A basal leaf is one that grows from the lowest part of the stem. Basal, in general, refers to the base of a structure. Plants with basal growth often grow in what is called "rosettes", meaning the leaves form a circle at the base of the stem, all growing to a similar length, and somewhat resembling the petals of a rose.
Why Do Some Plants Grow Basal Leaves in Rosettes?
There are several reasons for this occurrence.
The cluster of low sitting leaves can serve as protection for the roots of the plant, when the top of the plant dies back in winter.
Plants like lettuce and many succulents may grow in rosettes to conserve water. When they are ready to go to seed, the stem elongates, or bolts, and sets flowers and seeds. In this case, the original plant usually dies.
Many weeds, like the common dandelion and plantain, are protected by their basal rosettes because the leaves make it harder to pull the plant from the ground. The leaves give way, long before the root pulls lose.
What Plants Grow Basal Leaf Rosettes?
Some perennial plants form only temporary basal rosettes, for one reason or another. They eventually send up a stem with additional leaves and the basal rosette may disappear completely. These include: harebell (Campanula rotundifolia), oxeye daisy (Chrysanthemum leucanthemum), whorled stonecrop (Sedum ternatum), and Yarrow (Achillea millefolium),
However some plants, like the English daisy (Bellis perennis), maintain their basal rosette shape through their entire life cycle. These are called "perpetual rosette plants". The flower stems simply grow above the rosette.
Then there are perennial plants that send up new basal foliage once the older foliage starts to look tired and worn.
This new growth forms a rosette, such as the new foliage that emerges at the base of lungwort (Pulmanaria) plants, once the flowers begin to fade. When this happens, the older, fading leaves should be trimmed off, leaving just the basal rosette and allowing the plant to concentrate on storing its energy and food, while it prepares for next season. Other examples of perennials that send up new basal growth later in the growing season are: coral bells (Heuchera), Jacob's ladder (Polemonium), true geraniums, and dandelions.
Annual plants can also grow as rosettes. Many of them are weeds, like the dandelions and plantain mentioned earlier, but there are also some annual garden plants including: English daisy (Bellis perennis), fleabane (Erigeron annuus), and viper's bugloss (Echium vulgare).
Many rosette forming plants are biennials, which makes sense since biennial plants spend their first year soaking in nutrients and storing energy. They don't need long stems and lots of foliage competing with their roots. So it's not uncommon for biennials, such as foxgloves (Digitalis), to grow as basal rosettes their first year, conserving their energy, and then sending up a regular flowering stem in their second year of growth.
Although there is a lot of variety among plants that form basal rosettes, plants that maintain their rosettes tend to look tidy all season, since their resources are so focused in their minimal growth.