The spring warm-up finally begins

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By David Graper

SDSU Extension Horticulture Specialist

Do you like to watch the weather on TV? I think that most gardeners try to watch the weather whenever they can. Of course now we can get up-to-the-minute updates on our smart phones but I still want to catch the weather forecast every day if I can. The weather and especially the temperature are very influential in what happens in the garden and we have no control over, as much as we might hope we did.

Starting this past week, we finally saw some nice warm, spring-like temperatures. It is amazing to watch the transformations that occur with the warmer weather. The coats disappear to be replaced by t-shirts and shorts. Snow drifts finally melted and what looked like barren soil for the last few months suddenly shows signs of life emerging from below. Temperature is what mainly drives that transformation.

Air temperatures and soil temperatures are linked but as you would expect, soils warm up much more slowly than the air and show little variation over time while air temperatures can easily vary by 30 degrees or more in a single day. There are many factors that influence how quickly soils warm in the spring. The wet soils that many gardeners have in their gardens this year are slow to warm. Dry, sandier, more exposed soil will warm more quickly than clayey and shaded soils. Mulches and turf grass shade the soil and keep it cooler longer into the spring. This can be beneficial in keeping some plants dormant a little later into the spring to avoid spring freeze damage to tender shoots. If you look around, you will probably see where tulips have been planted up close to a home, particularly on the south side of the home, the tulips will soon be growing quite rapidly. While in other sites, that are more shaded or mulched, we will see later tulip emergence and growth.

Soil temperature can play a more important role in the early spring growth of many different kinds of plants than air temperature. This is particularly true for seed germination of vegetables as well as weeds. Radish seed can germinate in soil as cold as 40° while watermelon seeds need soil temperatures close to 75°. Prostrate knotweed can germinate at a range of soil temperatures ranging from 35° all the way up to 85°. Crabgrass, a common lawn weed, will start germinating when soil temperatures reach about 62° in the top 1-2” of the soil, which usually coincides with the time when lilacs begin blooming.

Vegetable seedlings and transplants also need certain temperatures to germinate as well as grow and produce fruit later in the season. Spinach, peas, radishes, cauliflower and head lettuce can be grown with minimum soil temperatures of 40 to 45° but have an optimum soil temperature of 60 to 65°. But they also will not tolerate high air temperatures that can occur in mid-summer so they need to be planted early in the spring or in some cases, may be planted again in late summer for a fall crop. Broccoli, onions, cabbage, chard, leaf lettuce, potatoes, carrots, turnips, beets and kohlrabi are in the next group that can be planted and will tolerate warm summer temperatures. Warm season vegetables like beans, sweet corn, squash and pumpkins need at least 50° soil temperatures while cucumbers and muskmelons need at least 60° and tomatoes, eggplant, pepper and watermelons need 65-75° soil temperatures.

It is quite easy to measure the soil temperature in your garden and around your yard. You can purchase an actual soil thermometer or just go to your local grocery store or kitchen aisle and look for an instant read thermometer, which is ordinarily used to check if your steak or roast chicken is done to the right temperature. These inexpensive thermometers range in price from about $5 to $30 and are also available online. Generally you will want to stick the probe about 2” down into the soil to get a reading. Take readings at several locations to get a good average of what the soil temperature is for your garden. Just clean it up well before you put it back in the kitchen drawer or your steak might have a little bit of a gritty taste the next time you use it. Or, better yet, buy one that you use just for measuring garden soil temperature and keep it with your other gardening tools.

For more information on planting vegetables see:



A good time for repotting

Do you have an old houseplant that has just gotten too big for its pot? Does it seem like it always dries out? Does it tip over easily because it is too top-heavy? Has the pot cracked open because of too much root pressure inside? Are roots growing out of the top of the pot? If you answered yes to a few of those questions, it might be time to give your plant a larger pot. Now is a good time to do it too because plants are likely going to begin a new growth spurt with the longer days and increased sunlight intensity. I suggest taking the plant into the garage for repotting to avoid a mess on the floor.

Before you start the process, you will need to get a larger pot. Generally look for a pot that is a “size” larger than the current pot. So, if it is in a 6” pot now, look for an 8” pot. Generally you want about an extra inch of space around the root ball in the new pot for smaller plants and 2” or more space around the root ball for larger plants. I always recommend getting pots that have drainage holes. This allows excess water to drain out of the bottom of the pot when watering. You will probably need to use a pot saucer to catch the extra water but it will keep it from getting onto your window sill or floor. It is a good idea to empty the saucer after all the water has drained out, so that it is not reabsorbed into the potting soil.

You will also need to get some more potting soil. I do not recommend using ordinary garden soil as potting soil. It is usually too poorly drained for use in a pot and may come with extra things you don’t want like weed seeds, insects, worms, and pathogenic fungi or bacteria that can make your plant sick.

Remove the plant from the old pot. If it seems to be stuck, try tipping it upside down and tapping the rim of the pot on a counter. Place some pre-moistened potting soil in the new pot and check the depth by setting the plant in the pot. When finished you will want about an 1” of open space left at the top of the pot for a 6-8” pot or about 2” of open space left for larger plants. This extra space will hold water while watering and allow it to soak down into the potting soil. When you have the right depth, continue adding more potting soil, firming it around the old root ball. If you don’t firm it well enough, when you water the plant, the water will just run down around the old root ball instead of soaking into the old root ball and new potting soil that you added. Give the plant a good thorough watering, so that you see water coming out of those drainage holes. Let it drip out then replace it back in your home for additional years of life and growth.

