Why Seattle waits for May to plant tomatoes

The big picture: Warming temperatures are changing where and when different plant varieties can grow successfully, according to the U.S. Department of Agriculture Plant Hardiness Zone Map, which was updated last year.

• The new hardiness map shows that the region will likely transition to USDA Zones 8b and 9a by 2040, but for now we are still in Zones 8a and 8b.

Zoom in: Usually it's safe to plant tomatoes in mid-May in the Seattle area, but every year and each location is unique, said Grivas.

• Wait until the soil warms above 50° to tidy up and until nights are reliably above 50° to plant tomatoes, she said. Planting too soon can stall growth and encourage blossom-end rot.
• Seattle itself may be a little warmer than outlying areas due to the urban heat island effect.

Here are a few tips from Grivas to help tomatoes flourish this year:

• Select varieties with a days-to-maturity of 65–80 days for the most reliable harvest.
• Keep starts inside and and replant in a bigger pot, burying the stem so only two sets of leaves remain.
• If planting outside, plant in full sun in a hole twice as deep and wide as the existing pot. If doing so now, put the plants under frost cloth.