I don't know if it's the same where you garden but for me for the last few years Spring has been replaced by a sort of doubled up half hearted Spring arrangement. I have been having a remarkably early start to Spring several weeks ahead of expectation but then instead of things improving slowly into early summer they have gone awry. Instead of a steadily increasing warmth I have experienced a premature (and although incredibly brief almost as good as the usual!) summer and then a very premature autumn where Summer ought to be. Indeed too regularly the end of June and July have been cooler and greyer than most of the recent March and April days.
Now as to whether or not this is climate cooling or not I don't really care except it is playing havoc with some of my crops. Indeed I have now reassessed when I sow and plant out some and this is aided by the increasing provision of seed varieties suitable for summer and autumn sowing. A second or successional sowing and planting season is also rather handy as it spreads some of the work load and the harvest through other periods alleviating the late spring bottleneck. And of course there have always been a few crops that prefer to be grown in the long but decreasing days anyway.
Cabbages and cauliflowers for over wintering are usually sown in summer but there are none that crop properly before then from such a sowing. However if it is just a few green leaves to add to a mixed salad or to juice that you are after then almost any brassica can do. Specific varieties such as Vanguard have been bred to give good crops of greens either in the autumn or later in spring but I have found that most of the hardy cabbages conventionally over-wintered for use in the spring such as Pixie, Excel and Spring Hero can be sown more closely and then thinned for greens in several stages leaving the remainder to grow on and head.
Likewise the Curly kales, indeed any kales, make plenty of young leaves almost continuously, especially if they are cloched in some way once the autumn gives way to winter. They can be sown earlier but summer sowings still make handy plants and they can be grown closer together saving space. A cover will also keep the leaves more succulent and appealing, and free of mud! Similarly the conventional sprouting broccoli's are sown months earlier but there are also annual sprouting broccoli's that sown thickly now can give several flushes of succulent stems and I reckon these to be very good value, again for better and for longer with some sort of cover or fleece.
Chinese cabbages almost have to be sown now or later as they unfailingly bolt if sown any earlier. Well they bolt for me anyway unless I irrigate thoroughly whenever I sow and grow them. If you live on the wetter side of the country these may be easier for you -but then you probably have more slugs than me, and oh do slugs love Chinese cabbages. Much easier to grow and keep slug clean is Pak-choi; this is a Chinese cabbage that does not head up the leaves as such but more resembles a head of celery with crunchy swollen stems -it is superb for stir fries. This is a versatile and useful green and ought to be more widely grown; given plastic cover it can be had fresh right throughout most winters. Shungi-ku the Chinese edible chrysanthemum can be sown and grown alongside and adds a piquant leaf to the stir fry, it also helps fend off some pests with it's strongly aromatic foliage.
Spinaches are also usually sown about now for an autumn crop as just like the Chinese cabbage they have an unerring ability to bolt if sown before midsummer, or if they get dry. As for all these summer sowings careful watering is essential and even though modern varieties such as Vivat are more mildew and bolt resistant than older sorts either or both will still be caused by dryness. I find if I want spinach I do best to grow it in a cool damp trench as for leeks or celery and provide a drip watering system.
Needing similar treatment is Florence fennel, Finocchio, this is not widely grown, probably because of it's strong aniseed flavour. It is similarly prone to bolt if sown early or if dry but given the trench as for celery and direct sowing, thinly, it can be got to produce those crunchy celery like stems which are good with fish and can go in the right stir fry. If you want whiter stems blanch the base with peat or similar for a fortnight or so before pulling otherwise it will tend to be green.
Parsley, Chervil, Dill and Rocket can all be sown now for use throughout the autumn and into winter. I grow a lot of these to use their leaves in mixed salads especially Rocket, which like mustard and cress, can be sown almost every week of the year and is very valuable nutritionally. It was banned from some monasteries as it was thought to provoke lust! Spring onions can also be sown anytime and are always useful, they will stand long into winter when chives have become unavailable. (However for over-wintering or Japanese onions that you wish to bulb up then you should wait till mid-August to sow.)
In fact about the only salading you can't sow now, well you can sow but it will probably not germinate, is lettuce. This needs cool temperatures to germinate so if you want to try to grow some then keep the site or container shaded and cool. Although there are several sorts of winter heading lettuce (they also need cloche cover of course) the loose or oak leaf non heading sorts are much better value, and more reliable. Lettuce do tend to do much better from an early autumn sowing so sow successionally in several lots to be sure.
The very similar to lettuce Chicories grown for leaf, and Corn salad, Valerianella, can be sown in several batches from now on, as can the equally useful but unknown Miners lettuce, Claytonia/Montia. These are all extremely hardy saladings which grow from now till they flower in spring. They will survive in the open ground but as for all the others you get a more palatable crop if the worst of the weather and mud is kept off them. (Watch out for slugs hiding under the Miner's lettuce which turn pink as a give-away.)
Turnips and radish are also a lot better sown now than earlier as they grow so much more quickly and are crunchier and sweeter. Of course they can't get huge but then that is not desired except for the Swedes and these last are hard this side of the country anyway regardless of when I sow them. However I find a bit of bone meal makes a vast difference to the turnips. The Black Spanish and the Oriental radishes should be sown now for use grated in winter salads and these are a very hardy and useful crop if you like their heat! Kohl rabi is a better option, it does not grow hot or woody so easily as the other roots and as it is more closely related to the brassicas it is probably more nutritious.
Beets and chards ( and perpetual spinach) may bolt but if kept moist can do very well from a summer sowing though they will not make the size earlier sowings achieve. If you sow the white or golden beets the leaves are also useful as a spinach even if they do not root up much. As with the spring sowings birds are an especial pest for these so I sow inside plastic bottle tube cloches (no top or bottom) these keep wind, slugs and birds off but do not heat up and are grand protection for all these summer sowings
French beans and Peas especially offered for July sowing have so far failed to deliver on their promise most years. I already sow in June rather than May for summer crops but July sowings, certainly those after the first week or so, are too often lost to an early frost or mildew. However as they are legumes they are also beneficial to the soil and as the ground is free after the early potatoes go it is worth sowing them anyway, maybe I'll get a decent crop this year with a bit of luck.
Potatoes are likewise a good idea but not awfully successful. They say plant your earlies late and your lates early but I don't reckon they meant this late. Specially chilled tubers are available commercially for summer and autumn planting which can crop within two months or so of starting off. There is only one problem with the plants and their cropping which is the blight which has ruined them every year in the open ground. However those I have grown in big pots under cover have done well. To save on the cost of prepared seed I keep some Early sets from the spring in my kitchen fridge which seem to work just as well as the commercial ones.
And of course right now don't forget to be pegging down strawberry runners so you can plant them out as soon as possible, and before we know it will be time for the autumn sowings, it never stops.........