I'm not sure how much you know about growing roses as it's a very deep/broad subject. So I've responded with some info that you might already know. If so, I apologize in advance, and hope the rest of the info will help.

I believe the Austin advice you mention is not for climbers. It's for bush types, grown as bushes. The newest Austin recommendation (if you want a super full, big looking bush) Plant 2 to 3 of the same bush variety, 18" apart. (so for 3 plant in a triangle with 18" between all three). Then you get a 'statement' sized, big bush once it all grows together. This is advice is based on growing English roses in the UK, and sometimes is not a good idea here in the states. Here, our vastly varied climates dictate otherwise. Since you have climbers, this advice doesn't apply, especially as heat will also be an issue.

HEAT is a thing. More frequent waterings are needed. Most people use drip irrigation, but some have overspray from sprinklers or water by hand, etc. The point is... Black Spot and Powdery Mildew will be more prevalent if water gets/stays on the leaves. Due to heat stress and fungal pressure, folks tend to plant their roses fully apart, so there is air circulation around all sides of the mature plant helping to keep plants cooler and dryer. Gertrude Jekyll is noted by Austin to be good for 'Hot & Dry' climates. Constance Spry is not on that list.

The climbers are a different deal. Some of the English climbers have really really stiff canes, harder to bend. Those they recommend for large arches, pergolas, etc. Other climbers have more flexible canes (usually) and those are recommended by Austin for 'pillars' obelisks (8' minimum height) and standard arches. Trellis can be large or small, very wide like an espalier or more narrow. Fit the rose to the available space and growth habit.

ALL the climbers bloom best on lateral wood, so the main canes should be fanned or trained horizontally to encourage laterals to sprout/grow up from the horizontal laying cane. Heirloom Roses has a great How to Prune Your Climbing Rose video which also shows how to train a climber. There are other great youtube 'how to train a climber' videos as well. Use twine, zip ties, wire, whatever you prefer that works well for you.

I've read many times that climbers will still grow wide, some very wide... as well as tall. If you look up the rose on 'helpmefind.com' it will tell you the ranges of height/width, notes about what that rose does in heat, wet, etc. There are also member comments from those who have or who are growting that rose and what they have found to be the case in their climate. Great resource and much more reliable than any catalog or the Austin website. (The Austin growth information is based on what their roses do in the UK... ).

Gertrude Jekyll is a well known/grown Austin pink SHORT climber. Austin recommends it for Obelisk, Pilar or Doorway (horizontal wires on a wall). It's well known to be super prickly. https://www.helpmefind.com/gardening/l.php?l=2.1822 Strong, old rose fragrance. Large (4.25" diameter), very double, old-fashioned, quartered bloom form. Rebeat bloomer: Blooms in flushes throughout the season. Note this: Height: 4' to 10' (120 to 305cm). Width: 4' to 6' (120 to 185cm). In a warm climate, it will grow a bit bigger than in the north, so expect the top end of those ranges for your rose. No way you want to plant TWO of them close together. Staying on top of the cane training/tying and pruning for ONE in a warm climate will be a fun task. It's prickly, so be sure to have a way to get to all sides of it as it grows.

GJ will need to be trained horizontally in some fashion to get lots of blooms. I saw that a member make the following comment, "This rose really responds well to the old Victorian practice of pegging - something I highly recommend, having used it on several roses now. Just bending and tying the long stems into the plant itself to form a network of canes crisscrossing back and forth to make a bushy shape, has made such a difference to this old plant, transforming it from a bean pole of a few stems and few flowers, to a beautiful large bush covered in flowers. She has so many blooms on her these days, she is like a different plant. And she tolerates shade really well too." Someone else wrote of 'winding' her around a tripod (which I take to mean a french type of obelisk, a tuteur). Again training the main canes horizontally around/on some sort of support.

