May marks the transition from cool spring planting toward the warmth necessary for summer harvests in Oregon. This month requires caution, as the potential for late frost lingers while the soil begins to warm enough for tender crops. Successfully navigating May requires understanding local microclimates and knowing which plants should be started from seed versus moved as transplants. The goal is to safely establish warm-weather favorites while maximizing the final yields of cool-weather vegetables.
Understanding Oregon’s Diverse Planting Zones in May
What to plant in May depends fundamentally on the gardener’s location within Oregon, as the state features varied climate zones. The most reliable indicator for planting success is the local average last frost date and the soil temperature, not the calendar date. Monitoring the soil temperature with a thermometer is important, as seeds and transplants rely on thermal conditions for successful germination and establishment.
Gardeners in the Willamette Valley and Western Oregon lowlands generally see their last frost risk by mid-May, allowing for earlier planting of warm-season crops. Even within this region, elevation and proximity to water influence planting safety. For heat-sensitive plants, the soil temperature should consistently reach 60°F, with 65°F to 70°F being ideal for crops like tomatoes.
Along the Oregon Coast, the cool, maritime influence slows soil warming compared to inland areas. Coastal gardeners often wait longer for transplants to avoid cold, damp soil. Conversely, the High Desert and higher elevations of Eastern Oregon face the latest last frost dates, extending into late May or early June. In these regions, planting of tender crops should be delayed or heavily protected.
Seeds to Direct Sow Outdoors
May is the prime time for direct sowing numerous warm-weather crops that thrive when planted directly into the garden soil. These plants establish robust root systems quickly when they germinate in place, often outperforming transplants of the same varieties. The most common warm-weather direct sows include bush and pole beans, which should be planted after the soil has warmed to at least 60°F to prevent seed rot.
Crops demanding warm soil conditions for successful germination and growth include:
- Cucumbers
- Summer squash
- Zucchini
- Pumpkins
For gardeners in cooler microclimates, delaying the sowing of these cucurbits until the last week of May ensures the ground is sufficiently warm. Sweet corn is another staple that needs warm soil, and it is best planted in blocks rather than single rows to ensure adequate wind pollination.
Gardeners can continue succession planting of cool-season crops that tolerate the increasing heat of late spring. Sowing small batches of carrots, beets, and radishes every two to three weeks ensures a continuous supply before the peak summer heat arrives. Leaf lettuces and Swiss chard can also be planted now, especially if given partial shade to delay bolting. When direct sowing, seeds should be planted at a depth roughly two to three times their diameter and kept consistently moist until germination occurs.
Transplants Ready for the May Garden
May is the moment for moving warm-weather transplants, which were started indoors to gain a head start on the relatively short Oregon growing season, into their permanent outdoor homes. Tomatoes, peppers, and eggplant are the most notable tender crops, as they cannot tolerate frost and require soil consistently above 60°F. Melons and basil also fall into this category and should be transplanted only when the risk of a temperature dip is completely gone.
A process known as hardening off is a necessary step for all indoor-started transplants before they are set into the ground. This involves gradually acclimating the young plants to outdoor conditions like direct sunlight, wind, and cooler nighttime temperatures over a period of seven to ten days. Failure to properly harden off transplants can result in sunscald, leaf damage, and stunted growth.
When transplanting, the root ball should be handled gently and placed into the prepared garden bed. Ensure the plant is set at the same depth as it was in its nursery pot, except for tomatoes, which benefit from being planted deeper. May is also the final window for planting any remaining cool-weather starts like broccoli, cabbage, and Brussels sprouts for a spring harvest. These plants are resilient but will quickly bolt and become unproductive once the intense summer heat sets in.