In western Washington, plant sweet potato slips outdoors from late May into mid-June, once the soil has warmed. That is well after the last freeze, not on it. Frost is a poor guide here, because cold, wet spring soil holds this crop back long after the freeze danger has passed. East of the Cascades, the season runs on a different clock.
The short answer for Washington
The frost dates for western Washington come from NOAA’s 1991-2020 Climate Normals for the Seattle reference station. They mark the band where the chance of a 32°F freeze sits between 30 and 70 percent. In spring, that band runs from February 27 to March 18. In fall, it runs from November 13 to November 30. You can read the underlying numbers in the NOAA station data. They describe odds of a freeze on a given date, and any single year can land outside them.
A long, mild gap between those dates sounds like a generous season. For sweet potatoes it is misleading. The plant is tropical. It stalls when soil is cool and stops growing well in dim, cloudy weather. Late winter and early spring in the maritime Northwest are both. A slip set out in March sits in cold mud and does little.
When should slips go in the ground?
Wait until the soil feels warm to the hand at planting depth and nights are no longer chilly. In the maritime Northwest that usually means late May at the earliest, and mid-June is common. Gardeners who warm the bed ahead of time with clear or black plastic can move a week or two sooner. Start the slips indoors about six weeks before your target date, by sprouting a store-bought or saved root in water or damp sand in a warm room.
How much season does that leave?
Plenty, if you pick the right plant. Sweet potatoes are counted in days from transplanting the slip, and most varieties need a long season. A long-season variety set out in mid-June reaches that count in fall. An even longer-season type set out the same day reaches it later in fall. The first fall freeze band doesn’t begin until November 13, so the calendar itself leaves room. The limit is heat and light in September and October, when the days shorten and the nights cool quickly.
That is why the quicker varieties are the safer bet. Our guide to the best sweet potatoes to grow in Washington goes through which types size up fastest in a cool summer.
What will not work here
Planting by the frost calendar is the most common failure. A gardener who sees a last-freeze window in late February or March and sets slips out in April has put a tropical vine into cold, saturated soil. The slips sit, yellow, and often rot at the base. Sweet potatoes want warm ground. Last frost tells you only that freezing has stopped, and spring soil takes far longer to warm.
The second failure is the slowest varieties. Summer heat is mild in this part of the state. The reference station sees only 3.0 days a year reaching 90°F. That is good for lettuce and poor for a crop that sizes up on warm nights. A late type that needs 120 days of steady warmth is a gamble even when planted in mid-June. In a cool summer it finishes small, or it is still bulking up when October’s short days arrive.
Third, leaving the plants to the rain. Over the whole year the station averages 39.3 inches, but only about 3.0 inches falls across June, July and August. June gets 1.45 inches, August 0.97, and July just 0.6. The wet season comes in the other months, so a summer garden is effectively a dry-season garden. Newly set slips need regular water for the first couple of weeks. Established vines are fairly drought tolerant, but a month of July with almost no rain is a long time without a deep soak.
Soil changes how often that watering is needed. A sandy bed lets summer rain through within a day and may need water every few days. A clay bed holds it longer but sets up hard, which is bad for roots that need room to swell. A rain gauge on your own plot tells you far more than a regional average does. These are normals from one station over 1991-2020, and a slope, a city lot or a rain shadow shifts them.
Last, flat, heavy ground. Dense clay that stays wet into June cools the root zone and invites rot. Planting on raised rows or mounds drains the bed and warms it sooner. It is the one cheap fix that helps nearly every western Washington garden.
What changes across Washington
Everything above describes the mild, wet, maritime side of the state. Washington has no single answer. The USDA Plant Hardiness Zone Map (2023 edition, built on 1991-2020 normals) spans zones 4a through 9a inside the state’s borders. That range is the distance between a sheltered Puget Sound shoreline and a cold mountain valley. The zone is based on one thing, the average coldest night of the year. It says little about how warm a summer is, which is what sweet potatoes care about.
