Washington State University Extension

Garden Tips

Soil Temperature RSS feed


written by
Marianne C. Ophardt
WSU Extension Faculty
for the Tri-City Herald, Kennewick, WA
published 3/7/14

It=s March, we are setting the clocks ahead this weekend, and the daytime temperatures have reached 50 degrees and above, but winter may still have a few last gasps before we can say spring has arrived and planting can start.

St. Patrick=s Day is a traditional day for some to plant potatoes and peas, but smart gardeners wisely check the soil temperature before planting their veggie seed out in the garden. If the soil is too cold, the seeds will just sit there and may rot before getting a chance to sprout and grow.

To check the soil temperature, invest in a soil thermometer. You should be able purchase one for about $10 to $15 at a local garden store or from an on-line garden supply company. Take the soil temperature in mid-morning by inserting the thermometer=s probe two inches into the soil for small seeded crops (eg. lettuce) and four inches into the soil for large seeded crops (eg. squash, beans). The probe of some of the soil thermometers have markings that indicate inches to make this easier.

Seeds of early spring cool-season crops can be planted when the soil temperature is 40 degrees or above. This includes lettuce, peas, kale, radishes, arugula, and spinach. When the soil reaches 50 degrees, plant seeds of leeks, onions, Swiss chard, and turnips. Wait until it reaches 60 degrees for planting beans, beets, broccoli, Brussel sprouts, cabbage, carrots, and cauliflower and 70 degrees for cucumbers, squash, melons, and corn. The soil temperature should be consistent for several days before deciding these optimum temperatures have been reached.

Seed potatoes are best planted when the soil temperature is 45 degrees or above and daytime temperatures are consistently in the 65 degree range and nighttime temperatures in the 55 to 65 degree range.

If you are anxious to plant your garden, you can warm the soil up a little faster by covering the garden with a sheet of clear plastic. To keep the wind from wreaking havoc with the plastic, lay it out smoothly and then pull it taut, firmly burying all the edges in trenches.

If you choose to keep the plastic in place, you can plant seeds and transplants by making holes in the plastic, but weeds will grow profusely under the plastic. In addition, the clear plastic will heat the soil to plant damaging or stressful levels during the very sunny, hot part of summer unless your garden plants are big enough to shade the plastic by then.

Clear plastic works better than black plastic for warming the soil because it allows sunlight in during day and then traps the heat that builds up, much like a greenhouse. I recommend warming the soil up with clear plastic, but removing it before planting. Gardeners also find that the soil in raised beds warms up faster and situating your garden so it receives full sun and faces south will also help.

The last average date of frost for our area is May 1. In your hurry to get your garden planted, keep in mind that tomatoes, peppers, eggplants, beans, cukes, squash, and melons will need protection if frost is in the forecast. Row cover fabrics can provide several degrees of protection.

If you haven=t done so already, plan out your garden now and purchase your seed and a soil thermometer. Spring is on its way! (I hope.)

Published: 3/7/2014 10:16 AM


written by

Marianne C. Ophardt

WSU Extension Faculty

for the Tri-City Herald, Kennewick, WA

Spring is here! What a difference some warm weather and sunny days makes. While it’s still prudent to keep an eye on the weather forecast, now is finally the time to get busy out in the garden.


In the vegetable garden, it looks like the danger of frost is past. If you have transplants you’ve been wanting to get planted, you can get started. Don’t forget about “hardening” those transplants if they’ve been sitting in the house or garage a while. Sudden exposure to full sun and wind can damage plants and delay their growth and production.

You can harden your transplants by putting them outside in a sheltered location where they’ll get two to three hours of direct sun and be protected from harsh wind. Over the next week, gradually increase the time exposed to direct sun and decrease their watering, but don’ allow them to wilt.

If you purchase your transplants from a nursery where they’ve already been exposed to sun, cool temperatures, and wind, you don’t need to harden them before planting. However, avoid purchasing older, larger transplants. They’re slower to resume growth after transplanting and end up being less productive than younger, smaller transplants. Look for healthy, medium-size, stocky, dark green plants that aren’t already flowering.

The air temperatures have finally warmed up for us, but the soil is still cool. I would wait another week or more to plant seeds for crops of melon, squash, and cucumbers. They need soil temperatures of 70 to 75 degrees for good germination.


If you haven’t already fertilized your lawn for the first time this year, now is a good time.

