Q&A: How can you tell when soil is warm enough? And other gardening questions

CBC

17-05-2025

Listeners were asked to submit their gardening questions ahead of the long weekend
Media | Getting ready to garden this long weekend? Here are some expert tips
Caption: Victoria Day long weekend is nearly upon us and it marks the unofficial start to gardening season. Perry Grobe owns Grobe Nursery and Garden Centre in Breslau. The gardening guru shared some tips with CBC K-W's Aastha Shetty about how to get started planting this weekend.
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The long weekend is here! For many people, this is the weekend gardening begins.
Perry Grobe is the owner of the Grobe Nursery and Garden Centre in Breslau.
The gardening guru has some tips, tricks and answers to gardening questions that listeners submitted earlier in the week. Host Craig Norris spoke to Grobe on CBC K-W's The Morning Edition.
This interview has been edited for length and clarity. Audio of the interview can be found at the bottom of this article.
Craig Norris: What should people do to prep the soil in either a garden or a planter box?
Perry Grobe: In the case of the garden in the ground, the thing that degrades the most is organic matter. So adding any kind of compost product, peat product, anything like that to try and hold moisture, improve aeration and have more microbial activity is of benefit.
In containers, the potting soil is a little different there. Some folks will change some of the soil out from one year to the next. It's not essential to do that, but those type of soils that are there are meant to drain freely. And oftentimes if you add a lot of garden soil to a pot, you're going to have troubles with the plants going forward. So if you're going to use any kind of an organic amendment, you'd be using a granulated one, perhaps like a granulated compost as opposed to real live compost.
Norris: We asked listeners for some questions for you and we got more than a few in our inbox. I want to start with Deborah. She wrote in with this question: Two years ago, our plentiful and robust tomato plants suffered a devastating late season blight. When is it safe to replant tomatoes in the same soil? Will the pathogens be eliminated now after two years?
Grobe: Unfortunately with pathogens like late blight they can go for quite some time. I'm not quite convinced two years will do it. The trick with late blight… is one, don't obsess about using heirloom varieties cause sometimes they tend to be the worst for it. Sometimes you have to use a hybrid or should have some hybrids mixed in because they tend to be a bit more resistant.
Second thing is you're trying to protect soil splash from the existing soil up on the leaves, the lower leaves because if it's in the soil, if you can reduce the splash, you don't have as much of a chance at getting into the plant itself. Now how do you do that? Well, you can do it with mulches.
You have to be a little careful with timing and temperature with mulches. In my home, what I do is I put landscape fabric down and then I cut a slit, put the plant in, take off the lower leaves and that tends to reduce the amount of late blight infection. And the third thing is of course using a fungicide that's geared for blights. And most of those that are available have to be used right at the outset, when the plant is relatively small as a protectant. Once you have it, it's very hard to eradicate it.
Norris: Sue in Guelph wrote that her garden is flourishing with many plants and shrubs that she's purchased over the years, including a Rhododendron 'PJM Elite Star'. Her question is about caterpillars. Sue asks: I have a small number of boxwoods that I'm planning to protect from the boxwood tree moth by applying the non-toxic BTK. The moths appeared for the first time last summer. When can I expect the first caterpillars to appear?
Grobe: We're actually coming into that time. It's generally the middle to end of May. So monitoring for that particular larva would be done. Now, the actual overwintering site for the box with tree moth is easy to see once you know what you're looking for. It looks kind of like an egg sack for a spider on the back of a leaf. The caterpillars really are… not hard to see once you know what you're looking for. The product that she's using to control it essentially works by ingestion. So it gets onto the leaf, it gets ingested by the caterpillar and it gives them a belly ache that he doesn't recover from. And the timing to do that is, oftentimes if you see flight, it's generally within a week or two after that.
Norris: Gloria wrote in. She's looking for advice for a small tree, not a shrub, for her large, full sun perennial garden. She'd like it to be no more than 20 feet tall when it's done. Disease resistant, easy to grow and attractive for at least three seasons. Any recommendations?
Grobe: The tricky part with that is of course that plants generally have their flowering period and then they're finished. So that part of it is usually a three-week period regardless of species. The off-seasons, when it's not in flower, that's where it gets a little trickier because you have foliage colour. So in her note, she mentioned crabapple and crabapple is certainly one of the things that has the potential to have more seasonal interest because you can have the fruit hanging on through the winter, you can have purple leaves in the summer time… You have to find varieties that are scab resistant or mildew resistant, which are the most common issues faced by crabapple. You can use Serviceberry, which is a smaller tree that flowers always looks nice. Fringe Tree looks nice and it's smaller.
The problem is the four season interest. She had indicated Magnolia — and Magnolia I have to say, everybody notices it when it's in flower. They don't really notice it too much when it's not. And that's the challenge. Everybody would like to have something that blooms all summer and but we oftentimes don't have anything that follows that.
Norris: Shirley asked a question about grass. Shirley said: we are killing the grass in our backyard to get rid of Creeping Charlie by covering it with heavy tarps. How long will this take? When it looks dead, should we dig up any grass and weed remnants and discard them?
