The Polytunnel in mid April 2021

This week I thought I would concentrate on the polytunnel or greenhouse as there is a lot to be done in April both in terms of sowing new seed and in potting on tomato, chilli and sweet pepper plants sown in March. The polytunnel is warm enough now for vigourous growth from 'outdoor' cool climate crops like lettuce, Oriental salads, beeteroot, garlic or early carrot but still too cool for heat loving plants like tomato, chilli, pepper, cucumber or squash. The transition (when early spring crops make way for summer crops) begins in May for tomatoes and chillis followed by more tender plants like cucumber, courgette, squash and melon at the end of May and into June.

Some of you may remember my polytunnel plan published in February which shows my succession plan for this year, shown above. The crops above the blocks refer to what is planted now, the crops in the circle show what will be planted in May and June. The boxes below include follow on crops which will be sown or planted in the Autumn to either produce crops in winter or early the following spring.

Potting on tomatoes and peppers The photo above was taken yesterday of one of my tomato plants in a 9cm pot showing a vigourous root system that will soon be too large for the pot. As I have another 3 weeks to go before I will be planting them in the polytunnel beds, I need to pot them on to a larger pots to prevent the roots getting constricted and becoming pot bound.

If I knew my beds would be ready at the beginning of May I would pot on to 12cm pots but as I have a number of crops which won't be ready to harvest (e.g. garlic), I will go for a larger 3 litre or 19cm pot to buy myself some more time.

When potting on, tomato plants are deep planted with the stems buried up to the first set of leaves (or further if they have become very leggy) to strengthen the plant and further expand the root system. The same goes for peppers but as they are not as vigourous growers as tomatoes, they will be fine in 12cm pots.

Growing Squash I will confess I am not generally a big fan of eating squash (or baby food) but I know many people (including my wife) is so I usually grow one or two varieties to keep the peace. What I do like, a lot, is the incredible choice of shape and colour available in squash fruit and they fact that they store so well (winter squash, not summer squash) over winter. I will have to wean myself onto them.

If you are thinking about growing Squash, be prepared for the fact that they take up a huge amount of room (up to 5 square metres), they are definitely not for the small garden.

There are 2 distinct growth habits including bush and trailing types. The bush varieties require significantly less space but are still planted 1.2m apart while the trailing varieties will need a 2m spacing and even then, will easily outgrow that.

Summer or Winter Squash Apart from bush or trailing varieties, squash is also categorised as summer or winter varieties. They are both sown in April or May but summer squash is faster to mature and is harvested in the summertime while winter squash is harvested in October, before the first frost.

A courgette or marrow is technically a summer squash, other varieties include patty pan (above), vegetable spaghetti or Sunburst F1. Summer squashes don't store well so need to either be used fresh or processed and preserved. Winter squash form thicker skinned fruits that can be stored over winter and include the well known butternut squash but also (and arguably better flavoured) varieties like Crown Prince, Delicata or Uchiki Kuri. In the top photo, the large white/green squash is 'Crown Prince', the long yellow and green striped one beside it is 'Delicata' and the bright orange mini pumpkin on the bottom is 'Uchiki Kuri'.

Growing Squash Both summer and winter squash need a warm and sheltered garden with plenty of sun to ripen. 'Plenty of sun' is not something I am familiar with in Co.Sligo so I grow squash in the polytunnel otherwise they can be hit and miss. If you are a little closer to the equator than I am and want to grow squash ouside, the bright orange 'Uchiki Kuri' is a good place to start as it copes better with cooler summers and is relatively compact so is more likely to ripen in a shorter season.

Squash are sown in early-mid April for polytunnel growing and mid April to early May for growing outiside. Either way seeds need to be sown in small 7cm pots and need at least 20˚C to germinate. They are potted on to a larger 12cm post 3-4 weeks after sowing and need to be protected from cold at all times.

If planting outside remember that even the lightest frost will kill your plant so always err on the side of caution with your planting date. For polytunnel growing plant into beds in mid May, for outdoor crops planting in mid June is safest provided the weather is settled, if it is in way cold, protect with a layer of fleece for the first couple of weeks.

