How can we grow the best tasting fruit possible? It’s a question that is surely at the forefront of every gardener’s mind as the summer progresses. We are not commercial growers, who have other criteria such as appearance, yield and shelf-life to consider. Even a small plot is capable of producing more than enough fruit for our needs, and we can pick and eat or freeze within a matter of hours, so no need to worry about storage and transportation. It’s flavour that drives us to grow our own, and for this guide our focus is on raspberries - how fruit ripens, and what we can do to get the tastiest berries possible. Much of what we’ll look at applies to all fruit, so don’t be afraid to try some of the principles with any other fruit you grow, and see if you can make this year’s crops the best yet.
First, we need to go over some basic botany, to understand the processes involved when fruit ripens, what influences these processes and then how we can manipulate them for our own benefit. Don't worry, this isn't a return to the dark days of home-schooling! We won’t get too technical and the solutions are all ones we as gardeners are very familiar with.
Plants make sugars through photosynthesis- it’s a complicated process but essentially the leaves use carbon dioxide from the air, and sunlight, to form carbohydrates (sugars). Early in the season, these take the form of starches which make the young small raspberries. These pack together well (like Lego building blocks), which is why underripe fruit is firm to the touch. They also don’t taste very sweet, so making the fruit unattractive to birds whilst the seeds are still immature.
Once the seeds are fully formed and viable (so would grow if planted), the ripening process can begin in earnest. This is done by the plant producing ethylene, a gas hormone which starts to change the texture, taste and appearance of the fruit. The starch in the berries is broken down into simple sugars - fructose, glucose and sucrose, the sugars we find so appealing. Cell walls disintegrate so the fruit becomes softer. Pigments in the fruit become more noticeable - the main way we can tell many fruits including raspberries are ripe is due to the colour change. Sugars in fruit are measured by something called the Brix level. Originally developed to help winegrowers pick grapes at the right level of sweetness to get the desired alcohol level in their wine, it’s now widely used by commercial fruit growers to measure the levels of sugar, vitamins and minerals in fruit. The higher the Brix value, the sweeter and more nutritious the crop.
As an aside, fruit can be divided up into two groups. Some fruit continue to ripen after picking (called climacteric fruit). Bananas are the most obvious example, picked when still green they are exposed to ethylene after transportation and will continue to ripen in the fruit bowl. Other fruit (non-climacteric) only ripen whilst attached to the plant. All berries, including raspberries, are in this latter group, so only pick when they are perfectly ripe. They won’t get any better, they will just start to deteriorate within a short space of time. ( I write this as someone who as a student once left a globe artichoke on a sunny windowsill in the hope that it would ripen and become edible. In my defence, I was studying French rather than horticulture!)
So how can we influence all of this, and try and pack as much flavour as possible into our berries. The most obvious answer is the amount of sunlight, to allow the plant to photosynthesise and produce those vital sugars. We can’t change the weather, so we have to accept that flavour will vary slightly from year to year depending on the vagaries of the Great British Summer. However we can make sure we give the prime, sunniest spots to fruit we want to be as sweet as possible, and relegate the less-sweet fruit (such as gooseberries, Morello cherries, damsons etc) to more shady positions. Try and keep neighbouring plants and trees well-pruned, so as not to cast too much shade. Even harvesting fruit on a cloudy day has been shown to reduce Brix levels.
Another factor that is only partially under our control is water. Raspberries enjoy a cool, moist root run, so need plenty of moisture especially early in the season. However a lot of water when the fruit is ripening is counter-productive - it’s makes the berries too soft and dilutes the sugars. If you have to irrigate because you are on very free-draining soil, only do so up until mid-summer. As with many plants, a thick mulch applied in Spring when the soil is damp is the best way to ensure water levels remain consistent.
Where it becomes much more interesting is how feeding plants affects flavour, and there are several ways we can apply fertilisers to boost Brix levels and get the tastiest fruit. Plants need three main macro-nutrients to grow well - nitrogen (N), phosphorus (P) and potassium (K). You’ll see these mentioned on just about every plant feed you can buy. For fruit growing, the most important two are nitrogen and potassium, and the ratio of them available to the plant at any one time. Nitrogen is used mainly for growth, for producing new stems and healthy foliage. Plants need plenty at the start of the growing season, so they can maximise photosynthesis. However as the summer progresses, it’s the potassium which becomes most important. Many scientific studies have shown that a high potassium/ low nitrogen ratio as fruit ripens increases sugar content and flavour.
Many gardeners use a ‘one size fits all’ approach to using fertiliser- a handful of a general purpose feed such as Growmore at the start of the season, and the job is done. Others rely on an annual mulch of manure. While these help, and are certainly time efficient, it’s not the best way to get the very best flavour. Instead we should switch to a two phase strategy, giving a little nitrogen (pelleted chicken manure is excellent) early in the year, but then switching from May onwards to a high potassium feed (you can buy Sulphate of Potash, but the easiest way is to use a liquid tomato feed). That way we give the plant the right nutrients at the right time, and should get the highest levels of sugar.
Using a seaweed foliar feed over the summer has also been shown to improve Brix levels. They are very low in N, P and K, the macro-nutrients, but are very high in all the micronutrients and trace elements plants need to thrive. They stimulate root growth, and a better root system increases the uptake of water and minerals - so the plant can pack the fruit with sugary carbs. In short, the healthier the plant, the tastier the fruit.