflies, they are drought tolerant and grow well in the shade. In short a Heuchera is low-maintenance and have multi-season interest. On top of all that deer, slugs and snails leave them alone most of the time.
I think a Heuchera is not very suitable as groundcover. They are clump forming but do not spread very much so they do not cover the ground. On top of that the stems will become woody and the clump falls open. The plant has to be dug up and divided then.
In dry, shady areas where other plants struggle, Heuchera does not. Normally Heucheras need well-drained and somewhat moist soil. Once established they tolerate dry soil, but they generally do not grow quite as large.
Nowadays there are varieties that may prefer a sunny position so be sure to check before you buy one. Generally Heucheras need more moist if they are placed in a sunnier spot. Most of the times the foliage looks better in more shady conditions with 4 hours or less direct sunlight.
The foliage of the Heuchera is essentially evergreen but they do not look very appealing in the winter. It depends on the severity of the winter for a large part. If it turns very cold the leaves will die back completely. In spring new leaves will emerge replacing the old ones and often a second flush in late summer.
A small downside to Heuchera is that it is short-lived unless it is divided every 3-4 years as the crown becomes woody. You can also dig it up and place it deeper into the ground, burying the woody stems. The best time to do this is in the early spring.
Heucherella is the result of a cross between Heuchera and Tiarella. Hence the name Heucherella. Their common name is foamy bells. Heucherellas do not exist in the wild.
The leaves and flowers have a mix of characteristics from both genuses. The characteristics depend on the characteristics of the parents which makes them very divers. New cultivars are introduced constantly.
Heucherellas may have more of the trailing characteristic of the Tiarella. This is why they may spread better than Heucheras which may make them better groundcover but not very much. Often the Heucherella blooms more abundantly than the Heuchera. The flowers attract butterflies and bees and are sterile.
The Heucherella likes well-drained soil. The Heucherella is more moist tolerant than the Heuchera. Once established the Heucherella is quite drought tolerant. Heucherellas are essentially shade groundcovers. Nowadays there are varieties that may prefer a sunny position so be sure to check before you buy one.
It is best to divide the Heucherella every 3-4 years to maintain vigor.
The Heucherella is evergreen in mild winter climates.
The Tiarella is commonly named foam flower. Like with the Heuchera and Heucherella new cultivars of the Tiarella are constantly introduced with a variety of leaf colors. The Tiarella grows in deciduous woodlands and mountain terrains in the wild.
The genus is native to eastern North America: Tiarella cordifolia and Tiarella trifoliata. There is also one native to East China and the Himalayas: Tiarella polyphylla.
There are many synonyms used but none are officially accepted: Tiarella cordifolia var. cordifolia, T. cordifolia var. collina (also Tiarella wherryi), T. cordifolia var. austrina and T. macrophylla.
There are varieties of the Tiarella cordifolia (heart-leaved foamflower) that spread by runners. Those runners produce plant offsets that take root and bloom in their second year. If the runners grow too far you can cut them back. If the plant becomes too uneven you can cut it back to about 2-4 inches (5-10 cm) from the ground.
There are also varieties of the Tiarella cordifolia that are clump forming and can be propagated by dividing the clumps in the spring or autumn.
The Tiarella polyphylla can also be propagated by division in the spring or autumn.
The Tiarella thrives in moist, humus-rich, well-drained soil and in full to partial shade. The Tiarella prefers a moistier habitat than the Heuchera. This perennial is evergreen in mild winter climates. The Tiarella can remain undivided for years without losing vigor. This makes the Tiarella suitable as groundcover in my opinion.
The Tiarella can produce seed, but you will need more than one species or cultivar. The plants are generally self-sterile.