It has been a cool spring so far, with our weather transforming into true spring warmth slowly, marked by the cherry blossoms at the University of Washington Quad and the tulips in bloom up in Mount Vernon. Officially the last frost dates and low night temperatures for our area in the Puget Sound lowlands are dwindling down. The later we get into April the better the chances are that our heat loving and tender plants we are growing or purchasing for our veggie gardens can get set out. There are still a few items we want to be most cautious of, those cold sensitive plants like peppers, eggplants, tomatoes, and especially basil. Part of the issue is the air temperature, both day and night time lows, but we also need to be vigilant of current soil temperatures which affect seed germination if planting anything in the garden directly and affect transplant success. If you live a little higher in elevation like in the eastside highlands you will want to be even more cautious.
So how do you know what to do? Assessing air temps, upcoming weather, and regional soil temperature is possible if you know where to look for information.
Understanding Frost Dates
Frost dates include a range of dates which each relate to a percentage of probability that you might hit low temperatures which can harm plants, often at night but potentially during the day. The farther into spring you get the lower the probability and especially for daytime temps. The National Gardening Association has a useful webpage with a chart on first and last frost dates to help you plan.
Plants and Soil Temperature Tolerance
All plants have climate parameters in which they thrive. Many of our vegetable garden crops are not native to the Pacific Northwest and many need warm summer temps to thrive. But not all of them! We tend to group what we plant in the veggie garden into cool and warm weather crops. Some of them can straddle multiple seasons but there are some general guidelines that can help you plan. One really useful tool is the Tilth Alliance Maritime Northwest Garden Guide. This month by month guide advises on what can be planted by seed directly, indoors to transplant later, or be planted out under frost protection earlier to stretch the growing season. There are also some great special articles that can help you take the best care of your plants.
A good general list for spring planting (February – April, early May) directly into a garden whether by seed or transplant can include:
- Brassicas – broccoli, cabbage, kale, radish, mustard, collards, and numerous other cole crops that thrive in cooler soil and in some cases winter over for early next season spring harvests.
- Peas – snap, snow and shelling type peas all do well planted early in the spring. You can also do a fall sowing of snow and snap peas that will net an earlier harvest in the following spring.
- Lettuce and Other Greens- Lettuce seed can germinate in soil temperatures as low as 35 degrees F! It will do better when the soil warms into the 50 degree range and if you start them indoors in pots and then set them out in March and April. Swiss chard and spinach can be seeded directly or set out as starts as can radicchio and other chicories like endive. Protect these tender starts from voracious slugs which also thrive in cool and moist conditions. Check out some advice on how to manage slugs at the Northwest Center for Alternatives to Pesticides – NCAP.
- Root Crops – Carrots appreciate a little warmer soil temperatures to germinate well so waiting until April will benefit you, but you can also throw a blanket of row cover over the bed to warm the soil and encourage them to sprout if you want to get them in earlier. Beets are pretty tolerant of cooler soil and air temps, as are parnsips and turnips. Leaving the row cover on after they germinate will prevent adult flying pests from getting to them to lay eggs – like carrot rust fly, leaf miner fly, or imported cabbage worm butterfly.
- Herbs – Cilantro is very cool weather tolerant; you can seed it directly into nooks and crannies among your other veggies where it will provide an abundant set of flowers to attract pollinators if you let some of it bloom. Italian parsley does well from seed but try curly varieties from starts anytime from March on. Try seeding dill directly in April and May. Seeds in the carrot family, like dill and parsley, take a couple of weeks to sprout so be patient! Woody evergreen herbs like lavender, sage, rosemary, thyme, and oregano can go in the garden anytime of year as long as it is not too hot or the ground is frozen but buy them as plants or start them in the winter indoors to grow to size to plant out. Chives and chamomile are also cool tolerant and chives will grow as perennials in your garden for years to come, dying down in the winter and then bursting out with fresh tasty leaves in the spring followed by pretty purple flowers. The flowers are also edible!
