Why Ferns Feel a Dry Spell So Sharply

Ferns are woodland creatures at heart. On a shaded forest floor they enjoy steady humidity, cool soil and a carpet of slowly rotting leaves, so their roots have never needed to travel far or dig deep. Instead, most garden ferns build a dense, fibrous mat of roots in the top 15cm or so of soil, which is exactly the layer that dries out first when the weather turns hot and rain stops falling.

Their fronds work against them too. They are thin, divided and full of surface area, which is wonderful for catching dappled light but means they lose water quickly in wind and sunshine. So a fern will often droop before your shrubs have noticed anything is wrong. The good news is that ferns are also remarkably tough once established. Give them deep, occasional soakings rather than daily sprinkles and they will ride out most British dry spells without lasting harm.

Water Deeply and Less Often

Little and often is the wrong approach for ferns. Light daily splashes only wet the surface, encouraging roots to stay shallow and making the plant more dependent on you. A thorough soak once every five to seven days in hot, dry weather pushes water down to the root zone and encourages a deeper, more resilient root system.

As a rough guide, a mature clump in a border will appreciate 10–15 litres at each watering, poured slowly so it soaks in rather than running off. Aim at the soil around the crown, not over the fronds — wet foliage in still, warm air invites fungal problems and wastes water that evaporates before it reaches the roots.

• Check first. Push a finger or trowel 15cm into the soil beside the crown. If it feels cool and damp, wait; if it is dry, water properly.
• Go slowly. Fill a watering can, deliver half, let it sink in, then finish. Rushing simply channels water away from where it is needed.
• New plantings need more. Ferns put in this spring or last autumn want a soak every two or three days in their first summer, until they are clearly growing away.
• Pots are different. Container ferns dry out far faster. Stand the pot in a bucket of water until bubbles stop rising, then lift it out to drain.

Mulch the Root Zone

If you do only one thing after reading this, make it mulching. A 5–7cm layer of leafmould, composted bark or well-rotted garden compost over the soil around your ferns keeps the root zone cool, slows evaporation and steadily feeds the plant as it breaks down. Leafmould is particularly good for woodland planting, because it mimics the litter layer these plants evolved with.

Apply mulch in late spring, once the soil has warmed and is still damp from rain, and top it up in mid-summer if it has thinned. Keep it clear of the crown itself — piling material against the base of the fronds can cause rot — and feather it out towards neighbouring plants. A mulched fern bed may need watering half as often as a bare one.

Timing, Water Quality and Hosepipe Bans

Early morning is the best time to water, while the soil is cool and there is time for foliage to dry before evening. Early evening is a reasonable second choice. Midday watering loses a surprising amount to evaporation and can scorch wet fronds in strong sun, so it is worth avoiding.

Rainwater from a water butt is ideal: soft, free and at ambient temperature. Most ferns cope perfectly well with tap water, though in hard water areas hart's tongue fern and hard fern can develop pale deposits on their fronds over time. When a hosepipe ban is in force, a 10-litre can per established clump, once a week, will keep a shaded border going. Use greywater sparingly, only on well-established plants, and never on edible leaves.

Plant in Communities and Shade the Soil

Ferns look better and suffer less when grown with companions that shade the ground between them. Epimedium, hardy geraniums, Pulmonaria, Tiarella, Helleborus and wood anemones all knit together into a living mulch that keeps soil temperatures down and moisture in. They also fill the gaps where sun would otherwise bake bare earth.

Be realistic about competition. Planting ferns directly beneath thirsty trees such as birch, willow or poplar means fighting for every drop, so either water more generously or choose a spot a little further out from the trunk. Tough, drought-tolerant choices include Dryopteris, Polystichum, Blechnum and Asplenium, which will shrug off a dry August far better than moisture-lovers such as royal fern or ostrich fern.

Reading the Fronds and Keeping a Simple Routine

Ferns tell you clearly when they are thirsty. Fronds that curl inwards, droop or develop a greyish sheen are asking for a deep soak that evening. Crisp brown tips and edges mean damage has already happened — trim them later, but keep watering, because the roots are still alive and working.

• Check the soil weekly through summer, ideally on the same day each week.
• Water deeply when the top 5cm feels dry, then leave the plant alone.
• Top up mulch once in midsummer, keeping it clear of crowns.
• Give pots and new plantings a little extra attention.

Do not panic if a few fronds brown off in a hot spell. Established ferns hold dormant buds at the crown, and cutting away tired growth in late winter usually triggers a fresh flush of croziers in spring. Keep the root zone cool and damp, water thoroughly rather than frequently, and your ferns will unfurl new fronds long after the dry spell has passed.