East of the Cascades the climate is high desert. The frost-free season is far shorter than the Seattle window, with colder winters and sharper cold snaps. Summer days run hotter, and the dry air means watering matters even more. Sweet potatoes can be grown there, but the dates in the western calendar do not carry over. Spring freezes come later and fall freezes arrive sooner, and a fast variety is more than a preference. It can be the whole plan.
At altitude, or in the northern valleys, shift in the same direction. Plant later, choose the earliest maturing types, and use plastic mulch and row cover to hold heat. Small, sheltered sites at the base of a south-facing wall or fence often run a few weeks warmer than open ground. Don’t guess your own zone from the state range. Look up your address on the USDA Plant Hardiness Zone Map and read your zone from there. Then ask a neighbor with a long-running vegetable garden when they set out tomatoes and peppers. Sweet potatoes follow nearly the same timing, a little later.
For a broader look at the crop in the state, see growing sweet potatoes in Washington.
Where to check this for your own county
The best local source is Washington State University Extension, the land-grant extension service for the state. Each county has an office, and many have Master Gardener volunteers who answer planting questions for free. They know the frost patterns, the soils and the microclimates in your area.
When you get there, look for two things. The first is a local vegetable planting calendar, which will list warm-season crops such as tomatoes, peppers and sweet potatoes by county or region. The second is the varieties that have been trialled near you. A variety that sizes up well in a warm Yakima Valley summer may fall short on the cooler coast, and a trial in your own county is the best evidence you can get before buying slips.
A county recommendation outranks a state one. This article is the state version: it uses a western Washington reference station and a state-wide hardiness range, so it can only point the right way. Counties differ by weeks, and so do the fields on either side of a hill. If extension staff give you a date that differs from the one here, follow theirs.
If your county has no sweet potato guidance at all, ask about tomatoes, peppers and melons. The planting dates for those warm-season crops are a good stand-in, and the staff can tell you how local gardeners have fared with slips.
Checking this against your own garden
Your own plot outranks every date above. Three observations are enough to calibrate it, and none needs a tool.
- Soil warmth by feel. Dig a hole in the morning and hold a handful. If it feels cool and clammy, wait. If it feels pleasantly warm, the bed is ready. Test several spots. A raised bed or a patch against a south wall warms first, and a low corner that stays wet will lag.
- New leaves on the slips. After planting, watch for fresh growth at the tips. New leaves mean the roots have taken hold. If the slips stay flat, or the leaves turn yellow or purplish, the ground is too cold or too wet for them.
- Vine growth by midsummer. Vigorous vines running across the bed by July point to a warm spot. Stunted vines, even with water, suggest a cool, shaded or windy site, and next year you may want to plant earlier under plastic or in a different corner.
Keep notes on the date you planted, the date the first new leaves appeared and the date you harvested. After one or two seasons you will have a calendar more accurate than any regional table.
Some gardeners also time this by the Moon: when to plant sweet potatoes by the moon, as almanac tradition rather than fact.
Common questions
Can I plant sweet potatoes in March after the last frost?
No. The last-freeze window of February 27 to March 18 only means freezing becomes unlikely. Soil stays cold and wet for weeks after that, and slips set out then tend to stall or rot. Wait until late May or June.
Will sweet potatoes finish before the first fall frost?
In western Washington, usually yes. The first freeze band begins November 13, and a long-season variety planted by mid-June is ready in fall. Slower varieties are riskier because cool fall nights and short days slow the final swelling.
Do I need to water through the summer?
Yes, especially at first. June, July and August together bring about 3.0 inches of rain, and July alone averages 0.6 inches. Water newly planted slips often, then soak deeply less often once vines spread. A rain gauge shows what your plot actually gets.
Does eastern Washington follow the same dates?
No. East of the Cascades the climate is high desert with a much shorter frost-free season, so the western dates do not apply. Choose the fastest varieties, warm the soil with plastic, and check with your county extension office for local timing.