With warmer weather, lawn irrigation for the season has started. Check for even coverage using straight sided soup cans. Place the cans uniformly throughout the zone you’re checking, staying within a foot of the perimeter and at least two feet away from spray nozzles. Next run your regular cycle for that zone. After the cycle is complete, compare the depths of water in the cans.

If the depths vary widely, try to determine the reason. It could be poor design, but the cause could simply be an obstacle, such as a tree branch or tall ornamental grass that’s stopping the water stream, or you may need to adjust the flow of some sprinkler heads or replace worn ones. More even distribution can help you save water.


It’s always a good idea to check your drip irrigation system each spring, especially if you didn’t drain or blow out your system in the fall. You may have cracked lines or clogged emitters. Experts recommend flushing your mainline before you start your drip in the spring along with cleaning the filter to remove any dirt. After flushing, cap the system and turn the water on to check the emitters. If they aren’t operating correctly, either unclog or replace them if needed. Many of your plants on drip have probably grown. Consider if you need to add more emitters to provide them with enough water.

Drip irrigation is a great way to conserve water and to reduce weeds in your garden. Do you want to learn more about drip irrigation? A super new on-line publication is available from WSU Publications, “Drip Irrigation for the Yard and Garden” by Dr. Troy Peters, WSU Irrigation Specialist. This free publication discusses how to set up a drip irrigation system for a small yard or garden space. You can find it at:

Published: 5/14/2011 2:31 PM


written by

Marianne C. Ophardt

WSU Extension Faculty

for the Tri-City Herald, Kennewick, WA

Not everything offered for sale in a seed catalog is a seed or a plant. Many seed companies also offer a variety of nifty garden gadgets, equipment, and supplies. Here are some that I found while thumbing through the seed catalogs in my mailbox.

Compost Thermometer: Twice a year I teach a composting class where I talk about the composting process. When built correctly, a compost pile quickly heats up due to the activity of aerobic bacteria working at breaking down the organic matter. The temperature of the pile can go above 150 degrees in just a few days. If the pile gets too hot, above 170 degrees, it will kill off these valuable bacteria. A compost thermometer allows a gardener to take the internal temperature of the pile and decide when to turn it. Made out of stainless steel, the compost thermometer has a long (18-20″) stem and a dial with a range of 0 to 200-220 degrees. These typically cost about $30 to $80. You can find one for $32.50 from Territorial Seed Company (

Soil Thermometer: Also available from Territorial Seed Company is a soil thermometer, a diminutive device compared to a compost thermometer, with a 6.5 inch stem . It comes in handy when you’re starting seeds indoors or directly into the soil outdoors, since soil temperature is critical for good seed germination. Warm season vegetables tend to germinate best if the soil is 65 to 70 degrees. Gardeners are often tempted into thinking things have warmed up enough for planting because of balmy weather. Using a soil thermometer tells you if the soil is warm enough for sowing seed.

You might also want to know that you shouldn’t plant spring flowering bulbs in the fall until the soil temperature drops below 60 degrees; the roots of trees and shrubs will continue to grow until soil temperatures drop below 40 degrees in the fall; crabgrass will start to germinate when the soil temperature reaches 55 degrees for four or more days in a row; cool-season grass seed can be planted when the soil temperatures are in the 60 degree range; and seed potatoes tend to rot in cold, wet soils below 50 degrees, sprouting best in soils of 55 to 70 degrees.

Seed Starting Supplies: It’s logical that many seed catalogs also offer all sorts of seed starting equipment from flats, to seed starting potting mix, mini-greenhouses, bottom heat mats, plant labels, hand seeders and more. Rootrainers caught my eye. Thirty-two Rootrainer cells come with a tray. Each of the cells is 1.5 inches square and 5 inches deep, deeper than many other seed starter cells. These deep cells encourage a longer root system and help improve transplant survival. Each set of four cells can be opened up so you can check the progress of the roots and so you can easily remove the seedling for transplanting. These can be found in Park Seed Company’s catalog (

Park Seed Company also has a complete special mini-greenhouse for starting seeds on your windowsill called a “Bio-Dome”. This is a seed flat with 60 individual cells and a plastic dome cover with vents. Each is about 15 inches long and 9.5 inches wide and comes with 60 cells that are 2.25 inches deep. The $25 cost per dome is a bit pricey, but it’s reusable from year to year.

Yes, there are plenty of wonderful seeds for sale in seed catalogs, but there’s also a wealth of other handy gardening treasures to be found.

