Here is a map of the last average frost days for your area. Just remember that it's an average, so there will be about as many years with frosts after this as before. So unless you can cover things up if you do happen to get frost, or you don't mind losing things if there is a frost, don't plant out frost-tender plants yet. I'd look up the plants and see how they deal with cold, since there are some annuals which are fine with frost and some that aren't. If you haven't already, you can start hardening plants off by putting them outside daily, starting with a short amount of time and increasing it a bit each day. I usually start mine in a spot that is shaded until mid afternoon, and move them into a bit more sun and breeze daily. I typically plant out on a cloudy or rainy day to reduce shock.

The Scabiosa is a perennial and should be frost tolerant, so I wouldn't worry about planting that out. Mine from previous years is showing green now even though we are still getting some night time frost.

Your Impatiens is frost-sensitive as is Nicotiana, so I'd wait on those for a week or two.

I haven't planted any of your others, so I don't know about them.

How warm the soil is will vary according to a bunch of factors: moisture levels, how much sun it gets, whether it has been covered with mulch, and what your night time temperatures have been dropping to, so I don't know what to tell you about soil temperature. I am currently planting out cold-tolerant plants, but not warmth-loving varieties, but I am also 80 miles north of you and in an inland valley that is a spring frost pocket. Hopefully someone experienced in your area will weigh in with their experience.

Thank you, NHBabs. I looked at the 10-day weather forecast and we have no low temperatures predicted below 42. (I know weather predictions turn out to be occasionally wrong!) Our average (50%) last frost date here is April 21-30.

Just about everything has been hardening off for the last 10 days.

I guess my main issue is that I don't know where to "look up" the frost sensitivity of these various flower plants. I haven't been able to find any sort of document on the web describing in any semi-precise way the frost sensitivity of various flower plants. The seed packets are sometimes helpful but don't give the kind of even moderately-detailed information I'd like. They more often tell you when to plant the seed either indoors or outdoors (in relation to average last frost date) but nothing about when to transplant the plant outdoors. For instance, how did you learn/know that the Impatiens balsamina is frost-sensitive as is the Nicotiana? From direct experience? Or on the web somewhere? Or...?

Thanks much.

How did I know? . . . Partly experience with those two particular plants, but you can look up plants on Missouri Botanical Garden plant finder. If it says the flowering season is June-frost, I will assume that it is frost sensitive. Here's the page for Nicotiana sylvestris; look at the bloom time line.

NHBabs, thanks for that link to to MBG plant finder. That is definitely going to come in handy. Yep, I see "June to frost" indicated for the bloom time so I see what you're talking about. Could that also just mean that the plant grows slowly, or is slow to flower? I have only the basal leaves now; maybe it means that it needs May to put up a flower stalk. Ah, I just noticed that in the "Culture" section of the MBG article it notes " Set out seedlings or purchased plants after last frost date." I guess we could ask if that means the 50% frost date or the 100% frost date, or ... Well, we're almost definitely (famous last words) past our last 32-degree day, but...

Maybe I'll plant out half of my plants and see what happens over the first few days.

I use the MBGpf a lot! It has the most honest plant size figures I've ever found as well as truthful problem listings, unlike most sites. It's the "to frost" part of the bloom time line I was looking at. That comment says to me that the plant turns to mush when there's frost.

I don't plant a lot of indoor starts any more as my schedule makes it difficult for me to take care of them, but I know that heat-loving plants such as tomatoes don't benefit from being planted out early since the cool temperatures of soil and air tend to stunt growth. For tomatoes, research says that having cooler days and warmer nights makes them stockier, so I often put them in a wheelbarrow and they go out during the day and in at night. I don't know if this applies to any of your varieties, but damage can happen for some plants at temperatures above 32.

You could plant out, but cover with floating row cover to give them a few degrees of extra warmth.

My Nicotiana reseeds itself around the garden in a non-thuggish manner, so they bloom later than yours will, but they will fill the garden with color and scent in August-frost, so I am happy with them.

Thanks much for this, NHBabs. I'm about to go outside where after about a week of hardening out I left (almost) all my non-warm-season annuals outside overnight for the first time. I'm crossing my fingers. The low temperature at 5 am was 43, so I think they'll have made it.

I wrote an email to Missouri Botanical Gardens asking them to clarify what exactly it means when a description indicates "Set out seedlings or purchased plants after
last frost date." Is that the 50% frost date, or the 10/90% frost date? and do we mean 32 degrees or 36 degrees, and...?

I hate to be geeky but it seems we have to be sometimes (at least as a beginner ... maybe when I'm more experienced I'll be able to act more out of intuition and experience).

What it means is that your seedlings can NOT tolerate ANY frost and if you want them to make it you need to be well past your last frost date. I don't do annual flower starts, but I do have a few dozen veggie starts (tomatoes, peppers, zucchini, pumpkin, watermelon, cantaloupe, cucumber etc) and none of them tolerate frost at all. They won't go out until mid-May at the earliest. Especially with the forecast showing lows in the mid-40s and highs not even reaching 60, heat-loving plants (which tend to be the veggies I listed and the annuals you have) just sit miserably in the ground not doing much of anything when it's cool like this. There's a reason Memorial Day weekend is garden center plant sale heaven around here - it's FINALLY reliably warm enough to plant all the plants!

IMHO, average last frost date is a meaningless concept for home gardeners. It has a use for farmers who can get a premium for bringing in some crops early. For home gardeners, when people say 'average frost date' they almost always means '90% frost date'. That is late enough that a 10 day forecast will be enough to tell if planting out will be safe or not.

