May is when the ground begins to warm up in most regions. It is still too early to consider tender plants but perfect for planting hardy ones. Some areas of the garden may still be too wet so check the soil conditions first. Plants coming straight from the greenhouse need to acclimate. First, leave the plants outside during the day, then leave the containers outside overnight. After that, they can be planted into the garden.
Hardy Perennial Flowers
Hardy perennials grown in containers can be planted in May. There will be little foliage visible so you need to know what you are buying. This is when the scientific name can be valuable. If you end up with a different plant than the label states most nurseries will replace the plant. A huge advantage to planting in early spring is the large selection of plants. You will know which ones are hardy if the nursery has them growing outdoors in an unprotected area. Some examples of perennials to plant early are hellebore, delphinium, penstemon, salvia, bergenia, primrose, coral bells, peony and violet. The assortment will be different depending on where you live. Plants receive less damage or transplant shock when installed during their dormant state.
Bare Root Flowers
In early May you can purchase and install bare root plants. You will find plants such as bleeding heart, astilbe, calla lily, iris or daylily sold this way. You can also buy canna lily and dahlias but they are subject to late freezes. Only plant them early in milder climates. Roses are also sold bare root. They may be in a bin of sawdust or sold in a box. One advantage to bare root plants is seeing how mature the roots are. How quickly a plant establishes is directly related to root health. In most cases bare root plants are cheaper too. The plants are in their dormant state so they may take awhile to wake up once planted. Some instructions encourage you to soak the roots in water overnight. Since these plants are in a dehydrated condition, this is a good idea.
Perennial Flowering Herbs
Perennial herbs can produce attractive flowers too. These include lavender, bee balm, yarrow and coneflower. Herbs are sold in 4-inch or 1-gallon containers. Gallon sized herbs can be divided before planting. Lavenders are very hardy but do not like standing water, check for adequate drainage. French or Spanish lavenders are slightly more tender. Once the weather warms up they will grow rapidly. Make sure you allow enough space for spreading plants like bee balm. Ornamental oregano's like (Origanum herrenhausen) or (Origanum rosenkuppel) are grown specifically for their attractive blooms, as well as ornamental forms of edible sage (Salvia officinalis).
Foliage Plants that Bloom
Some plants are grown for their foliage but also produce nice flowers. New varieties of hosta such as "Venus" have large fragrant white flowers. Bear's breech (Acanthus mollis) has large bold leaves but the purple and white flower spikes are equally as attractive. Bergenia is grown for its perfectly round and even colorful leaves. Since bergenia is an early bloomer the pink, white or red flower spikes really stand out. Coral bells (Heuchera) may come in more foliage colors than any other perennial. Some of the earliest coral bells also have vivid red flower spikes. All of these flowering plants are good border or filler plants.