May Garden Calendar

OSU Master Gardeners in Newport, Oregon
Portland Rain Garden Example

Produced by OSU Extension, each month provides reminders of key garden chores, such as fertilizing, pest control, planting, and maintenance.

Recommendations in this calendar are not necessarily applicable to all areas of Oregon. For more information, contact your local Extension office.

Sustainable gardening

The Oregon State University Extension Service encourages sustainable gardening practices.

Preventive pest management is emphasized over reactive pest control. Identify and monitor problems before acting and opt for the least toxic approach that will remedy the problem. The conservation of biological control agents (predators, parasitoids) should be favored over chemical controls.

Use chemical controls only when necessary and only after thoroughly reading the pesticide label. First consider cultural, then physical and biological controls. Choose the least-toxic options (insecticidal soaps, horticultural oils, botanical insecticides, and organic and synthetic pesticides — when used judiciously).

Planning

  • Prepare and prime irrigation system for summer.

  • Use a soil thermometer to help you know when to plant vegetables. Wait until the soil is consistently above 70 degrees Farenheit to plant tomatoes, squash, melons, peppers and eggplant.

  • Place pheromone traps in apple trees to detect presence of codling moth. Plan a control program of sprays, baits, or predators when moths are found.

Maintenance and Clean Up

  • If needed, fertilize rhododendrons and azaleas with acid-type fertilizer. If established and healthy, their nutrient needs should be minimal. Remove spent blossoms.

  • When selecting new roses, choose plants labeled for resistance to diseases. Fertilize roses and control rose diseases such as mildew with a registered fungicide.

Planting/Propagation

  • Plant dahlias, gladioli, and tuberous begonias in mid-May.

  • Plant chrysanthemums for fall color.

  • Plant these vegetables (dates vary locally; check with local gardeners):

    • Oregon coast: Snap beans, broccoli, Brussels sprouts, cantaloupes, pickling cucumbers, dill, kale, parsnips, peppers, pumpkins, summer and winter squash, sweet corn, and tomatoes.
    • Western Oregon: Mid-May, transplant tomato and pepper seedlings.
    • Western Valleys, Portland, Roseburg, Medford: Snap and lima beans, Brussels sprouts, cantaloupes, slicing and pickling cucumbers, dill, eggplant, kale, peppers, pumpkins, summer and winter squash, onions, potatoes, tomatoes, and watermelon.
    • Lower elevations, eastern Oregon (dates vary widely): Snap and lima beans, beets, celery, sweet corn, slicing and pickling cucumbers, dill, kale, kohlrabi, onions, parsley, parsnips, peppers, white potatoes, pumpkins, summer and winter squash, and tomatoes.
    • Central Oregon and higher elevations of eastern Oregon: Direct seed carrots, corn (late May), chard, kohlrabi, and potatoes. Transplant Brussels sprouts, cauliflower, cucumbers (late May), leeks, or peppers.
    • Columbia and Snake River valleys, Ontario: Cantaloupes, dill, eggplant, kale, okra, peppers, sweet potatoes, tomatoes, and watermelon.

Pest Monitoring and Management

  • If an unknown plant problem occurs, contact your local Master Gardener hotline or plant clinic, for identification and future management options.

  • Manage weeds while they are small and actively growing with light cultivation or herbicides. Once the weed has gone to bud, herbicides are less effective.

  • Trap moles and gophers as new mounds appear.

  • Leafrolling worms may affect apples and blueberries. Prune off and destroy affected leaves.

  • Monitor aphids on strawberries and ornamentals. If present, control options include washing off with water, hand removal, or using registered insecticides labeled for the problem plant. Read and follow all label directions prior to using insecticides. Promoting natural enemies (predators and parasitoids that eat or kill insects) is a longer-term solution for insect control in gardens.

  • Spittle bugs may appear on ornamental plants as foam on stems. In most cases, they don't require management. If desired, wash off with water or use insecticidal soap as a contact spray. Read and follow label directions when using insecticides, including insecticidal soap.

  • Control cabbage worms in cabbage and cauliflower, 12-spotted cucumber beetles in beans and lettuce, and maggots in radishes. Control can involve hand removal, placing barrier screen over newly planted rows, or spraying or dusting with registered pesticides, labeled for use on the problem plant. Read and follow label directions when using insecticides.

  • Tiny holes in foliage and shiny, black beetles on tomato, beets, radishes, and potato indicate flea beetle attack. Treat with Neem, Bt-s, or use nematodes for larvae. Read and follow label directions when using insecticides.

  • Prevent root maggots when planting cole crops (cabbage, broccoli, collards and kale) by covering with row covers or screens, or by applying appropriate insecticides.

  • Monitor rhododendrons, azaleas, primroses and other broadleaf ornamentals for adult root weevils. Look for fresh evidence of feeding (notching at leaf edges). Try sticky trap products on plant trunks to trap adult weevils. Protect against damaging the bark by applying the sticky material on a 4-inch wide band of poly sheeting or burlap wrapped around the trunk. Mark plants now and manage with beneficial nematodes when soil temperatures are above 55 degrees Farenheit. If root weevils are a consistent problem, consider removing plants and choosing resistant varieties (PDF).

  • Control slugs with bait or traps and by removing or mowing vegetation near garden plots.

  • Monitor blueberry, raspberry, strawberry and other plants that produce soft fruits and berries for Spotted Wing Drosophila (SWD).  Learn how to monitor for SWD flies and larval infestations in fruit.

Trade-name products and services are mentioned as illustrations only. This does not mean that the Oregon State University Extension Service endorses these products and services or intends to discriminate against products and services not mentioned.