In contrast, Constance Spry is a ONE Time bloomer, and huge. Austin notes it is THE largest English climber and has "many thorns". Good for a 10' wall, a fence, or a Large Arch. Austin says it grows up to 20'! Yet on Helpmefind (https://www.helpmefind.com/rose/l.php?l=2.41146.2) they list it as a ONCE BLOOMING rose. Light pink to rose-pink, rose-pink center. Mild, myrrh fragrance. Average diameter 4". Large, full (26-40 petals), cupped, globular, old-fashioned bloom form. Once-blooming spring or summer. Height: 4'11" to 9'10" (150 to 300cm). Width: 6' to 10' (185 to 305cm). USDA zone 5b and warmer. Can be trained as a climber. Disease susceptibility: susceptible to Mildew, susceptible to rust . Prune after flowering is finished.

A member comments: ' I had 'Constance Spry' here in my tropical low desert garden for several years. The plant was huge, but it never produced a blossom." Another member said, "I live in SoCal-- Ventura County, inland from the coast. Constance Spry has been on my arch for fifteen years and has produced maybe five roses."

And another member replied, "Constance Spry only blooms once a year which means it blooms on old wood instead of new wood. Do not prune the plant till after it flowers... even then, if you don't know how to prune a climber you risk loosing some of next year's blooms. On top of that, it's a climber; climbers usually spend the first several years building biomass instead of blooming. Give the plant more time. It's an added benefit to train the canes horizontally to encourage more blooms, otherwise, if allowed to shoot upwards, you will only get blooms on the very tips of the canes instead of the whole length of it. If after five years it still doesn't bloom, shovel prune it... either it doesn't like your climate or you're better off growing something less finicky. By the way, DO NOT prune it like a regular Hybrid Tea, i.e. cutting all the canes down to one foot every autumn, that is a sure recipe for failure."

After reading the comments from actual people growing this rose in hot conditions, it sounds like Constance doesn't like hot climates. In any case, it is a huge, thorny plant - so seems best to only plant one...

I want to second what Kim said above. When I grew a grafted Gertrude Jekyll in full sun in Seattle (a much less challenging environment for a rose than Texas, so your experience may differ) I had one plant and it wanted to be a large, well-leafed bush. I pruned it thoroughly to maintain it at about 4-5 feet. Gert has THORNS -- serious ones. I wouldn't want to deal with a jungle of densely planted Gertrude Jekyll's myself because of that. Based on what I've seen, I don't think that you need to plant a cluster of Gertrude Jekyll to get a good, full plant.

This method can be used in just about any bush type rose. I do it a lot with Floribundas. This system makes a great statement in the garden.

I'm amused whenever a breeder labels a rose as a "small climber" to describe an unkempt, rangy rose with bad manners! (which describes many of the earlier Austins)

🤣🤣🤣

Constance Spry along with other roses in my San Francisco garden in 1992. It was on Bosworth in Glen Park, right at the Bart Station. I never had this success with it here in Austin, I m trying again. There were 10-20 ft canes.

Just a few thoughts. I’ve had Gettude J before & she’s very thorny & grows big. I woukdnt plant her close to other roses as pruning will be very thorny-difficult. Also, in humid weather roses get black spot-for this reason I would not plant rose bushes to overlap at all-they need air between olants to dry out leaves at much as possible. In my humid climate if the roses over lap other roses or any plant-they overlapping onto other leaves area gets black spot first.

So I plant my roses with about 6 inches to a foot inbetween each bush. creates lors if air circulation.

In dryer climates I see roses and all types of flowers overlap beautifully.

I just built a new arbor for my climbers - this one was $150 at lowes.

I agree with Kimberly. In my experience Austin's recommendations for their roses are STILL based on the assumption that you are growing them in the coolish climates in the UK, not in very warm climates. Please note the comment above about growing them in San Francisco, which has a cool summer (foggy) climate, and a cool but mild winter climate. The success in SF discussed above may be because of the similarity with cool climates in the UK. I don't think trying to repeat that success (or the spacing of the roses) in Austin, TX would work. In my garden, 'Graham Thomas' gets 4 times taller and 3 times wider than Austin said it would. I did not realize that when I planted it 25 years ago, assuming it was a smallish/medium sized shrub.