- Potatoes – this tomato family member is the one that needs to be spring planted for best crop success. Try using fabric planting bags filled 1/4 full of soil and plant your seed potato into the soil and roll the bag sides down to get some light into the growing area. When the sprouts emerge add a little more soil, taking care to not completely bury the leaves, let grow some more, add some soil, roll the sides of the bag up, and continue this process until the bag is full of soil and the tops are all growing up out of the top of the bag.
A good general list for late spring and summer planting (May and June) directly into a garden whether by seed or transplant can include:
- Tomato and related Solanaceae family – pepper, tomatillo, eggplant, ground cherry, Wonderberry, flowers like nicotiana and petunia (these flowers are not edible), are examples of this family of plants. These plants need warmer soil, 60 degree at minimum, to thrive in the garden. You can set things out earlier with protection and measures to warm up soil and keep night temperatures from harming them. Try a hoop house or build mini greenhouses around the plants until summer sets in.
- Squash family – pumpkin, winter squash, summer squash like zucchini, cucumber, melons, bottle gourds, winter melon, and bitter melon, are all related and need warmer soil and air temps to do well in the garden. Some of them have very long growing seasons and need consistent temperatures to produce well. Melons, bottle gourds, winter melons (wax gourds), and bitter melons especially need a long growing window and warm days through the summer. Choose the shortest season varieties you can find (look for days to harvest on the seed package or plant label). Grow them on an upright or A-frame trellis to lift them up to where the plants can get some radiant heat from the sun, mulch with dark colored mulches to warm the soil and try some cloches covered in floating row cover to keep them warmer. You can plant all these seeds directly into the soil when all danger of frost is passed except you should start those longer season starts earlier indoors.
- Beans – both dry shelling beans and fresh picked beans need warmer temps. Try direct seeding into mounds, with five to six plants per hill for pole climbing varieties and in rows about 6 inches apart for bush style beans. Build a sturdy bean tent for them to climb. There are many colors of pods and seeds to choose from with a difference in texture in taste. Try a few different kinds. Also try some long bean varieties which have a longer date to harvest window – you might want to get those started indoors to set out when you direct seed your other beans. Protect from slugs when they are just sprouting.
- Corn – local lore says that in the PNW we need corn to be “knee high by the fourth of July” in order to have a successful crop. Corn is a grass family plant and grows rapidly once germinated and especially if you supplement with a nitrogen product like fish fertilizer. Warm the soil with black weed cloth that lets water through ahead of direct seeding, or start plants a few weeks before you want to set them out. Group closely together for best pollination – these are wind pollinated crops. When you see tassels appear at the tops of the corn stalks you are seeing the male flower parts – you will find the female parts, or silks, on the tips of the cobs themselves. The pollen from the tassel falls or is blown into the silks. When two varieties of corn are tasseling at the same time there is potential for cross pollination and the corn cob that develops will be a mix of genetic characteristics of both varieties on this season’s corn. This can make the corn less tasty than what you are aiming for. Plant varieties close together that do not tassel at the same time or bag and hand pollinate. If you have lots of land you can grow them at a distance from each other but that might mean as far apart as 150 feet or more.
- Herbs – Basil, shiso, lemongrass, plus all the cool season herbs can be planted out in late May through June. These three herbs in particular need warmer nights and thrive during the heat of the summer. Keeping a floating row cover hoop house over them with the ends open for air circulation can warm them even more.
To track soil temperature in your area try visiting the Washington State University’s AgWeatherNet page or invest in a soil or compost thermometer and check your soil directly.
You can find floating row cover at local garden centers, nurseries, hardware stores and use it to warm soil and build summer cloches. This fabric also keeps our flying pests at bay, preventing them from laying eggs and causing damage from their larval stages. This is especially useful for brassicas, carrots, and the beet family plants like chard, spinach, and beets.
Our soil temperatures as of this posting are in the mid fifties so many things can be planted but just be a little more patient for those warm season crops needing warmer soil temps. We are not there yet, but soon will be.
