What to do in your Southern California garden in April
Replace fading cool-season annuals with heat lovers such as celosia, dahlias, marigolds, petunias, salvia, verbena, and vinca. Try starting cosmos, sunflower, and zinnia from seed, even if you're a novice gardener. They're all super-easy, make good cut flowers, and attract the beneficial insects you want in the garden.
Planting perennials in your rose beds adds complementary textures, forms, and colors, and provides interest when roses are not in bloom. According to Wen Wang, rosarian at Descanso Gardens in Flintridge, good choices include catmint, cranesbill, feverfew, French lavender, lamb's ear, Shasta daisies, snow-in-summer, and veronica. We also like bearded iris, scented geraniums, and 'Indigo Spires' salvia.
Container-grown roses are in full bloom and in plentiful supply at nurseries this month. Three outstanding options ― all floribundas ― are worth seeking out: 'Julia Child', a delicious butter yellow; smoky purple, clove-scented 'Ebb Tide'; and 'Tuscan Sun', a deep apricot blend.
Coastal gardeners (in Sunset climate zones 21-24) can continue to plant quick-maturing, cool-season crops, including chard, leaf lettuces, radishes, and spinach. Inland (zones 18-21), switch to warm-season crops such as beans, corn, cucumber, eggplant, melons, peppers, summer and winter squash, and tomatoes. In the high desert (zone 11), wait a few more weeks; frost is still a possibility.
Lure bees to pollinate your fruits and veggies. The following bee magnets need only moderate water: Agastache, ‘Mönch’ aster, catmint, germander, lavender, rudbeckia, and Salvia chamaedryoides. See nature.berkeley.edu/urbanbeegardens for more choices.
Cosmos, sunflowers, and zinnias are quintessential summer flowers ― neither fussy nor thirsty ― and are great if you’re new to growing seeds. They also draw bees and beneficial insects. Other nonthirsty annuals include celosia, marigold, portulaca, sanvitalia, and sweet alyssum.
Plant beans, corn, cucumbers, eggplant, lima beans, melons, peppers, squash, tomatoes, and other warm-season crops. Delay planting two to four weeks in the high desert (zone 11) where frost is still a possibility. Baker Creek Heirloom Seed Company is a great seed source for less common varieties.
If pots are too packed with bulbs, some brown and leafless, it's time to repot or divide cymbidiums. Knock the root mass out of the pot and separate it into clumps by hand or with pruning shears. Keep at least three healthy bulbs with foliage; repot those in fresh potting medium designed for orchids.
Feed trees, shrubs, groundcover, perennials, and other permanent plants. Try using a fertilizer containing iron on all plants, not just the chlorotic ones. That's Steve and Donna Brigham's practice at Buena Creek Gardens nursery in San Marcos. They use Best Super Iron (9-9-9 with 11 percent iron) to lower pH and brighten flower colors. Apply at half strength and water well after application.
Warm days and cool nights are ideal conditions for powdery mildew. Prevent this fungus by hosing off foliage in the morning several times a week to wash off spores. To treat it, spray foliage with a baking-soda formula, such as 1 tablespoon baking soda plus 1 tablespoon canola oil to a gallon of water.
Keep the aphid population in control by stripping the pests from plants by hand. Wear thin disposable rubber gloves. Or dislodge the pests from plant foliage with a strong blast of water from a hose.
Search for snails on strappy-leafed plants such as agapanthus and daylilies, then hand-harvest and dispose. Or trap by allowing them to collect on the underside of a slightly elevated board.