As heatwaves in the UK appear to be becoming increasingly frequent and intense, it’s essential to take extra care of exterior commercial landscaping and any outdoor potted plants you have at home. Standard maintenance isn’t enough anymore. This guide offers important strategies for watering, soil management and plant protection to help your outdoor greenery thrive even in extreme temperatures.
In this guide, we cover:
- What are the best watering tactics for exterior plants in a heatwave?
- How do you rehydrate dried-out organic potting compost?
- Which container plant tactics protect against extreme heat?
- How can you make potting soil more heat-resistant?
What are the best watering plans for exterior plants during a heatwave?
When a heatwave hits, watering becomes a routine maintenance task rather than an exercise in precision. Simply spraying water on your pots and containers will not work. In extreme heat, improper watering techniques can waste water, damage plant foliage or leave root systems completely dry even when the surface looks wet. The timing and technique of watering are crucial for making every drop count.
- Early morning is best: Water early in the day when ground and container temperatures are coolest. Water can soak down to the roots before daytime temperatures cause rapid evaporation.
- The dusk alternative: Late evening is your second-best window. While it keeps water in the soil overnight, aim to keep the foliage dry, as excessive leaf moisture overnight in warm weather can create ideal conditions for fungal issues like powdery mildew.
- Drench deeply, don’t sprinkle: Shallow, frequent surface sprinklings encourage plant roots to stay near the top layer of soil, where they quickly bake and dry out. Instead, apply water slowly and deeply to encourage roots to grow downwards into cooler, more protected territory.
- Avoid foliage: Water droplets on leaves under direct, intense sunlight can act like tiny magnifying glasses, causing severe leaf scorch and cosmetic damage.
- Watch out for “hose scald”: Water left in a hose in sunlight can reach scalding temperatures (40℃ or higher). Always flush out warm, stagnant water onto paving or drainage grates before watering sensitive plant roots.
Automated irrigation for commercial landscapes
Manual watering across expansive commercial grounds is often labour-intensive and unsustainable during peak heat. Transitioning to automated systems ensures efficiency and consistent plant health:
- Automated drip irrigation: Sub-surface or targeted drip-line systems deliver precise volumes of water directly to root zones at pre-programmed times (e.g. 4:00 am), eliminating water waste from wind drift and surface evaporation.
- Smart weather controllers: Integrating smart sensors enables irrigation systems to dynamically adjust watering cycles based on real-time temperature, soil moisture levels and rain forecasts.
- Water efficiency compliance: In the event of regional UK water restrictions or hosepipe bans, automated micro-drip systems are frequently granted commercial exemptions due to their high water-use efficiency compared to traditional hoses.
How do you rehydrate dried-out organic potting compost?
When organic potting compost dries out completely, the organic matter forms a waxy, water-repellent coating that stops the soil from holding water. This hydrophobic soil is one of the most frustrating challenges of a heatwave (and can be an issue for neglected houseplants).
Another consequence is that the soil shrinks away from the inner walls of the pot or planter, creating an outer gap. When you pour water into the pot, it takes the path of least resistance and slides straight down the sides and out of the drainage holes without hydrating the dense, dry root ball inside. There are three methods for rehydrating, depending on the pot size.
- Submersion (for small to medium pots):
If a portable planter has gone completely hydrophobic, submerge the lower half of the pot in a tub or bucket of water for 15 to 30 minutes. This forces the soil sponge to rehydrate from the bottom up via capillary action. Don’t leave the pot in the water for longer, as it can damage the roots.
- The slow-drip (for large planters):
For pots too heavy to submerge, pour water a little at a time using a fine rose attachment on a watering can, allowing small amounts to seep through the surface before adding more.
- The washing-up liquid surfactant
For large, immovable commercial planters, you can also create a safe rewetting agent using mild washing-up liquid to help the process:
- The recipe: Add 1 to 2 drops (about 1/4 teaspoon) of washing-up liquid per 5 litres of water (one standard watering can).
- How it works: Soap acts as a surfactant, breaking the surface tension of the water and allowing it to be absorbed by the dry, waxy soil particles and spread evenly across the root zone.
⚠️ Safety warning: Use only basic, mild dish soaps. Never use dishwasher gels, laundry detergents, or antibacterial soaps. These contain harsh enzymes, high sodium levels, chemicals that strip protective root membranes or antibacterial compounds that can harm beneficial soil microorganisms.
Which plant container tactics protect against extreme heat?