Jackie

another picture of the arch - I have followed the advice here on how to plant them and the distance. I just ordered the Gertrudes two weeks ago and they were shipped and arrived the next week (a couple of days ago). I am still waiting for Sprys which I ordered two months ago.

@Bob Atchison - Pallasart - Hagia Sophiab, your pics from your prior garden a magnificent!!! What is that gorgeous climber in the last pic, if you remember>

The pink climber is Constance Spry and I don't know what the white one is, it was growing in my neighbor's garden, beautiful isn't it!

My first Gertrude plant, trained horizontally...even the laterals that sprout up from the horizontal canes want to reach for the sky. I kept after the laterals.

2nd Gertrude plant

Kimberly has a lot of relevant things to say. Constance Spry was a pretty big plant here in NC. I have a second plant of it which is showing good vigor after a slow start. It's own root, first one was grafted. I AM finding that a pretty hefty proportion of old Austins are not vigorous own root. Some do very well own root but many don't. Or they might take quite a while to get to a nice size. So they can do OK but slowly and likely will not be as big as a grafted plant. If yours are grafted I'd chuck the idea you need to plant close.

2nd Gertrude plant since first didn't show up

The two Gertrudes arrived over a week ago from DA and I planted them right away. You can see they are already sprouting new canes from the base. I am still waiting for the Sprys. They were a later edition to the online catalog and sold out right away. DA customer service said the Sprys would ship once they were dug up (?) and were ready to go. They said they would go out by the end of the month. I put down alfalfa this year which is supposed to promote new thick canes.

I’m a overplanting kind of gardener 😆 it all depends on the rose you are growing. I second growing in a arbor or another structure. Bellow I have 1 bush, 1 climber, another bush and 1 more climber. They did pretty well considering the tight space. Disease is a problem when they are too close together

What roses are those, CeresMer?

Climate conditions matter significantly on a lot of the advice your getting here. I live in Semi Arid North Texas about 5 hours north of you in Austin. I also live on a hilltop with a good bit of wind. I have some beautiful David Austins. I have none that are huge and rangy. My Gerts have been in the ground for 5 years now and neither are huge. You'll have more humidity then I but similar rainfall. In central Texas Blackspot and Powdery mildew pressures aren't anything like our west coast, east coast, and Southeast friends have to put up with. I grew up not an hour from Austin and my mother had little trouble with blackspot. Nothing like I had when I lived in Galveston.

For me none of my Austin's have gotten huge, at least not like they did on the gulf coast. In a wet year, My Olivia Rose Austin gets about 6 feet tall, but my Gerts have stayed 4 foot or shorter. 2 of them are partly shaded by a rose tree and I keep considering moving them. The other one is in full sun and its about 4.5 feet tall.

When I first started growing roses I spaced them 3 feet apart. I have since decided I prefer the roses growing together rather and intermingling some as opposed so I try to plant them 1 foot apart to 18 inches depending on the bush.

Ancient Mariner is currently my largest rose bush at about 5 foot tall and 4 foot around. I have a albrighton rambler that is only 4 foot tall.

All that to say your individual conditions will impact things greatly. We are warm weather and long growing season, but our heat and more arid climate seems to prevent the Austins from becoming the beasts they become in other areas of the US.

Kingcobb were your Gertrudes climbers?

The biggest one is. I know climbers take longer to take off, but it's also a climate issue. Roses arguably grow healthier in dryer climates, but just like trees, they don't get as large.

After waiting for two months DA told me they had oversold Spry -I had to call them to find out. I have replaced Spry with James Galway. Maybe I made a mistake ordering the Sprys since they won't bloom until the second year and are very thorny. Everyone seems to like Galway.

I have Galway, and it’s growing well. Of course, this is only year 1. Lol. Yes, Galway was highly recommended to me as well. Good luck! No matter what you end up with, I’m sure it will be beautiful.