It is important to keep plants weed free in the early stages as they will be difficult to keep on top of once the plants fill out. If weather is bad at flowering time and there are few insects about, you may need to hand pollinate by removing the male flowers and rubbing them into the centres of the female ones. You can tell male and female flowers apart because the males have a straight stem under the flower while the females have a little bulge as shown above.

If trailing varieties are getting out of hand (and they will) they can be controlled by stopping (cutting off) the ends of the vines once the desired number of fruit have formed. Limiting fruit production to 4 or 5 per plant also makes sense, especially in cooler gardens as it is more likely to ripen and will be of a higher quality. If stopping plants, you should leave 2m of stem and leaves past the last fruit to allow the plant to photosynthesise and process enough energy to grow and ripen the remaining squash.

Sowing Cucumbers Cucumbers are another one of those crops that are far superior than the shop bought alternative. Your own cucumber will be fatter and juicer and, crucially, can be picked at just the right time so you can enjoy perfectly ripe fruit.

Before sowing cucumber seed, it is important to know if your plants are open pollinated or all female F1 varieties. Like the squash above, open pollinated cucumbers have male and female flowers but, unlike squash, we don't want them to pollinate each other. The problem is that a pollinated female cucumber develops a thick skin and becomes bitter (some people would risk a gag there but not me) so the male flowers must be regularly removed which can become a chore. The all Female F1 varieties don't have this issue (for obvious reasons) and usually produce a better quality fruit so I would recommend them every time. The best variety I have grown is 'Passandra F1', which produces massive yields of juicy, medium sized fruits.

The most common issue with starting plants off is loosing them due to cold so delaying sowing until mid to late April will make a big difference. Like squash, cucumber seeds will need 20˚C to germinate and will need to be kept warm until they are ready to plant in the polytunnel in mid to late May.

Sweetcorn for polytunnel growing Sweetcorn may not be a crop you would immediately think of for polytunnel growing but in temperate climates results can be patchy outside (depending on the summer) and in a Northerly garden like mine it won't grow at all. If you have can afford the space for it it is actually very well suited to a polytunnel, especially if grown through squash and will reliably turn out a delicious crop of sweet cobs.

As with cucumbers, I think I would recommend one of the modern F1 varieties as they are significantly sweeter. Sweetcorn is generally split into 3 categories; traditional sweetcorn, modern supersweet F1's and more recent tendersweet F1 types which are a cross between traditional and supersweet varieties.

I think the best all round variety is probably supersweet Sweet Nugget F1, especially if you have a garden warm enough to grow outside. For indoor growing in a short season I find tendersweet 'Swift F1' very good because, as the name suggests, it is quick to mature.

For indoor growing, sweetcorn is sown in mid April in 7cm pots on a heat bench or South facing windowsill at 20˚C and planted in the polytunnel beds towards the end of May. If growing outside, sow indoors in early May and plant outside in early June.

Sweetcorn is wind pollinated so, as there is little wind in the tunnel, it is another plant where we need to lend a hand. The male flowers are on the top of the plant and disperse their pollen to the sticky, silky threads of the female flowers (the corn cob) below when disturbed by the wind. To hand pollinate, simply shake the top of the plant when the male flowers have opened or collect pollen in a cup and transfer to the female flowers with a soft brush. You need to do this every couple of days when the plants are producing pollen. It is important to be thorough as every tassel on the the female flower is directly linked to a single sweetcorn kernel on the cob; if the strand doesn't get pollinated, that kernel will be missing from the sweetcorn.

You should also grow sweetcorn in a block of plants rather than a row so they can pollinate each other effectively, remember uneven pollination leads to 'toothless' cobs.

Sowing indoors for planting out in May The greenhouse, polytunnel or windowsill should also be a busy place now full of trays of seedlings for planting out in May. Crops to start indoor now include Brussels sprouts, cabbage, calabrese, cauliflower, chard, coriander, courgette, dill, kale, kohlrabi, leeks, lettuce, parsley, perpetual spinach, pumpkins, scallions, squash, swede and turnip.