While plants growing directly in flower beds can draw moisture from deeper soil layers, container-grown plants operate in a closed system. With limited soil volume and root systems sandwiched against hot container walls, potted plants are the first to show signs of heat stress during a UK heatwave.
Protecting containers requires a combination of smart positioning, root insulation and adjusted maintenance routines.
Relocate to a cooler place
- Move out of the afternoon sun: High temperatures combined with direct afternoon light (typically between 12:00 pm and 4:00 pm) create the highest thermal stress. Move portable containers, hanging baskets and troughs to the east or north side of buildings or place them under tree canopies and overhangs.
- Cluster containers together: Group individual pots into clusters on patios or entrance courtyards. As plants naturally transpire, grouping them creates a localised microclimate with higher humidity and reduced moisture loss. Furthermore, the foliage of taller plants helps shade the exposed pots and soil of smaller neighbouring containers.
Insulate and cover
Direct sunlight on a dark container can heat the soil inside to over 35°C, overheating delicate root tips and preventing water absorption.
- Raise containers with “pot feet”: Placing pots directly on sun-baked concrete, stone, or asphalt transfers high surface heat into the root ball. Elevate pots using clay pot feet, bricks, or cork risers. This creates a thermal gap, allowing cooling airflow beneath the base.
- Use double-potting: For sensitive or high-value specimen plants, place the primary growing pot inside a larger decorative planter, filling the gap between the two with damp bark, compost or spherical clay pebbles. This outer wall acts as a thermal buffer, keeping the inner root system significantly cooler.
- Shield dark planters: Black, dark grey or metallic containers absorb heat rapidly. Cover vulnerable outdoor containers with light-coloured hessian, shade cloth or bamboo screening to protect them from the heat.
Adjust plant maintenance
When plants are struggling against high temperatures, standard maintenance practices need to be paused or modified.
- ❌ No fertiliser: It is tempting to feed a wilting plant to help it recover, but applying fertiliser during a heatwave causes further damage. Plant food stimulates new foliage growth, forcing the plant to draw up more water at a time when the roots are already struggling to absorb moisture. Save plant feeding for cooler, settled weather.
- ✂️ Deadhead spent blooms, but don’t prune: Snip off fading flowers and developing seed pods so the plant doesn’t waste vital energy on reproduction. However, avoid pruning healthy foliage. Even slightly scorched leaves provide essential shade for the inner stems and topsoil below. Major structural trimming should wait until autumn or next spring.
How can you make potting soil more heat-resistant?
Standard multipurpose composts, especially lightweight, peat-free blends, are designed for general growing, but they dry out rapidly and shrink under high thermal stress.
There are several ways to improve the drought tolerance of the potting medium, by altering the soil mix or adding substances that absorb water, improve re-wetting and keep root zones cool.
- Loam-based soil mixture: Combine standard multi-purpose compost 50/50 with a loam-based potting soil (such as John Innes No. 2 for medium pots or No. 3 for large, permanent planters). Loam contains clay and sand particles. Clay absorbs water and nutrients, slowing down evaporation. Loam-based soils do not shrink away from container walls when dry, thereby preventing edge-gap runoff during watering.
- Coconut coir: Incorporate 20–30% coconut coir (the fibrous outer layer of a whole coconut) into your container soil. Coir absorbs up to 10 times its weight in water and evenly distributes moisture throughout the pot. Unlike dried peat moss, which becomes hydrophobic when dry, coir rehydrates quickly.
- Biochar (lawn/garden grade): Blending 5% to 10% biochar (a highly porous, pure form of charcoal) into your potting soil creates micro-reservoirs around the roots. These capture excess water during watering and slowly release it back to the roots as the surrounding soil dries out.
- Vermiculite or water-retaining crystals: For high-exposure containers and hanging baskets, mix in expanded vermiculite or horticultural hydrogels. These absorb water and slowly release it to the surrounding roots.
- Mycorrhizal fungi: When planting up containers, dust root balls directly with mycorrhizal fungi powder. These beneficial fungi form a symbiotic network with the roots, effectively expanding their surface area and allowing the plant to extract moisture (and nutrients) from every part of the pot.
- Top dressing: Finish every pot with a 2-3 cm top dressing of horticultural grit, slate chips, bark mulch or expanded clay pebbles. This protective barrier reflects direct heat and reduces surface evaporation by 25–50%, depending on the material.