The safe assumption is that annuals are not frost tolerant. There are annuals, like petunias, that are strongly heat loving and don't perform well in cool summers. Since I'm not familiar with most of the seeds you are growing, I can't speak from personal experience how much heat they want/need. The snapdragon and scabiosa are short lived perennials, and can handle some frost. I've grown balsam, and had it reseed for a couple of years. I seem to remember it wanted a longer growing season to really take off than I could provide.

I was running out of space inside so I decided to move some of my hardier annuals outside in a wagon and bring them in if needed. I looked up a list of hardy annuals to see which are my seedlings fell into that category. I already knew that stock is quite hardy and can be put out when pansies go out. You may also see on some of your seed packets the designation HA for hardy annual. Others are listed as half-hardy annuals or HHA. This could help your decision making in the future. Other plants are considered tender.

suzabanana, thanks very much for this. Your last sentence is important for me to remember. Our weather prediction is for lows to be increasing from 43 to 47 for the rest of the week through the weekend, then beginning next Tuesday morning the lows will rise into the 50s. But we've had rain for the last two days and it'll be rainy and drizzly for the rest of the week.

I think overcast, drizzly weather is actually good for planting, isn't it? I planted all the Stock, Snapdragons and Bells of Ireland yesterday and they all look fine now. Today I planted some of the Nicotiana and will wait a few days for the rest. Next week I'll plant the Balsam (Impatiens). Ironically, both the Scabiosa and White Lace (Orlaya) -- which as you say wouldn't at all mind being planted in these temperatures -- are still a bit small and I want to let them develop a bit before I put them out. I think they're both very slow growers.

asarum, thanks for your note. I checked my seed packets (from Botanical Interests in Colorado) and I don't see any two-or-three-letter designation like that. That would actually be really helpful. What seed company do you use?

I just browsed on the web to see if I could find a formal definition of hardy, half-hardy, tender, etc. and I found something very informative here.

btw, I'd be glad to know where you saw that list of hardy annuals.

Thanks.

Rick, I just looked through my large collection of flower seeds from many companies and am dismayed to see that this information is not included very often. I am sorry for steering you in the wrong direction! But you can search online in the future using these terms. I also have a book, from Seed to Bloom by Eileen Powell that provides this information as well, but in some cases is confusing because for instance, annual poppies are sometimes HA and sometimes HHA because one entry includes multiple types of annual poppy (papaver). It is a great reference.

Here is a link for a list I found on the winter-sowing forum right here. Hardy annuals for winter-sowing

The thing about winter sowing is that the seeds are sown in winter, but germinate and come up at a time that is right for them. For instance, I suspect that the cosmos I had last year have self-seeded. Some may be coming up now in that area, but the plants coming up out of the ground right now are too small to be sure. It could be that what I am seeing is young weeds. The cosmos if any many come later. A nursery would stress that it is too early to put out cosmos and out of an abundance of caution, I am keeping my cosmos in the pots till around mid-may here in Weymouth, right near the coast. I use the winter sowing list with caution because it applies to seeds not to young plants.

Thanks, Asarum. OK, I just ordered a copy of From Seed to Bloom. Sounds like pretty close to what I've been looking for (I've been surprised at how hard it is to find this kind of information in one place.)

I meant to do winter sowing this last winter and just somehow didn't get around to it. It's interesting that that article uses bachelor's buttons as a starting-off place. I sowed a lot of bachelor's button seed last year (late: June 17) and was graced with many beautiful flowers through the summer and fall. It's a really wonderful flower, especially when planted en masse. Those BB's were one of the real success stories of last year's garden. And now, come the next spring, scattered throughout my yard are probably 30-40 volunteer bachelor's buttons seedlings/plants, some at a fairly advanced size.

I also grew Cosmos last year, and loved them. Another elegant flower.

Hmm... I'm noticing that even though that article begins with a nice drawing of a bee visiting a bachelor's buttons plant, BB's are actually not included in the list of hardy annuals suitable for winter sowing. That's surprising as it reseeded quite well for me.

I've always been intrigued by what determines whether the seed of any particular annual, dropped by the plant's mature seed pod in late summer or fall, starts to germinate a few weeks later in the warmth of late summer or fall, or doesn't and instead over-winters to germinate and grow the following spring. I remember seeing many BB (ok: Centaurea cyanis) seedlings around their parent plants around October of last year and thinking, "Oh, my dear brother -- you shouldn't be growing here now. It's not going to work out well for you in the long term. Have you heard of something called winter?" I'm guessing they eventually all learned their lesson.

But other seeds from the same parent plant get smart and decide to remain on or slightly beneath the ground during our cold winters, don't mind getting snowed upon, and then in April celebrate their great fortune and good wisdom by growing into nice healthy plants in May. And then in June they celebrate their beauty by bursting forth a glorious blue, or purple, or white flower.

I know that many plants know enough science to insert into their seed some sort of chemical that allows the seed to go dormant when it needs to. Do all annuals that grow on the east coast have a fall-dormancy chemical in the seed? Some? Most? Even if many do, I would guess that sometimes it just doesn't work. Maybe the dormancy chemical is only effective for some percent of the seeds, and that some other seeds do germinate, or try to germinate, in the fall when it's a fairly good bet that germination will not lead to a full plant. We all know that's why nature insists that flower plants produce so many seeds: to guard against the unexpected or unwise.

But every spring when I see these brave seedlings, already nicely-sized for early May -- way, WAY bigger than any plant growing from seed I'd been able to plant even in earliest spring -- I think: yes, this is one of the real advantages of winter sowing (or really, fall sowing), here done by nature, not by any human. She really knows what she's doing, or at the very least, how to make up for any inability on her part to foretell the future.