Grobe: The answer is it could take several months for that to take place. Most weeds and twining weeds like that are really hard to eradicate at the best of times. And generally, yes, you try to remove debris that you see afterwards for sure. You don't necessarily have to compost it, but it would be good practice if you're planning to put new turf down as opposed to planting beds, they just strip off all of that stuff and dispose of it.
Norris: Let's go to Pat in Paris. She says she has an apple with six different varieties grafted onto the main trunk. She says it produces apples but has been affected by the apple leafcurling midge insect. She put down landscape fabric to prevent grubs, and she's wondering what she should do next.
Grobe: I think she's done pretty much everything that can be done. The only other thing potentially would be is if the litter is there at the end of the year to get rid of it and at the first signs that you see the actual midge you want to take those tips out because if it's done at the right time when they're just starting to form, you get rid of the worst of them such that they don't have a chance to get back to the soil to reproduce. So that would be the simplest way to deal with that.
Norris: Lisa said: I live in an apartment and I'm looking to make a very small herb or vegetable garden. I'm south facing, get sun from about 11:00 a.m. to sunset. What would you recommend for a small yet high-yield patio garden?
Grobe: So the first thing is try to get the biggest pots you can possibly have up on the patio because the soil volume that you have there is going to play a big role as to how successful the things are going to be. South facing, you're gonna have tons of sun, tons of heat, and you may not be there all day to supplement with water and you're going to need to because you're gonna pots often dry out.
I used to have an extensive patio or balcony garden before I had a house and I tried lots of things. And the big thing is how you're going to deal with the watering is the thing that's really the issue… You got to think very carefully what you would like. Herbs are something that, for example, if you do, you're gonna harvest for every meal or potentially every meal. Tomatoes, the payoff is big if it's a patio tomato or a cherry tomato where you can eat cherry tomatoes all summer. Makes sense. Some vegetables like beets or whatever, you're not, you know, you're not going to do it because there'd be better uses for your time, effort and things.
So figure out how you're gonna water, get the biggest pot, make sure you're using a potting soil or one that's geared for pots. And then the nice thing is that you can plant regardless of frost. It doesn't matter. You'll actually have an extended season. You'll probably get more food off of it into the fall than most people will.
Norris: Wendy told us there's a whitish gray substance on her Ivory Halo Dogwood shrub which, in her words, defies any description. She sent a photo. You have seen the photo. She wants to know what it is and how she can get rid of.
Grobe: So this question was the toughest of the ones I had because in fact it looked like a couple of different things. I actually went to a few guys that I count upon for help with when I have problems I haven't seen before and the consensus is that it looked like a bit of sun scald injury that potentially could be the early signs of canker. Missing from her description of whether the plant itself is suffering at all. If it's not really impacting the growth of the plant, feeding the plant and keeping it is the thing to do. Try to keep it as vigorous and healthy as possible. If, however, it is declining or not doing well, dogwoods are inexpensive. Go get another one.
Norris: How can you tell if soil is warm enough to plant?
Grobe: The best way to know whether the soil temperatures are warm enough for zucchini is to go out there in the morning and take your pants down and sit on the ground in your bare bottom and if it feels alright then it's the right time. Now your neighbours might say something when that happens, but if it's warm enough for you, it'll be warm enough for the plants.
For vegetable gardening, I usually don't plant the plants themselves until June. But nobody really wants to wait that long. And it's very difficult to keep the plants in the small pots. And I get that. So the thing to do is to cheat and that is to try and warm the temperature of the soil up artificially by covering it. Because if you can do that for three, four, five days ahead of planting, you'll find you'll have better success. Do it now and then next week you'll be able to.
Plant sales
Local nurseries and greenhouses will be open and busy this weekend. There are other plant sales happening around the community, too.
Here's a list of a few of them:
Friday 1 p.m. to 4 p.m. at the Red Chevron Club in Guelph, there will be a seedling sale by Brookfront Farms and Sustainable Market with up to 20 varieties of tomatoes, 10 herb varieties, plus lots of broccoli, cauliflower, cabbage, cucumber, pepper, eggplant, flowers. Cards accepted but cash is preferred.
Saturday 8 a.m. start, the Preston Horticultural Society's plant sale will be at 212 Highland Park in Cambridge. Organizers stress no early birds but the first 25 customers will receive a free plant (culver root).
Saturday the Garden Club of Kitchener-Waterloo will hold their plant sale at First United Church in Waterloo.
Saturday 9 a.m. to 1 p.m. at Harcourt United Church the Guelph-Wellington Master Gardeners will host their annual plant sale with all the usual items for gardens and flower beds, plus shrubs, trees and even some tropical houseplants. There will also be 20 master gardeners on site to help. Cash, debit and credit cards accepted.
Saturday 9:30 a.m. to 12 p.m. at the Ken Seiling Waterloo Region Museum is the annual heritage plant sale. The plants are a selection of the flower and vegetable varieties found in the gardens of the Doon Heritage Village. Purchases are cash only.
LISTEN| Perry Grobe of Breslau's Grobe Nursery answers your gardening questions
Media Audio | The Morning Edition - K-W : Perry Grobe of Breslau's Grobe Nursery answers your gardening questions
Caption: The long weekend is nearly upon us and while rain is in the forecast, many will be at garden centres around our area to get plants for their gardens and patio boxes. Perry Grobe of Grobe Nursery and Garden Centre in Breslau offers some tips and tricks and answers your questions.
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