- Expanded clay pebble base layer: For deep commercial planters or large garden pots, place a 15% base layer of expanded clay pebbles (LECA) or coarse lava rock at the bottom before adding soil. Excess water pools in this drainage layer during watering and is drawn back up into the soil via capillary action as the planter dries out later in the day.
Summary of soil amendments for heat-resistant potting soil
| Soil amendment | Best used for | Primary benefit |
|---|---|---|
| John Innes (loam) | Large planters & permanent containers | Prevents shrinkage, retains water & nutrients |
| Coconut coir | All pot & trough mixes | High moisture holding; easy re-wetting |
| Biochar | Long-term container installations | Permanent underground water & nutrient bank |
| Vermiculite/hydrogels | High-exposure containers & hanging baskets | Absorbs and releases water over several days |
| Mycorrhizal fungi | New container plantings | Expands effective root surface area for moisture extraction |
| Mulch/grit top-dressing | All exterior containers | Cuts surface evaporation by 25– 50% |
| Expanded clay pebbles (LECA) | Bottom layer of deep commercial planters | Capillary action draws pooled water back into the soil |
The benefits of commercial exterior plant management
Outdoor landscaping affects brand identity, tenant wellbeing and property value. In a heatwave, the risks to these assets increase.
Brand image and first impressions
Exterior plants are one of the first things visitors see, setting the tone for the entire site.
- Avoid signs of neglect: Dead or wilting plants suggest poor maintenance and negatively impact visitor perception.
- Protect capital assets: Plant displays can be expensive to replace.
- Tenant satisfaction: Healthy green spaces help retain tenants and improve workspace quality.
Benefits of professional grounds maintenance
Heatwaves are now a regular occurrence. During heatwaves, maintenance teams can be overwhelmed with building safety and cooling issues, and plant maintenance becomes a secondary priority.
Working with a specialist such as Ambius UK ensures consistent care:
- Expert monitoring: Technicians spot early signs of heat stress and pests before damage is permanent.
- Drought-resilient planting: Specialists select species best suited to extreme weather to maintain site standards.
- Reliable care: Scheduled watering and soil maintenance let your internal teams focus on core operations.
Conquer the heat waves
Summer heatwaves in the UK are becoming more common, but extreme heat and dry spells do not have to result in scorched leaves, dried-out plants, or wasted landscaping budgets. By moving from reactive “panic watering” to a proactive care strategy, outdoor greenery can easily survive and thrive through the hottest months.
From creating drought-tolerant soil mixes with loam and coir to insulating container root zones and using gentle surfactants for hydrophobic soil, your outdoor displays can maintain their health and vibrancy regardless of high temperatures and dry weather.
🏢 For facilities, operations and property managers
👉 Contact the Ambius UK Exterior Team Today to request a grounds evaluation or discuss custom planting solutions for your property.
🌿 For general plant enthusiasts and gardeners
Looking for more expert advice on plant care, sustainable greening, and container gardening?
Explore the Ambius Knowledge Centre for in-depth guides, seasonal tips and inspiration on keeping both indoor and outdoor spaces thriving year-round.
[Note: the link above goes the the page “Looking after plants” because there is no overarching “Knowledge Centre” page as a hub page for all the content in it, even though it is a main item in the menu. The “Articles and Insights” page goes to 2019 news!]
FAQs
1. When is the best time to water my outdoor plants during a heatwave?
The early morning is the “gold standard” for watering. Watering at dawn, when ground and container temperatures are coolest, allows moisture to soak deep into the root zone before daytime heat causes rapid evaporation. Late evening is a secondary option, provided you keep foliage dry to avoid fungal issues.
2. What should I do if my potting soil has become “hydrophobic” and repels water?
For small to medium pots, submerge the lower half in a tub of water for 15-30 minutes to rehydrate from the bottom up. For large, immovable planters, add 1-2 drops of mild washing-up liquid to a 5-litre watering can; this acts as a surfactant to break the surface tension of the waxy, dry soil.
3. Why should I avoid fertilising my plants during very hot weather?
Applying fertiliser during a heatwave can cause further damage because it stimulates new growth. This forces a plant that is already under stress to draw up even more water than the roots may be struggling to supply. It is best to save feeding for cooler, more settled weather.
4. How does “double-potting” help protect container plants in hot weather?
Double-potting involves placing your primary growing pot inside a larger decorative planter and filling the gap between the two with damp bark, compost or clay pebbles. This outer wall serves as a thermal buffer, shielding the inner root system from direct sunlight and keeping it significantly cooler.
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