The areas under trees are usually filled with tree roots. Some of those roots are near the top of the soil, which makes the place root bound. Digging up the area can injure those roots and hurt the tree. Annual flowers are ideal for putting in root bound areas because you can plant them from seed, or in tiny holes to get six-packs of seedlings. Annuals are shallow rooted, so won’t disturb the roots of their trees.
In warm-winter climates, U.S. Department of Agriculture plant hardiness zones 8 through 10, spring starts as early as December. If the place is root bound but at a sunny location, cool-season flowers work well. Think about the seasons. The place could be in colour during the summer but in full sunlight during the spring because it is under deciduous trees. Cool-season annual flowers comprise Lobelia (Lobelia inflata) which grows to from 6 to 30 inches high and is covered with small blossoms in pale blue or white. Calendula (Calendula officinalis) contains bright orange and yellow daisy-shaped blossoms from 3 to 4 inches round. The blossoms are edible. The bush grows to 18 inches high. Sweet peas (Lathyrus odoratus) typically grow from 5 to 6 feet high and also need a support for their tendrils to grasp. But there’s a dwarf variety that only grows to 18 inches and does not need a support. Other spring picks comprise stock (Matthiola incana), pansies (Viola x wittrockiana) and snapdragons (Antirrhinum majus).
Summer gives you a wide variety of blooms from amaranthus (Amaranthus) to zinnias (Zinnia elegans). When the spring blooms are fading, pull out them. Pulling up the plants will make the place a little uneven. Gently rake the area smooth using a grass rake so you don’t disturb the tree roots. Plant the seeds of this summer annuals. Amaranthus have purple, red, pink or yellowish blooms. The blossoms may form small fuzzy balls or even be tufted on upwards spikes. Zinnias have a selection of colours, all except blue; sizes from 1/4 to 4 inches in diameter and grow from 6 inches to 36 inches high. Other summer annual blossoms include cosmos (Cosmos bipinnatus), growing up to 48 inches high, and marigolds (Tagetes patula).
Trees such as oaks don’t do well with frequent watering during the summer, particularly in Mediterranean climates where oak trees have adapted to slopes that don’t get much rain. Plant drought-resistant annual flowers underneath the oaks. Love-in-a-mist (Nigella damascena), verbena (Verbena), moss rose (Portulaca grandiflora) and bachelor button (Centaurea cyanus) are all drought resistant.
Planning and Planting
Before planting, soak the area so it’s moist to a depth of 6 inches. Test it out by plunging a screwdriver into the soil. Let the area dry off a little so you’re not planting in mud, but rather damp soil. How long that takes depends on the weather. In foggy coastal areas it could take as much as a week. In dry inland locations, the soil could be prepared the next day. Scatter the seeds and then cover no over 1/2 inch of soil. The exception could be big seeds, like sweet peas; poke a hole in the soil 1 inch deep, slip the seed in the hole and then cover. Water with fertilizer that’s water soluble to soak the bed.