Apply permaculture methods when choosing companions for crabapple trees. In permaculture, certain botanicals work together as "guilds" to conserve water, ward off predatory insects and animals, choke out weeds, build soil, produce edible produce and encourage beneficial insects and birds. Many shrubs, bulbs, herbs, and flowering perennials and annuals make great understory plants for crabapple guilds. Don't despair if your crabapple tree is currently underplanted with lawn--or even weeds. Just clear, dig, plant and mulch small patches as you go, and within one or two years the new plants will have chocked out competing grass or weeds.
Plant two staggered rings of mixed beneficial bulbs around the crabapple tree, one a few feet away from the trunk and the second at the drip line (the ground under the outer edge of the mature tree's branches). Although commercial orchard growers resort to grass to suppress weeds and prevent soil erosion, in reality grass is a poor companion for fruit trees. Instead, choose flowering bulbs that suppress weeds without robbing crabapples of nutrients. Cheerful daffodils also deter deer and gophers. Members of the onion family (garlic, garlic chives, Egyptian walking onion and ramps) repel insects and provide produce. Daylilies and camas add beauty and contribute more edible food. Steam or boil daylily buds as you would green beans, toss fresh daylily petals into salads, and slow-cook camas bulbs, long an important root crop for Native Americans, in a fire pits or crock pots.
Make an informal circle of purple-flowered comfrey outside the bulb rings. Comfrey, a true multi-purpose herb with medicinal and culinary properties, can be cut to the ground several times a season, providing free, soil-enriching mulch. Other "mulch makers" to interplant with comfrey include clover, cardoon, artichokes and nasturtiums. According to permaculture expert Toby Hemenway, soil microorganisms encouraged by the diversity of mulch materials make harmful fungi like apple scab less likely to attack crabapples.
Incorporate plants that attract pest-eating birds and insects. Plant flowering bushes and perennial flowers on the outskirts of the crabapple understory where they can receive sun. Hemenway notes that many bird species eat harmful larvae and their eggs from the crabapple's bark. Attract these birds with butterfly bush, salvia and red hot poker flowers. Plant bee balm, yarrow, lavender, clover, coriander, fennel and dill to attract predatory wasps, which also eat harmful larvae.
Establish "mining plants" in the crabapple guild. This family of perennials boasts long tap roots that burrow deep into the subsoil, bring up nutrients for neighboring plants to share. Plantains, dandelions, chicory and yarrow all mine potassium and several trace elements from several feet below the topsoil.
Add nitrogen-fixing plants. The root systems of beans, clovers and lupines release nitrogen into the soil, enriching the tree's growing area. While lupines would probably win most beauty contents, planting a dozen bush beans in the first few years adds food to the family table as well as nutrition to the crabapple guild.
Finally, plant shade-tolerant fruiting shrubs and groundcovers under the protection of taller crabapple varieties. Currant, gooseberry and black raspberry shrubs appreciate protection from blistering afternoon sun, and they in turn reward the homeowner with luscious fruit. Some strawberries and lingonberry varieties also thrive in many apple or crabapple guilds.