Published: 1/29/2011 3:42 PM


Marianne C. Ophardt WSU Extension Faculty for the Tri-City Herald, Kennewick, WA Invariably I get asked to talk about how to ‘put the garden to bed’ in the fall, but no one asks me to address how to ‘wake up the garden’ in spring. Apparently, there’s no need for that. Warm temperatures and longer days do a pretty good job of that without my help. Gardeners are just itching to get out and get the gardening season started. The timing is perfect, there are a number of garden tasks that require our immediate attention… PRUNING: Flowers and leaves will be popping out any day on many of our fruit trees, shade trees, roses and shrubs. If you haven’t done your winter pruning already, now is a great time before new growth makes it difficult to see the plant’s framework. Keep in mind woody plants don’t ‘need’ to be pruned. Always prune with a legitimate purpose in mind. Good reasons to prune are to remove dead or diseased wood; to correct ill-placed growth where limbs are crowded or rubbing each other; to get rid of growth that’s posing a hazard; to correct growth defects caused by narrow branch angles or past improper pruning, or to encourage better flowering and fruiting. You should have used the winter respite to clean, oil, and sharpen your pruning tools. Do it now, if you didn’t earlier. Now is a good time to consider investing in a quality pair of bypass hand pruners and bypass loppers, and possibly a small pruning saw too. Pruners with a bypass (hook and blade) mechanism make a cleaner cut than anvil types that crush the stem, leaving damaged tissue behind On multi-stemmed flowering shrubs such as forsythia or lilac, you may find you also need a small pruning saw. On these shrubs it’s recommended that you take out 1/3 to 1/5 of the oldest wood, cutting it back to just a few inches above the soil. You should also remove any crowded spindly growth. I do this every spring on my wild and gangly forsythia. It amazes me how much wood I can take out and it still grows and grows. And yes, now is the time to prune your floribunda and tea garden roses, as well as most other types of roses. The proper way to prune varies from type to type. Before you cut, find out the proper way to prune each type. Now is also the time to ‘prune’ or trim ornamental grasses and perennials. You can use hand pruners or scissors to cut back smaller clumps of grasses, but light-duty hedge trimmers will work best for larger clumps. To make cleanup easier, use twine to tie the top growth together and then cut the grass back four to six inches above the soil. New green shoots will soon appear. Trim back dead perennial stems and flowers, if you didn’t get it done late last fall. I also cut back my perennial herbs, like sage, to keep them a bit more tidy and within the bounds of their allotted garden space. The hedge trimmers come in handy for both these jobs too. IRRIGATION: Check out your irrigation system and make sure it works as soon as you have water available. While it wasn’t a harsh winter, damage may still have occurred to sprinklers and drip lines. Before hot weather arrives, check the coverage your sprinklers are providing. You can do this by placing empty straight-sided tin cans in different areas of each zone, radiating out from the sprinkler heads. Run the system for a set amount of time. Measure and compare the amounts of water (by depth) in the cans. If there is a wide disparity, you may need to adjust your heads or get new ones that can provide a more even pattern. The folks at the irrigation supply store should be able to help you with this. Despite our winter precipitation, yard and garden soils are fairly dry right now. As soon as you have irrigation water available give trees, shrubs, gardens, and lawns a thorough watering that moistens the soil to a depth of at least a foot. LAWNS: I know you’re tempted but it’s not time to fertilize the lawn yet. Wait until the first of May for that. It is definitely time to apply crabgrass and weed ‘preventer’ chemicals to your lawn. However, this may lead to a dilemma for you because many of the ‘preventer’ products only come in combination with fertilizer. Stand-a-lone preventers are available, but you’ll have to look a little harder for them. Timing is not as crucial with products containing the chemical dithiopyr because it will provide both preemergence control, as well as some postemergence control of young crabgrass plants. However, before you buy any product, make sure that the weeds you think are crabgrass really are crabgrass. Lots of people are battling Bermuda grass and bentgrass, thinking their problem is crabgrass. If the thatch in your lawn is greater than one-half inch thick, you should consider ‘dethatching’ with thatch removal equipment. This should be done before the middle of April. It also should be done before the application of any weed ‘preventers.’ GARDEN STRUCTURES: As you go about the yard and garden, check out your garden structures for any needed repairs. Now is when it’s easiest to fix arbors and trellises… without the green growth of vines and climbing roses to obscure what might need nailing or wiring. See if any furniture or garden benches needs a new coat of paint or stain. Clean bird baths out thoroughly. Check for loose stepping stones and patio pavers and secure them.

Published: 3/25/2006 11:24 AM



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