Question from a gardener in Brown County

Q What may be causing these store-bought “on the vine” tomatoes (see photo) to be sprouting internally inside the fruit! Plant growth hormone? Chernobyl? Need your help to explain this.

A This is an example of vivipary, which is Latin for “live birth.” Some seeds, including tomato occasionally, may have seeds that can mature and then go on to germinate while still in the fruit. It can be more common in some cultivars or certain storage conditions.

Question from a gardener in Meade County

Q If grass clippings from a lawn treated with weed control spray are put into my compost bin, will the herbicide affect the finished compost? It usually takes about a year for the compost to be finished. Thank you.

A This is a difficult question to answer with a definite question because it depends on a number of factors. Ordinarily lawn herbicides will break down by microbial and chemical activity in grass clippings when placed in an active compost pile over the course of a year. However, it depends on the herbicide that was used, the rate applied, how soon grass clippings were collected after application and how active the compost pile is. Banvel, for example, has a longer residual activity in the lawn than does 2,4-D. If it was applied at a high rate, that will take more time to break down. If the lawn was mowed several times before clippings were collected and added to the compost, that will be better than if they were collected the next time after herbicide application. A cool, slowly working compost pile has less microbial activity than an active pile where temperatures can reach 150 to 160°F. It also depends on what crops might be in proximity to the compost. Tomatoes and potatoes are particularly sensitive to most broadleaf herbicides.

So, will it be OK to use? It depends. You could test the compost by mixing some with potting soil in a ratio of 1:4 compost to potting soil. Then, plant some tomato seeds in the mix and see how they do, compared to planting some seeds in straight potting soil.

Question from a gardener in Yankton County

Q What do you consider the general soil temperature to plant a vegetable garden in SD, USDA Zone 4-5a? What do you consider the general soil temperature to plant flowers? Thanks

A Appropriate soil temperatures is really based on the plant you are trying to grow, not on the zone. For example, spinach is a cool season vegetable which can be planted once soil temperatures in the top 2-4″ reaches40 to 45F, but the optimal temperature is 60 to 65F. That is the same pretty much where ever you are planting it. Similarly, squash, a warm season vegetable, needs a minimum soil temperature of 50F and has an optimal temperature of 60 to 75F.

Take a look at this publication, particularly page 6 for more information on soil temperatures.

Annual flowers can be broken down in a similar way. There are cool season flowers like sweet alyssum, flowering kale and dusty miller that can tolerate cooler soil and air temperatures and then you have the majority that are cold sensitive that need warmer soil temperatures like marigolds, coleus etc. Check the seed packet for more information.


READY, SET, GROW is set for May 5, 8:30 a.m. – 2:00 p.m. It’s a free workshop open to the public. Sponsored by Coteau Prairie Master Gardeners, Codington County Extension Complex (Entrance H), 1910 West Kemp Avenue, Watertown, SD.

• 8:30-9:00 — Registration.

• 9:00 — Unique Plants /Veggies – Vikki Schaack, Medary Acres, Brookings.

• 10:00 — The Best Plants Tested at McCrory Gardens during 2016 & 2017 – David Graper, SDSU Extension horticulture specialist.

• 11:00 — Great Container Plants and More! – Lori Seim, Sioux Valley Greenhouses, Watertown.

• 12:00 — Box Lunch Available – Catered for $10, order at registration.

• 1:00-2:00 — Make Your Own Tufa Pot – from newspapers! Instructors – Deb Cass and Lori Seim, cost $7.

Plant sale fundraiser offers diverse, affordable plants

Minnehaha Master Gardeners is excited to announce our annual plant sale event. This fundraising event is designed to provide the community with a wide variety of beautiful plants at a low cost.

The plant sale will take place from 9 a.m. to noon on May 12 in The Woman’s Building at the W.H. Lyon Fairgrounds at 100 N. Lyon Blvd., Sioux Falls, S.D., 57107. The doors will open at 7:30 a.m. for those who want to be at the front of the line. Minnehaha Master Gardeners invites the Sioux Falls community to join in on this valuable spring gardening opportunity.

Local growers donate perennials, annuals, vegetables, and other plant varieties to the sale. The sale offers high-quality, locally grown bulbs, herbs, tomatoes, flowers, and more. These plants originate from gardens in this region, so they are well-acclimated to the climate and conditions of Minnehaha county.

Minnehaha Master Gardeners aims to work within the Sioux Falls community to broaden gardening knowledge and provide resources for community members. Certified Master Gardeners receive extensive education through South Dakota State University horticulture department, making us regional experts in plants, insects, garden diseases, and gardening techniques. To learn more visit our website at or call (605)-782-3290.

Through our annual plant sale event, Minnehaha Master Gardeners looks forward to providing community members with high-quality, local plants for their gardens.

Tomato with seeds germinating while inside the fruit. iGrow photo
Various kinds of thermometers can be used to measure soil temperature. iGrow photo
The new pot should have at least an inch more space all the way around with a larger plant. iGrow photo
Place some potting soil in the pot then position the plant near the center. iGrow photo
Prostrate knotweed is one of the earliest weeds to germinate. iGrow photo
When finished there should be at least 1 inch of space below the rim of the opt to allow room for watering. iGrow photo