Last Frost Dates - Plant Hardiness

[Susan BBPM]

I'm a little confused over plant hardiness and last frost dates.

The terminology I'm using here is UK-centric. I know it's a little different in the US, but the same principles apply.

I'm in the West of Scotland - my Dad always used to say you should never plant out bedding plants before 1st June. I've always found it's fine by early-mid may. However, can I rely on this always being the case when planning what to sow and when?

The Royal Horticultural Society define half-hardy plants as "H3" (equivalent to USA zone 9). They say they can tolerate as low as -5 to 1 degree. Well, which is it, because if it's -5 planting out first of may would be fine here. But if it's 1 may could be risky.

However, most gardening websites suggest a last frost date of late April. It's *rare* to get frosts after this date. So presumably to be absolutely safe you wait until after 1st June (after which date we never get frost).

But what about temperatures? Even if we don't get a frost - are the cool temperatures of May likely to slow the growth of half-hardy plants? Apparently things like marigolds (H2) like it a bit warmer so perhaps I should plan to put them out a bit later than, for example, Cosmos (H3)? Morning Glory is H1c - so I'm guessing that can't go out 'till end of June when it's really warmed up? (I planted it out last week of May last year and they all died).

Our temps don't reliably get above 10 degrees centigrade at night until well into June. Even early June is a little bit chilly.

@headfullofbees - I know you have similar climate to me. I'd appreciate your thoughts on this - I know your ideas are based upon years of experience.

For reference:

Hardiness Zones in the UK: https://www.rhs.org.uk/plants/pdfs/rhs-hardiness-rating.pdf

 

[Chuck]

Don't forget about soil temps. Too cool and transplants either slow down or completely stop growth.

 

[Meadowlark]

I use a combo of soil temps, weather forecasts, and plant types to define my planting dates.

I think way too many folks around here plant on emotion and way too early.

 

[Susan BBPM]

I use a combo of soil temps, weather forecasts, and plant types to define my planting dates.

I think way too many folks around here plant on emotion and way too early.

I moved out to the countryside from the city in winter 2009. It was an EXTREMELY cold winter that year - it froze solid from mid December until well into March. It was late April before we saw the first signs of spring - so several weeks late.

I remember saying to a local farmer "will this cause problems for your harvest?". He said "no - it'll catch up. In a few weeks time it'll be back to normal".

This year I'm going to do a couple of experiments - so a few bedding plants and veg sowed early and the rest at normal time. To see if the farmer is right!

 

[Chuck]

I use a combo of soil temps, weather forecasts, and plant types to define my planting dates.

I think way too many folks around here plant on emotion and way too early.

That is exactly what I do too. People are so concerned about calendar dates when it comes to planting times that they don't ever think about the soil temperatures. One will plant in a soil that is too cool and his transplants will just sit there and do nothing even though the calendar and the ambient temperatures say it is time to plant. I will sit back and look at long range weather and my soil temperature guage. By long range weather forecasts I don't mean what the temperature will be in two weeks according to the local weather person. I look at high and low pressure areas over Northern Canada, which way they are moving and upper level wind patterns. If everything looks OK I will transplant but soil temperature is still the #1 priority. And when sowing seeds only a matter of a couple of degrees can be success or failure in germination. So, IMO, zones and calendar dates are only of minor importance. What I find to be the most important bit of data of all is what my soil thermometer reads.

 

[Sheal]

I've lived in various parts of Britain and this is the furthest north. I have never planted anything out until overnight temperatures are at 7C/45F or above, wherever I've lived. Many plants will suffer below that.

 

[Meadowlark]

... I look at high and low pressure areas over Northern Canada, which way they are moving and upper level wind patterns. If everything looks OK I will transplant but soil temperature is still the #1 priority. And when sowing seeds only a matter of a couple of degrees can be success or failure in germination. So, IMO, zones and calendar dates are only of minor importance. What I find to be the most important bit of data of all is what my soil thermometer reads.

Exactly. I do the same and particularly try to watch the weather in Alaska as well as Northern Canada. Very good indicators.

 

[YumYum]

I've noticed if the daytime highs are 60F and the lows are 30F, then the soil temp is right about the middle of these two, so in this case the soil temp is about 45F. Same thing with water and the swimming pool we had.

 

[Oliver Buckle]

so a few bedding plants and veg sowed early and the rest at normal time.

This is me, ring the changes. When in doubt I try as many possibilities as I can, one or another is probably right, but it will probably not be the same one every time, nature is like that.

 

[Susan BBPM]

Why do so many sources say don't plant out container grown HARDY annuals until all risk of frost is past?
One source vaguely mentioned that HARDY (H4, H5 etc) plants raised indoors won't be tough enough to plant out before last frost date.

But surely if I germinate indoors and grow them on in cold frames or outside they should be good to go by mid-April? Certainly there's plenty of light by then.

 

[Meadowlark]

Everyone that is planting this spring should keep an eye out for the possibility of a sudden warming of the polar vortex. As the polar vortex weakens from the top (stratosphere) down (surface), it unloads colder air from the polar regions into mid-latitudes, where most of us live. Many experts are suggesting this is eminent.

When/if there is a sudden warming of the stratosphere over the polar regions, thousands of feet above the surface, the otherwise stable polar vortex fractures in the stratosphere and a process begins that gradually works its way from the stratosphere into the troposphere (where we live) that can begin a domino effect that favors significantly colder weather over warmer weather as we head into March.

 

[YumYum]

Everyone that is planting this spring should keep an eye out for the possibility of a sudden warming of the polar vortex. As the polar vortex weakens from the top (stratosphere) down (surface), it unloads colder air from the polar regions into mid-latitudes, where most of us live. Many experts are suggesting this is eminent.

When/if there is a sudden warming of the stratosphere over the polar regions, thousands of feet above the surface, the otherwise stable polar vortex fractures in the stratosphere and a process begins that gradually works its way from the stratosphere into the troposphere (where we live) that can begin a domino effect that favors significantly colder weather over warmer weather as we head into March.

Daffodils are blooming now in my area. Those are the first visible wild blooms in my area. I think it is early for that myself. That makes me wonder if it will warm up greatly, then out of nowhere be deep freezing for a week. Have you noticed anything out of place in your area?

 

[Meadowlark]

Have you noticed anything out of place in your area?

Just a large amount of rain spread over the last 14 days or so. More than an inch of rain a day...and more predicted next week.

My soil temps are running mid-50s F which is pretty normal for this time of year and potato planting time.

 

[Chuck]

Just a large amount of rain spread over the last 14 days or so. More than an inch of rain a day...and more predicted next week.

My soil temps are running mid-50s F which is pretty normal for this time of year and potato planting time.

My soil temps are warmer than normal. I measured 2 days ago and it was 58.4F, about 4 degrees warmer than normal. The rain here is very sporadic and lately 1/4 inch is the most I will get at one time. Folks here in Bandera county have been in a drought for a long time and water wells are starting to fail.